SS3 Chemistry First Term E-Note

 

SS3 Chemistry First Term E-Note

• ALKANOIC ACIDS
• ALKANOATES
• FATS AND OILS
• AMINO ACIDS
• NATURAL AND SYNTHETIC POLYMERS
• CARBOHYDRATES
• METALS AND THEIR COMPOUNDS
• CALCIUM AND ALUMINIUM
• TIN AND COPPER
• IRON

Week: 1

Topic: Alkanoic Acids

Alkanoic acids are also known as carboxylic acids. A carboxylic acid can he identified from the carboxyl functional group and the ‘-oic’ name ending.

Formula	longest carbon chain	C-C single bonds	functional group	Name	occurrence
HCOOH	C1 : meth	-an-	-COOH (oic acid)	methanoic acid (formic acid)	ants
CH3COOH	C2 : eth	-an-	-COOH (oic acid)	ethanoic acid (acetic acid)	vinegar
C2H5COOH	C3 : prop	-an-	-COOH (oic acid)	propanoic acid (propionic acid)	dairy products
C3H7COOH	C4 : but	-an-	-COOH (oic acid)	butanoic acid (butyric acid)	rancid butter
C4H9COOH	C5 : pent	-an-	-COOH (oic acid)	pentanoic acid (valeric acid)	valerian root

General formula of carboxylic (alkanoic acids) : C_nH_2n+1COOH or R-COOH

Examples of Carboxylic (Alkanoic) Acids

Structural Examples of carboxylic acids

Nomenclature (Naming Alkanoic Acids)

Alkanoic acids are named as follows:

·The number of carbon atoms in the longest carbon atom chain is noted

·The corresponding alkane is then named

·Finally, the name of this corresponding alkane is modified by removing the ane and replacing it with alkanoic acid

For example:

thylpentanoic acid     ,

2-ethylpentanoic acid      ,

benzoic acid

Preparation of ethanoic acid

The Complete Oxidation of Ethanol to Ethanoic Acid by acidified sodium heptaoxochromate(VI) solution. Ethanol undergoes oxidation first to ethanal and then to ethanoic acid.

The reaction of sodium dichromate(VI) solution with ethanol gives a carboxylic acid, ethanoic acid, a dilute solution of which is sold as vinegar.

Ethanoic acid can also be prepared by

•  distilling anhydrous sodium ethanoate, CH₃COONa with concentrated H₂SO₄
• or boiling methyl cyanide CH₃CN with an acid

CH₃COONa_(aq)+ H₂SO_4(aq)  → CH₃COOH_(g) + NaHSO_{4 (aq)}

CH₃CN_(aq) + HCl_(aq) → CH₃COOH_(g) + NH₄Cl_(aq)

Physical Properties

1.    Ethanoic acid is usually a colourless liquid with a characteristics sharp and pungent smell

2.    It is soluble in water and a dilute solution has the usual sour taste of acid

3.    It has a boiling point of 118⁰C

4.   Pure anhydrous ethanoic acid freezes into ice-like crystals at temperature below 17⁰C

5.   It turns blue litmus paper red

1. Dilute solution has a sour taste

Chemical Properties of Carboxylic (Alkanoic) Acids

1.    Carboxylic (alkanoic) acids are weak acids, the acid dissociation constant, K_a, is small.

2.    Soluble carboxylic (alkanoic) acids dissociate slightly in water.

Neutralization Reactions

Neutralization: acid + base → salt + water

Carboxylic (alkanoic) acid + base → salt (metal alkanoate) + water

RCOOH + MOH → RCOO^–M⁺ + H₂O

e.g. CH₃COOH + NaOH → CH₃COO^–Na⁺ + H₂O

Ethanoic acid + sodium hydroxide → sodium ethanoate + water

Soluble salts of long-chain (fatty) acids are soaps

e.g. C₁₇H₃₅COOH + NaOH → C₁₇H₃₅COO^–Na⁺ + H₂O

Stearic acid + sodium hydroxide → sodium stearate + water

Reaction with Carbonates

acid + carbonate → salt + carbon dioxide gas + water

Carboxylic (alkanoic) acid + metal carbonate → metal alkanoate + carbon dioxide + water

e.g. 2CH₃COOH + Na₂CO₃ → 2CH₃COO^–Na⁺ + CO₂ + H₂O

Ethanoic acid + sodium carbonate → sodium ethanoate + carbon dioxide + water

e.g. CH₃COOH + NaHCO₃ → CH₃COO^–Na⁺ + CO₂ + H₂O

Ethanoic acid + sodium bicarbonate → sodium ethanoate + carbon dioxide + water

Reaction with Active Metals

Acid + metal → salt + hydrogen gas

Carboxylic (alkanoic) acid + metal → metal alkanoate + hydrogen

e.g. 2CH₃COOH + 2Na(s) → 2CH₃COO^–Na⁺ + H_2(g)

Ethanoic acid + sodium → sodium ethanoate + hydrogen

Esterification Reactions

Esters are produced in a condensation reaction between a carboxylic (alkanoic) acid and an alkanol (alcohol).

This is known as an esterification reaction.

carboxylic (alkanoic) acid + alkanol (alcohol) ester + water

e.g. 2CH₃COOH + CH₃OH CH₃COOCH₃ + H₂O

Ethanoic acid + methanol methyl ethanoate + water

Uses

1.    It can be used as a solvent

2.    It is used in food industries as vinegar for preserving and flavouring food

3.    It is used In the manufacture of cellulose ethanoate which is used for making synthetic fibres, such as rayon

1.  It is used in making compounds like ethyl ethanoate, ethanoic anhydride (used in aspirin), cellulose ethanoate (used for packing), propanone etc

 Assessment

1. Ethanoic acid is usually a colourless liquid with a characteristics sharp and ……. smell
   a. pungent
   b. annoying
   c. foul
   d. dirty
2. Esters are produced in a condensation reaction between a carboxylic (alkanoic) acid and an alkanol (alcohol) known as …….
   a. Polymerization
   b. Esterification
   c. Dehydration
   d. Hydrolysis
3. Ethanoic acid reacts with active metal to give off ……….. gas
   a. Oxygen
   b. Hydrogen
   c. Carbon
   d. OH
4. Boiling point of Ethanoic acid is …….
   a. 117⁰C
   b. 119⁰C
   c. 118⁰C
   d. 120⁰C
5. It is used in food industries as vinegar for preserving and flavouring food. True/False

[mediator_tech]

 

Week 2

Topic: Alkanoates

Alkanoates or Esters

Ethyl Ethanoates

All esters are similar chemically although they may vary in degree of reactivity. Ethyl ethanoate is one of the simple esters. Its molecular formula is CH₃COOC₂H₂.

Properties

Ethyl ethanoate is prepared by the esterification between ethanol and glacial ethanoic acid at 150^oC in the presence of concentrated tetraoxosulphate (vi) acid.

C₂H₅OH_(aq) + CH₃COOH_(l) = CH₃COOC₂H_5(l) + H₂O_(l)

Preparation of Alkanoates

1. Acylation. Esters are prepared by the acylation of alcohols or phenols. The acylating agents can be any of the following:

Carboxylic acid / H 2SO 4 or Acyl chloride or Acid anhydride

(i) Condensation of alcohols with carboxylic acid This reaction involves esterification of alkanols by alkanoic acid

(ii) Esterification through acid derivatives

1. By Reaction of Acids with Diazomethane. Acids on being treated with ethereal solution of diazomethane yield methyl esters.

RCOOH + CH₂N₂ à  RCOOCH₃ + N₂

Acid                Diazo               Ester

methane

1. By Tischenko Reaction. When aldehydes containing a-hydrogen atoms are treated with aluminium ethoxide. They undergo condensation to produce esters.

Physical Properties of Alkanoates

1. Physical state. Esters are colourless, volatile and oily liquids with a characteristic fruity smell. The smell of the most of the flowers and fruits is due to esters present in them. The characteristic tastes and smells of different esters find applications in the manufacture of artificial flavouring and perfuming agents. The flavours of some of the esters are given below:

Ester                                        Flavour            Ester                            Flavour

n-Pentyl ethanoate           Banana            Amyl but)’rate         Apricot

Octyl ethanoate                  Orange             Isobutyl                     Raspberry

Ethyl butanoate                 Pineapple         Benzyl ethanoate   Jasmine

1. Solubility. Esters are sparingly soluble in water but are quite miscible in organic solvents like alcohols and ethers. In fact, most of the esters are themselves very good solvents for plastics and nitrocellulose.
2. Boiling points. The boiling points of esters are always less than the corresponding carboxylic acids because esters do not form hydrogen bonds.

Chemical Properties

1. Hydrolysis.Esters are hydrolysed slowly by water at boiling temperature. The reaction is catalysed by small amount of acid or base. The basic hydrolysis is also known as saponification. It is because of the fact that the esters with high molecular mass acids (C₁₂-C₁₇) give soap on hydrolysis with a base. Soaps are sodium or potassium salts of carboxylic acids with high molecular mass (C₁₂-C₁₇).
   Ethyl ethanoate can be hydrolyzed by water into its component acid and alkanol again. The reaction is catalyzed by hydrogen or hydroxide ions i.e dilute acid or alkaliCH₃COOC₂H_5(l) + H₂O_(l) → C₂H₅OH_(aq) + CH₃COOH_(l)
2. Reduction. Esters are reduced to alcohols by the reducing agents like (sodium/ethanol) or (lithium aluminium hydride).
3. Reaction with Ammonia. Esters on treatment with alcoholic ammonia yield acid amides. This reaction is known as ammonolysis of esters.
4. Reaction with Phosphorus Pentachloride. Esters are converted into acid chlorides and alkyl halides by heating with phosphorus pentachloride.

RCOOR’ + PC1₅       →   RCOCl + R’Cl + POC1₃

C₆H₅COOC₂H₅ + PCl     →    C₆H₅ COCl + C₂H₂OH

Ethyl benzoate                             Benzoyl chloride

1. Alcoholysis. An ester on refluxing with a large excess of an alcohol in the presence of a little acid or alkali, undergoes exchange of alcohol residues, i.e., alkoxy parts as shown below:

This reaction is known as alcoholysis or trans-esterification.

Uses of Esters

They are mainly used as solvents for cellulose nitrate and quick drying substances like paints, nail varnishes, lacquer and adhesives. The commonly known thinner water is a mixture of esters. Esters are used in perfumes and cosmetics and artificial flavouring for foods. Certain volatile esters are used as solvents for lacquers, paints, and varnishes; for this purpose, large quantities of ethyl acetate and butyl acetate are commercially produced.

1. Esters that are have fragrant odours are used as a constituent of perfumes, essential oils, food flavourings, cosmetics, etc
2. Esters are used as an organic solvent
3. Natural esters are found in pheromones
4. Naturally occurring fats and oils are fatty acid esters of glycerol
5. Phospoesters form the backbone of DNA molecules
6. Nitrate esters, such as nitroglycerin, are known for their explosive properties
7. Polyesters are used to make plastics
8. Esters are used to make surfactants E.g. soap, detergents

ASSESSMENT

1. How is Ethyl ethanoate prepared?
2. What are some of the uses of Esters?
3. What are the chemical properties of Esters? [mediator_tech]

Week: 3

Topic: Fats and Oils

Fats and oils are naturally-occurring esters of three long carboxylic acids  called fatty acid with a special type of alcohol called glycerol ( propane- 1,2,3-triol). Glycerol has three carbon atoms, each with an –OH group on. These fats and oils belong to a group of compounds called lipids.

Fats are solid at room temperature, whereas oils are liquids. Animals and plants produce oils and fats as an energy store. Fats and oils are the most abundant lipids in nature. They provide energy for living organisms, insulate body organs, and transport fat-soluble vitamins through the blood.

Structures of Fats and Oils

Fats and oils are called triglycerides because they are esters composed of three fatty acid units joined to glycerol, a trihydroxyl alcohol:

Saturated and Unsaturated Fats and oils

Fatty acids can be saturated or unsaturated

The saturated fatty acids have single bond in their hydrocarbons while the unsaturated ones have double bonds. Esters produced from saturated fatty acids are usually solids at room temperature and they are called fats while the esters produced from unsaturated fatty acids are liquids at room temperature and are called oils.

Sources of Fats and Oils

• Animal sources generally provide fats, for example, dripping from beef, lard from pork and tallow from lamb. These fats are solid at room temperature and only become liquid when heated.
• Vegetable sources generally provide oils. Ester oils can be obtained from olives and seeds such as corn seed, sunflower seed, peanuts and soya beans, liquids at room temperature.
• Marine sources can provide both fats and oils, Sea mammals providing fats and oils being obtained from fish.

Properties of Fats and Oils

1.    Both fats and oils are insoluble in water and they decompose at temperature above 300^oC.

2.    Since they are esters their main reactions are saponification and hydrolysis.

Uses of Fats and Oil

1.    Fats and oils are used as essential ingredients of food

2.  Sometimes they are used in natural form such as groundnut oil, palm oil for such industrial products as margarine

3.   They are used as raw materials for making soap e.g. palm oil and coconut oil

4.   They are used in making paint, candles and varnishers

Hydrolysis of Fats and Oils

Fats and oils contain ester links formed when glycerol has reacted with fatty acids. These ester links can be broken when they react with water, splitting the molecule back into an acid and an alcohol.

This process is known as hydrolysis of an ester. It can be described using the following word equation:

ester + water → acid + alcohol

Hydrogenation of Oils

The hydrogenation of oils is also known as hardening of oil. This is done by passing hydrogen into unsaturated oil at about 180^oC and 5 atmosphere of pressure and in the presence of finely divided nickel as catalyst. In the process, the unsaturated part of the oil is saturated and the oil becomes hardening into fat.

Margarine is made by mixing such hardening oils with vitamins, salt, skimmed milk and colouring materials.

Formation of Soap (Saponification)

Saponification is a process by which triglycerides are reacted with sodium or potassium hydroxide to produce glycerol and a fatty acid salt, called ‘soap’. In other word, Saponification is the alkaline hydrolysis of the fatty acid esters.

Natural soaps are sodium or potassium salts of fatty acids, originally made by boiling lard or other animal fat together with lye or potash (potassium hydroxide). Hydrolysis of the fats and oils occurs, yielding glycerol and crude soap.

In the industrial manufacture of soap, tallow (fat from animals such as cattle and sheep) or vegetable fat is heated with sodium hydroxide. Once the saponification reaction is complete, sodium chloride is added to precipitate the soap. The water layer is drawn off the top of the mixture and the glycerol is recovered using vacuum distillation. Dyes, perfumes and disinfectants are added as required before the soap is passes into bars.

Detergents

Detergents are substances which act with water to make things clean. The can be conveniently classified into two main types – soapy and soapless detergents

Soapy Detergents: are detergents made from soap. They are made by heating vegetable oils like palm oil with a strong alkali like sodium hydroxide. Soap is also biodegradable, i.e. it can easily be decomposed by bacteria into simple inorganic substances.

Soapless Detergents: Most of the soapless detergents are made from petroleum fractions. Usually the hydrocarbon is reacted with sulphuric acid, and the product neutralized with sodium hydroxide to obtain the soapless detergent. They are non-biodegradable and so create water pollution problems when their forms clog up waterways.

Detergent helps to remove dirt in different ways:

1.)  They help water to spread out and completely soak a surface. They are good wetting agents.

2.)  They form an emulsion with the dirt. They are emulsifying agents.

3.)  As a wetting agent, detergent help to reduce the surface tension of water, which tends to pull water molecules together and making water itself to be a bad wetting agent.

As an Emulsifying Agent

Detergent has two main parts;

1. A long hydrocarbon chain which is soluble in the grease or oil of the dirt. This part is not soluble in water and described as hydrophobic
2. An ionic or polar part which in not soluble in the dirt but is soluble in water. This part is described as hydrophilic, COO^– Na⁺ or COO^– K⁺

Mode of Action

When detergent is added to grease, the grease soluble hydrophobic tail dissolve in the grease and the hydrophilic parts that bring along with a negative charge are insoluble in the grease and so they remain outside. The surface of the grease becomes negatively charge, and if stirring is applied, very large of colloidal particles of grease and detergent are formed. They are held in aqueous solution by the attraction of the hydrophilic parts for the water molecules, but are prevented from coming together because similarly charged on surface of the particles cause repulsion between each other. Thus they form an emulsion of dirty water.

Differences between Soaps and Soapless Detergent

1) Soap from scum with hard water (which contains dissolve calcium or magnesium ions), while soapless detergent is not affected.

2) Soaps are made from edible animal or vegetable oils while soapless detergents are made from petroleum, which is cheaper and edible.

Assessment

1. ……… is a process by which triglycerides are reacted with sodium or potassium hydroxide to produce glycerol and a fatty acid salt, called ‘soap’.
2. Fats are ……. at room temperature.
3. ……… are substances which act with water to make things clean
4. Mention 4 sources of fats and oil.  [mediator_tech]

Week: 4

Topic: Amino Acids

Amino acids are the basic structural units of proteins. Amino acids are organic compounds which contain both an amino group and a carboxyl group, that is, any of a group of organic molecules that consist of a basic amino group (−NH₂), an acidic carboxyl group (−COOH). Each molecule contains a central carbon (C) atom, termed the α-carbon, to which both an amino and a carboxyl group are attached. The remaining two bonds of the α-carbon atom are generally satisfied by a hydrogen (H) atom and the R group. The formula of a general amino acid is:

The amino acids differ from each other in the particular chemical structure of their R group.

There are 20 naturally occurring amino acids of biological importance. The human body can synthesize all of the amino acids necessary to build proteins except for the ten called the “essential amino acids”. They can be supplied by a combination of cereal grains (wheat, corn, rice, etc.) and legumes (beans, peanuts, etc.). The 10 amino acids that we can produce are alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. Tyrosine is produced from phenylalanine, so if the diet is deficient in phenylalanine, tyrosine will be required as well. The essential amino acids (that we cannot produce internally) are arginine (required for the young, but not for adults), histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids are required in the diet. Plants, of course, must be able to make all the amino acids. Humans, on the other hand, do not have all the enzymes required for the biosynthesis of all of the amino acids.”

Peptides

Peptides are amides formed by the interaction between amino groups and carboxyl groups. The bond joining the α – amino group of one amino acid and the α  – carboxyl group of another amino acid is called peptide bond. Two amino acids react to form a dipeptide, three to form a tripeptide etc. If a numbe of amino acids are linked by peptide bonnds, a polypeptide is formed. A polypeptide chain has an amino acid end and a carboxyl end. It consists of a regular repeating part or main chain and a variable part. or side chain.

Proteins

 

Proteins are highly complex substance that is present in all living organisms. Proteins are of great nutritional value and are directly involved in the chemical processes essential for life. A protein molecule is very large compared with molecules of sugar or salt and consists of many amino acids joined together to form long chains, much as beads are arranged on a string. There are about 20 different amino acids that occur naturally in proteins. Proteins of similar function have similar amino acid composition and sequence. When proteins are eaten, they are hydrolysed in the stomach to give a large variety of amino acids, which are then carried by the blood stream to all parts of the body. Some are used to build cell proteins in the growing body or repair damaged cells. Proteins are made up of polypeptide chains. The amino acid sequence in the polypeptide chain of a protein is specified by genes.

Glycine and alanine can combine together with the elimination of a molecule of water to produce a dipeptide.

In each case, the linkage shown in blue in the structure of the dipeptide is known as a peptide link.

If you joined three amino acids together, you would get a tripeptide. If you joined lots and lots together (as in a protein chain), you get a polypeptide.

Occurrence

Proteins are found in all living systems as structural components and as biologically important substances such as hormones, enzymes and pigments. Proteins in our food can be divided into first class and second class proteins. First class proteins contain essential amino acids and they are mainly of animal origin. Examples are lean meat, fish, eggs, milk and cheese. Second class proteins are mainly vegetable proteins such as beans and peas.
The relationship between amino acid side chains and protein conformation

The defining feature of an amino acid is its side chain (at top, blue circle; below, all colored circles). When connected together by a series of peptide bonds, amino acids form a polypeptide, another word for protein. The polypeptide will then fold into a specific conformation depending on the interactions (dashed lines) between its amino acid side chains.

Some important proteins are:

1.    Insulin (a hormone)

2.    Haemoglobin (Oxygen-carrying pigment in the blood).

3.    Ribonuclease (an enzyme)

4.    Collagen (a muscle protein)

The hydrolysis is catalysed by an acid or base. The amino acids obtained on hydrolysis can be separated and identified by using paper chromatography.

Test for Proteins

1. Biuret Test: If copper (II) tetraoxosulphate (VI) is added to a solution of a protein and the resulting solution made alkaline with sodium hydroxide solution, a violet colour develops.

2. Million’s Reagent Test: Add a drop or two of million’s reagent to some egg – white solution in a test tube. The formation of a white precipitate which turns brick red on heating indicates the presence of protein.

3. Trioxonitrate (V) acid Test: Add three or four drops of concentrated trioxonitrate (V) acid to 2cm³ of egg-white solution. The formation of an intense yellow colour indicates the presence of proteins.

Properties 

Denaturation – Proteins usually form colloidal solutions. When such solutions are heated, the proteins precipitate or coagulate. This is due to the irreversible changes in the molecular shapes of the proteins, and the proteins are said to have been denatured. Proteins are easily denatured by

• temperatures above 40C
• certain organic solvents and chemical reagents
• variations in pH

Hydrolysis

Proteins can be hydrolyzed to give amino acids by boiling them with solutions of hydrochloric acid. Hydrolysis is carried out using suitable enzymes.

Assessment

Explain the denaturation of protein.

Mention 3 examples of proteins.  [mediator_tech]

Week: 5

Topic: Polymers

Polymers are giant molecules which are formed by joining together a large number of much smaller molecules thus, forming a long chain. The smaller molecule, which is the starting material, is known as monomer (meaning one unit). The molecular size of a given polymer is not fixed.

There are two types of polymers: synthetic and natural.

Synthetic polymers: are derived from petroleum oil, and made by scientists and engineers. Examples of synthetic polymers include nylon, polyethylene, polyester, Teflon, and epoxy.

Natural polymers: occur in nature and can be extracted. They are often water-based. Examples of naturally occurring polymers are silk, wool, DNA, cellulose and proteins. Rubber and many resins are also natural polymers found in plants.Polymerization is any process in which relatively small molecules, called monomers, combine chemically to produce a very large chain-like or network molecule, called a polymer. The monomer molecules may be all alike, or they may represent two, three, or more different compounds. Usually at least 100 monomer molecules must be combined to make a product that has certain unique physical properties—such as elasticity, high tensile strength, or the ability to form fibres. The formation of stable covalent chemical bonds between the monomers sets polymerization apart from other processes, such as crystallization, in which large numbers of molecules aggregate under the influence of weak intermolecular forces.

Two classes of polymerization usually are condensation and addition polymerization.

Condensation polymerization is the process whereby two or more monomers link together to form the polymer with the elimination of a small molecule. In condensation polymerization, each step of the process is accompanied by formation of a molecule of some simple compound, often water. Two most important condensation polymers are nylon and terylene.

Addition polymerization is the process whereby two or more of the same monomers link together to form the polymer without elimination of any small molecules. In addition polymerization, monomers react to form a polymer without the formation of by-products. Addition polymerizations usually are carried out in the presence of catalysts, which in certain cases exert control over structural details that have important effects on the properties of the polymer. Addition polymers include poly(ethene), poly(chloroethene), perspex, etc.

Other Terms

Linear polymers: which are composed of chain-like molecules, may be viscous liquids or solids with varying degrees of crystallinity. A number of them can be dissolved in certain liquids, and they soften or melt upon heating.

Cross-linked polymers: in which the molecular structure is a network, are thermosetting resins (i.e., they form under the influence of heat but, once formed, do not melt or soften upon reheating) that do not dissolve in solvents.

Note: Both linear and cross-linked polymers can be made by either addition or condensation polymerization.

Plastics

Plastics are natural/synthetic materials. They are produced by chemically modifying natural substances or are synthesized from inorganic and organic raw materials. They are giant molecules which are products of polymerization of simple unsaturated compounds like ethene, propene or substituted unsaturated compounds like styrene (phenylethene) and vinyl chloride chloroethene. Plastics also include synthetic fibres like nylon and terylene. Plastics are either:

Thermoplastic: This can soften upon heating and return to their original form. They are easily molded and extruded into films, fibers and packaging. Examples include polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC).

Thermoset or thermosetting plastics: Once cooled and hardened, these plastics retain their shapes and cannot return to their original form. They are hard and durable. Thermosets can be used for auto parts, aircraft parts and tires. Examples include polyurethanes, polyesters, epoxy resins and phenolic resins.

Polyethylene, LDPE and HDPE: The most common polymer in plastics is polyethylene, which is made from ethylene monomers (CH2=CH2). The first polyethylene was made in 1934. Today, we call it low-density polyethylene (LDPE) because it will float in a mixture of alcohol and water. In LDPE, the polymer strands are entangled and loosely organized, so it’s soft and flexible. It was first used to insulate electrical wires, but today it’s used in films, wraps, bottles, disposable gloves and garbage bags.

In the 1950s, Karl Ziegler polymerized ethylene in the presence of various metals. The resulting polyethylene polymer was composed of mostly linear polymers. This linear form produced tighter, denser, more organized structures and is now called high-density polyethylene (HDPE). HDPE is a harder plastic with a higher melting point than LDPE, and it sinks in an alcohol-water mixture. HDPE was first introduced in the hula hoop, but today it’s mostly used in containers.

Polyvinyl Chloride (PVC): PVC is a thermoplastic that is formed when vinyl chloride (CH2=CH-Cl) polymerizes. When made, it’s brittle, so manufacturers add a plasticizer liquid to make it soft and moldable. PVC is commonly used for pipes and plumbing because it’s durable, can’t be corroded and is cheaper than metal pipes. Over long periods of time, however, the plasticizer may leach out of it, rendering it brittle and breakable.

Polystyrene (Styrofoam): Polystyrene is formed by styrene molecules. The double bond between the CH2 and CH parts of the molecule rearranges to form a bond with adjacent styrene molecules, thereby producing polystyrene. It can form a hard impact-resistant plastic for furniture, cabinets (for computer monitors and TVs), glasses and utensils. When polystyrene is heated and air blown through the mixture, it forms Styrofoam. Styrofoam is lightweight, moldable and an excellent insulator.

Polypropylene (PP): In 1953, Karl Ziegler and Giulio Natta, working independently, prepared polypropylene from propylene monomers (CH2=CHCH3) and received the Nobel Prize in Chemistry in 1963. The various forms of polypropylene have different melting points and hardnesses. Polypropylene is used in car trim, battery cases, bottles, tubes, filaments and bags.

Nylon: Nylon fibre is obtained by heating hexane dioc acid (adipic acid) with hexane 1,6 diamine. Nylon was originally developed as a textile but is available in many forms with vastly different properties. Engineering nylon grades are easy to machine with good resistance to biological attack. Unfortunately nylons can absorb moisture from the atmosphere and can degrade in strong sunlight (they are unstable in ultraviolet light) unless a stabilizing chemical is added at the initial manufacture of the plastic. Nylons are easy to mould. Nylons also have a natural ‘oily’ surface that can act as a natural lubricant. Nylons are used for everything from clothes through to gears and bearings. It is also slippery and can be used to make washers, spacers and bushes.

EpoxyResin: is a thermosetting polymer formed from reaction of an epoxide resin with polyamine hardener. The resin consists of monomers or short chain polymers with an epoxide group at either end. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol A. Epoxy resin has various uses such as; the resins that are cured through exposure with ultra violet light are normally used in optoelectronics, fibre optic and dentistry. Industrial tooling applications normally use resin to make laminates, fixtures, castings and moulds. In the electronic world, epoxy resin is used to make transformer, insulator, switch gear and generators.

Terylene: This is a polyester that is also known as Dacron in the U.S.A. It is formed by the condensation of benzene- 1,4- dicarboxylic acid (terephthalic acid) and ethane- 1,2- diol (ethylene glycol), using an acid catalyst.

Terylene is another synthetic fibre which is mainly used in the manufacture of synthetic textiles. Terylene is mostly used for clothing, ropes, sheets, sails and many others.

Natural Rubber

Rubber is obtained from the rubber tree, Hevea brasiliensis. When the bark of the tree is cut, a thick white liquid called latex oozes out. if the latex is collected and heated, it changes into an elastic solid called rubber. This rubber is of little use because it is soft and sticky. Chemically, it consists of 2-methyl buta-1,3-diene monomers known previously as isoprene. When the monomers polymerize, they form a long polymeric chains which have only a limited number of cross-links between them. By adding sulphur and heating, the soft rubber becomes hard.

Vulcanization is chemical process by which the physical properties of natural or synthetic rubber are improved. In its simplest form, vulcanization is brought about by heating rubber with sulfur. Finished rubber has higher tensile strength and resistance to swelling and abrasion, and is elastic over a greater range of temperatures.

Synthetic Rubber

1,3-Butadiene is an important industrial chemical used as a monomer in the production of synthetic rubber. Buta-1,3-diene co-polymerize with phenylethene to form the tyre of synthetic rubber known as bunas or styrene butadiene rubber (SBR).

Assessment

1. ……… are giant molecules which are formed by joining together a large number of much smaller molecules thus, forming a long chain.
2. Explain Epoxyresin.
3. What do you understand by Polystyrene?  [mediator_tech]

Week: 6

Topic: Carbohydrates

Carbohydrates (saccharides) – Molecules consist of carbon, hydrogen and oxygen atoms. It is a major food source and a key form of energy for most organisms. When combined together to form polymers, carbohydrates can function as long term food storage molecules, as protective membranes for organisms and cells, and as the main structural support for plants and constituents of many cells and their contents. Carbohydrates are naturally occurring organic compounds. The general molecular formular is C_x(H₂O) _y.

Types of Carbohydrates

Carbohydrates can be classified into two i.e. simple and complex sugars.

1.  Simple carbohydrates: These are also called simple sugars. They are crystalline, soluble in water and have a sweet taste. Structurally, they can be further divided into monosaccharides e.g. glucose, and disaccharides e.g. sucrose

2.   Complex carbohydrates: These are also called polysaccharides. They are non-crystalline, insoluble and tasteless substances, e.g. starch and cellulose. Starches include grain products, such as bread, crackers, pasta, and rice.

Monosaccharides

A monosaccharide or simple sugar has a formula that is some multiple of CH₂O. For instance, glucose (the most common monosaccharide) has a formula of C₆H₁₂O₆. This is the smallest possible sugar unit. Examples include glucose, galactose or fructose. Monosaccharides cannot be split into smaller units by the action of dilute acids. Monosaccharides are classified according to the number of carbon atoms they possess: trioses have three carbon atoms; tetroses, four; pentoses, five; hexoses, six; etc. Each of these is further divided into aldoses and ketoses, depending on whether the molecule contains an aldehyde group (–CHO) or a ketone group (–CO–). For example glucose, having six carbon atoms and an aldehyde group, is an aldohexose whereas fructose is a ketohexose. These aldehyde and ketone groups confer reducing properties on monosaccharides: they can be oxidized to yield sugar acids.

Glucose

Glucose commonly known as grape sugar or dextrose  is present in fruits such as grapes, honey and also sap of plants. It is the main source of energy for mineral tissues and its present in the blood of animals.

Preparation

Glucose can be prepared  from the hydrolysis of starch with dilute acid.

(C₆H₁₀O₅)_n + nH₂O → nC₆H₁₂O₆

Properties

If Glucose is heated with concentrated teteraoxosulphate(vi) acid, it will be dehydrated to form a black residue of carbon.

Disaccharides

A disaccharide is a derived or condensed from two molecules of monosaccharides by the elimination of one molecule of water.

monosaccharide + monosaccharide ↔ dissacharide + water

Sucrose

Sucrose or cane sugar is the common granulated sugar which we use to sweeten food. It occurs naturally in many plants and fruits. e.g. pineapple, carrots, sorghum and sap of sugar maple tree.

Preparation

Sucrose is prepared from juices of sugar cane and sugar beet. The cane or beet is shredded and crushed between rollers and the juice is extracted with water warmed to about 80⁰C. The solution is then purified by treatment with slaked lime and carbon(iv) oxide. The purified solution is concentrated by distillation under reduced pressure. On cooling, the concentrated solution, brown crystals of sugar separate out. The remaining liquid called molasses still contains a reasonable amount of sugar and is used in ethanol production. The brown sugar obtained is impure. It is refined by treatment with slaked lime and carbon (iv) oxide, and decolorized with animal charcoal.

Properties

Sucrose is a colourless crystalline solid. It has a very sweet taste and dissolves readily in water but not alkanol.
Sucrose chars on strong heating or warming with concentrated tetraoxosulphate (vi) acid.
If sucrose is heated to a temperature of about 210 celcius which is above melting point but below its charring temperature, a yellowish – brown substance known as caramel. Caramel is used for flavouring and in confectionery.

sucrose + water ↔ glucose + fructose

Sucrose is used to sweeten food and beverages, and for preserving food. It is also used to produce ethanol by fermentation.

Polysaccharides

Preparation of Starch

The raw material to be used is peeled cassava tubers which should be washed and ground into pulp. Water is then added to the pulp to extract starch. It forms suspension and this can stay for sometime before the water above is decanted and starch residue is allowed to dry.

Physical Properties of Starch

1.    Starch is a white odourless, tasteless powder with the formula (C₆H₁₀O₅)_n

2.    It is insoluble in cold water but soluble in hot water forming a viscous solution which sets into a jelly on cooling

Chemical Properties of Starch

1.    Starch gives the familiar characteristics deep blue colour with iodine solution

2.    Hot dilute acids hydrolyse starch into maltose and glucose

3.    It does not reduce Fehling’s solution

4.    It decomposes on heating in the presence of the enzyme diastase to form maltose sugar.

Test for Starch

Add a few drops of iodine to some boiled starch, a dark blue colouration which disappears on cooling results.

Uses of Starch

1.    It is used for stiffening linen

2.    It is used to produce ethanol and glucose

3.    It is used mainly as food

Cellulose

Cellulose is the highest of the polysaccharides. It is the main component of plant cell walls and plant fibres. The principal industrial sources are cotton and wood each of which contains about 50% of cellulose. Other sources of cellulose for textile purposes are floxi china grass, hemp and jute.

Physical Properties of Cellulose

1.  It forms transparent fibres when it is pure

2.  It is insoluble in water and in most organic solvents

Chemical Properties of Cellulose

1.  Cellulose can be completely hydrolysed to glucose by hot acids

2. Hydrolysis of cellulose can also be carried out readily by the enzyme cellulose which is produced by micro organisms present in the digestive system of termites and herbivorous animals.

Uses of Cellulose

1.  It is used in the manufacture of explosives, surface coatings, paper, textiles and ropes

2.  In the manufacture of gum, cotton and explosives

Assessment

1. Chemical formular for starch is ………
   a. (C₅H₁₀O₅)_n
   b. (C₆H₁₂O₅)_n
   c. (C₆H₁₀O₅)_n
   d. (C₆H₁₁O₅)_n
2. A …………… is derived or condensed from two molecules of single sugar by the elimination of one molecule of water.
   a. Disaccharide
   b. Monosaccharide
   c. Polysaccharide
   d. Starch
3. Carbohydrates consists of ……
   a. C, H, N
   b. C, H, O
   c. C and H
   d. C and O
4. sucrose + water ↔ glucose + ………
   a. glucose
   b. galactose
   c. fructose
   d. maltose
5. Test for starch -Add a few drops of iodine to some boiled starch, a ……….. colouration which disappears on cooling results
   a. Blue
   b. Bluish-green
   c. Dark green
   d. Dark blue

Week 7  [mediator_tech]

Topic: Metals and Their Compounds

Metals

Physical Properties of Metals

The physical properties of elements are dependent on

• the arrangement of their atoms or molecules in crystal lattices when in solid state
• the bonds that bind the atoms or molecules in the solid, liquid or gaseous state.

Most metals are solid at room temperature and exist as crystal lattices in which their atoms are held together by strong metallic bonds. metals have the following physical properties

1. High melting and boiling points
2. Characteristic lustre
3. Malleability – can be hammered into sheets
4. Ductility – can be drawn into a thin wire
5. Sonorousity – give off a note when hit
6. Hard but not brittle with great tensile strength [mediator_tech]
7. Relatively high densities
8. Good conductors of heat and electricity

Some metals do not exhibit all the above properties e.g

Mercury is a liquid with a melting point of -39^oC. Sodium and potassium are light, soft metals with low melting points of 97^oC and 63^oC respectively.

Chemical Properties

1. ionization behaviour – metallic ions have few valence electrons and so have a great tendency to form positive ions by losing electrons. i.e. they are electropositive
2. reducing and oxidizing agents – metals are reducing agents because they donate electrons readily during chemical reactions.
3. reaction with acids – a metal is more electropositive than hydrogen readily displaces the hydrogen ion from an acid. This is a redox reaction with the metallic ions donating electrons to form metallic ions and the hydrogen ions accepting electrons to form gaseous hydrogen.
4. nature of oxides – most metals react with oxygen to form basic oxides which are mainly ionic compounds.Soluble basic oxides form alkalis. Some metals like aluminium and zinc form amphoteric oxides.

Occurrence of Metals

Element which have low chemical reactivity generally occur native or free or metallic state. Eg. Au, Pt, noble gas etc. element which are chemically reactive, generally occur in the combined state. Eg. Halogen, chalcogens etc. the natural materials in which the metals occur in the earth are called minerals. The minerals from which the metals is conveniently and economically extracted is called an ore. All the ores are minerals but all ores cannot be ores. Ores may be divided into four groups.

• Metallic core (siderophile) of the earth crust contains (Mn, Fe, Co, Ni, Cu, Ru, Rb, Pd, Ag, Re, Os, Ir, Pt, Au). Entire composition of metals in the earth crust may be given as, Au(8.3%); Ca(3.6%); Na(2.8%); K(2.6%); Mg(2.1%); Ti(0.4%); Mn(0.1%); Fe(5.1%) other metals (0.1%).
• Native ores: These ores contains metals in free state, e.g. silver, gold, platinum, mercury, copper, etc. These are found usually associated with rock or alluvial materials like clay, sand, etc. sometime lumps of pure metals are also found. These are termed nuggets. Irons is found in free state are meteorites which also have 20 to 30% nickel.
• Sulphurised and arsenical ores: these ores consist of sulphides and arsenides in simple and complex forms of metals. Some or ores are:

Metal              Name of the ore                     Compositions

Pb                    Galena                                     Pbs

Zn                    Zinc blender                            Zns

Ag                   Cinnabar                                  Hgs

Fe                    iron pyrites                              Fes2

Ni                    Kufer nickel                            NiAs

Cu                   Copper pyrites                         Cu2s

III. Oxidized ores: In these ores, metals are present as their oxides or oxysalts such as carbonates, nitrates, sulphates, phosphates, silicates, etc.

Important ores of this groups are listed below,

Oxides

Haemalite                                            Fe2O3

Magnetite                                            Fe2O4

Limonite                                              Fe2O3.3H2O

Bauxite                                                Al2O3.2H2O

Corundum                                            Al2O3

Diaspore                                              Al2O3.H2O

Chromite                                             FeO.Cr2O3

Chromeochre                                       FeO.Cr2O3

Tinstone (Cassiterite)                           Cr2O3

Chrysoberyl                                         BeO.Al2O3

Cuprite (Rubby copper)                       Cu2O

Pyrolusite                                            MnO2

Zincite                                                 Zno

Rutile                                                  TiO2

Llmenite                                              FeO.TIO2

Carbonates

Magnesite                                            MgcO3

Lime stone                                          CaCO3

Dolomite                                             CaCO3.MgCO3

Calamine                                             ZnCO3

Malachite                                            CuCO3.Cu(OH)2

Azurite                                               Cu(OH)2.2CuCO3

Cerussite                                             PbCO3

Siderite                                               FeCO3

Nitrates

Chile saltpeter                                     NaNO3

Salt petre                                             KNO3

Sulphates

Epsom salt                                           MgSO4.7H2O

Barites                                                 BasO4

Gypsum                                               CaSO4.2H2O

Glauber’s salt                                      Na2SO4.10H2O

Anglesite                                             PbSO4

Schonite                                              K2SO4.MgSO4.6H2O

Polyhalite                                            K2SO4.MgSO4.CaSO$.2H2O

Phosphates and Silicates

Lepidolite                                            (Li, Na, K)2Al2(SiO3)(F,OH)2

Petalite                                                            liAl(Si2O5)2

Triphylite                                             (Li, Na)3PO4,(Fe, Mn)3(PO4)2

• Halide ores : Metallic halides are few in nature, chlorides are most common . for example,

Comman salt NaCl.

Extraction of Metals

General Principles

Metals found in combined forms exist as positive ions.  During extraction, the metallic ions can be reduced to their corresponding metal atoms. This can be done electrolytically or by chemical and thermal methods. The method chosen depends on the stability of the ore which in turn depends on the position of the metal in the activity series.

1. Mining of ore containing rock

The composition of rock around the world varies greatly and locations with metal bearing ore have been sought ever since man was able to extract metals. Nowadays the search is still going on for important deposits of rock with high percentages of the mineral in question. This search is now taking place under the sea and in other inhospitable environments.

Recently, for example, rock containing an appreciable percentage of rare earth elements has been discovered under the pacific ocean. This is a particularly important discovery as virtually 99% of known working deposits are in china and rare earths are essential in the manufacture of the strong neodymium magnets needed for the computer industry

1. Separation, purification or preparation of useful ore

Very few metal ores occur in a pure enough form to be used directly in the extraction process. The first stage is to separate the useful ore from the rock. This may not be necessary in some cases, for example, the extraction of iron, but essential in the extraction of aluminium.  [mediator_tech]

This separation may be physical, such as floatation, or chemical such as digestion of the required compound in a strong base or acid followed by re-precipitation and filtration.

Most ores are either oxides or sulfides. The sulfides are usually converted to oxides by roasting in air. This tends to release sulfur in the form of sulfur(IV) oxide, a pollutant and acidic gas. However, it is also a useful gas in that it is used for the manufacture of sulfuric acid by the contact process.

1. Extraction of metal from ore

Metals are all electropositive and need to be reduced to become metallic elements. Hence, all extraction processes use reduction. For the less reactive metals chemical reduction suffices, but for the more reactive metals electrochemical reduction is needed.

1. Purification of metal

Metals that are extracted by reductive processes usually need to be further processed to make them industrially useful.

Uses of Metals

Metals are very useful to people. They are used to make tools because they can be strong and easy to shape. Iron and steel have been used to make bridges, buildings, or ships.

Some metals are used to make items like coins because they are hard and will not wear away quickly. For example copper (which is shiny and red in color), aluminium (which is shiny and white), gold (which is yellow and shiny), and silver and nickel (also white and shiny).

Some metals, like steel, can be made sharp and stay sharp, so they can be used to make knives, axes or razors.

Rare metals with high value, like gold, silver and platinum are often used to make jewellery. Metals are also used to make fasteners and screws. Pots used for cooking can be made from copper, aluminium, steel or iron. Lead is very heavy and dense and can be used as ballast in boats to stop them from turning over, or to protect people from ionizing radiation.

Assessment

1. Most metals exist in nature as
   a. crusts
   b. alloys
   c. ores
   d. felspar
2. Most metals are malleable with high densities and have high boiling points except
   a. Zn
   b. K
   c. Sn
   d. Ca
3. Method adopted in extracting a particular metal from its ore depends on
   a. the fragile nature of the metal
   b. the location of the ore in the earth’s crust
   c. the stability of the ore which depends on the position of the ore in E.C.S
   d. the availability of power in the country
4. Metals found in combined forms exist as ……… ions
   a. positive
   b. negative
   c.  neutral
   d. none of the above

Week 8

Topic: Calcium and Aluminium

Calcium

Calcium is too reactive to occur as a free metal in nature. It occurs abundantly in the combined state as calcium trioxocarbonate (iv) in limestone, marble, chalk, aragonite, calcite, coral, dolomite, calcium fluoride. etc In Nigeria, limestone is found at Nkalagu in Ebonyi, Ewekoro at Abeokuta and Ukpilla in Delta state.

Extraction

Since calcium are very stable, metallic calcium is commonly extracted electrolytically from fused calcium chloride a byproduct of solvay process. Some calcium chloride is usually added to the fused calcium chloride to lower the melting point from 850^oC to about 650^oC. The mixture is placed in a large crucible lined on the inside with graphite which serves as the anode of the cell. The cathode consists of iron rod which just touches the surface of the electrolyte. As electrolysis proceeds metallic calcium collects on the cathode which is gradually raised so that an irregular stick of calcium is formed on it. Chlorine is liberated at the cathode.

Chemistry of the Reaction

At the cathode – the calcium ions receive two electrons each to become reduced to the metal.

At the anode – two chloride ions give up an electron each to become atomic chlorine. The two atoms then combine to become liberated as a gaseous molecule.

Cl^– →  Cl^– + e^–

Cl + Cl →  Cl₂

Overall electrolytic reaction

Ca_2(l) + 2Cl^–_(l)  →  Ca_(s) + Cl_2(g)

Physical Properties of Calcium

1. Appearance – Silvery grey solid
2. Relative density is 1.55
3. Calcium is malleable and ductile
4. It has relatively low tensile strength
5. Melting point is 850^oC
6. Calcium is a good conductor of heat and electricity

Chemical Properties of Calcium

Reaction with air – Calcium is a very electropositive and reactive metal. On exposure to air, it rapidly tarnishes ad loses its metallic lustre due to the formation of white film of calcium oxide or quick lime on the surface of the metal. When calcium is heated in air, it burns with a brick red flame to form calcium oxide

2Ca_(s) + O_2(g) → 2CaO_(s)

CaO_(s) + H₂O(l)→ Ca(OH)_2(g)

Reaction with non-metals – on heating, calcium combines directly with nitrogen, chlorine, Sulphur and hydrogen

3Ca_(s) + N_2(g) → Ca₃N_2(s)

Ca_(s) + Cl_2(g) → CaCl_2(s)

Reaction with water – Calcium reacts slowly with cold water and rapidly with warm water to form calcium hydroxide and hydrogen

Ca_(s) + 2H₂O_(g) → Ca(OH)_2(aq) + H_2(g)

Reaction with Ammonia – if ammonia is passed over heated calcium, it reacts as follows

3Ca_(s) + 2NH_3(g) → Ca₃N_2(s) + 3H_2(g)

Test for calcium ions

Flame test – calcium compounds give an orange – red colour to a non-luminous flame. Moisten the unknown compound with a few drops of concentrated hydrochloric acid. Dip the tip of a clean platinum wire into the mixture and hold it in a non-luminous Bunsen flame. If a bright brick red flame through a blue glass is produced, the unknown ions of the compound are calcium ions.

With sodium hydroxide – Add a few drops of NaOH solution to an unknown salt. The formation of white precipitate which is insoluble in excess sodium hydroxide indicate the presence of calcium ions

Uses of Calcium

1. Calcium is used as a deoxidant in steel castings and copper alloys.
2. It is also used in the manufacture of calcium fluoride and calcium hydride.
3. It is used in the extraction of uranium
4. It is needed in the diet of young children for development of strong bones and teeth
5. Calcium metal is used as a reducing agent in preparing other metals such as thorium and uranium. It is also used as an alloying agent for aluminium, beryllium, copper, lead and magnesium alloys.
6. Calcium compounds are widely used. There are vast deposits of limestone (calcium carbonate) used directly as a building stone and indirectly for cement.
7. Gypsum (calcium sulfate) is used by builders as a plaster and by nurses for setting bones, as ‘plaster of Paris’.

Aluminium

Aluminium is the most common metal in the Earth’s crust, making up 7.5% by mass. Its main ore is bauxite-a clay mineral which you can think of as impure aluminium oxide. It is the most important element in group III.

Extraction of Aluminium
Aluminium is obtained largely from the ore bauxite (Al₂O₃.2H₂O). Its production is a two-step process: the purification of bauxite and extraction by electrolysis.

Purifying the bauxite (aluminium oxide) – the Bayer Process

Crushed bauxite is treated with moderately concentrated sodium hydroxide solution. The concentration, temperature and pressure used depend on the source of the bauxite and exactly what form of aluminium oxide it contains. Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres.

High pressures are necessary to keep the water in the sodium hydroxide solution liquid at temperatures above 100°C. The higher the temperature, the higher the pressure needed.

With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium aluminate (III) (NaAl(OH)₄).

Extraction of Aluminium by Electrolysis

After purification, aluminium oxide is mixed with cryolite (sodium aluminium fluoride) Na₃AlF₆ to lower the melting point from 2000º to 1000º, which saves money. This mixture is heated and the molten liquid used as the electrolyte. Both electrodes are made of graphite (carbon).  The anode (+ve) is graphite and the cathode (-ve) is a graphite lining to a steel case.

The anode disintegrates. The hot oxygen produced here reacts with the hot carbon anode to give carbon dioxide. Hence it must be replaced regularly.

Aluminium ions are attracted to the cathode (the negative electrode) and are reduced to aluminium by gaining electrons.

Al³⁺ (l) + 3e^– →  Al (l)

The molten aluminium produced sinks to the bottom of the cell.

The oxide ions are attracted to the anode and lose electrons to form oxygen gas.

2O^2- (l) → O₂ (g) + 4e^–

Note: The extraction of aluminium is an expensive process because the large amount of electricity needed to keep the electrolytes molten is expensive. Hence using cryolite saves energy and money, as it acts as a solvent for the aluminium oxide and melts at a much lower temperature.

Physical Properties of Aluminium

1. Aluminium is a silvery white metal which is comparatively soft
2. It is a strong, malleable metal element.
3. It has a low density.
4. It is resistant to corrosion
5. It is a good conductor of heat and electricity.
6. It can be polished to give a highly reflective surface.

Chemical Properties of Aluminium

• Action with air: Aluminium burns in air at high temperature to form the oxide and the nitride

4Al (s) + 3O₂ (g) ——> 2Al₂O₃ (s)

• Reaction with non-metals: On heating, aluminium combines directly with non-metals like the halogens, sulphur, nitrogen, phosphorus and carbon with evolution of heat

2Al (s) + 3Cl₂ ——> 2AlCl₃ (s)

• Action with acids: Aluminium reacts more rapidly with the concentrated hydrochloric acid to displace hydrogen but more slowly with dilute one

2Al (s) + 6HCl (aq) ——> 2AlCl₃ (aq) + 3H₂ (g)

• Reaction with alkalis: Aluminium reacts with both sodium and potassium hydrogen solutions giving hydrogen gas and soluble tetrahydroxoaluminate (III)

2Al (s) + 2NaOH (aq) + 6H₂O (l) ——> 2NaAl(OH)₄ (aq) + 3H₂ (g)

• Reaction with Iron (III) oxide: Aluminium reduces iron (III) oxide to molten iron. The reaction is used in thermit process and it gives out a great deal of energy

2Al (s) + Fe₂O₃ (s) ——> Al₂O₃ (s) + 2Fe (s)

Uses

1. Low density and strength make aluminium ideal for construction of aircraft, lightweight vehicles, and ladders.
2. An alloy of aluminium called duralumin is often used instead of pure aluminium because of its improved properties.
3. Easy shaping and corrosion resistance make aluminium a good material for drink cans and roofing materials.
4. Corrosion resistance and low density leads to its use for greenhouses and window frames.
5. Good conduction of heat leads to its use for boilers, cookers and cookware.
6. Good conduction of electricity leads to its use for overhead power cables hung from pylons (low density gives it an advantage over copper).
7. High reflectivity makes aluminium ideal for mirrors, reflectors and heat resistant clothing for fire fighting.

Assessment

Explain the method of extraction of Aluminium in few lines and bullet points.

Mention 3 uses of Calcium

Week 9

Topic: Tin and Copper

Tin

Tin does not occur naturally as a free element. The main source is the mineral cassiterite or tin stone, SnO_2. In Nigeria was mined in 1930 at Jos Plateau.

Extraction from its Ores

Tin ore  is crushed and washed with water. This is called the concentration of the ore. The tin ore is roasted in air to remove impurities, such as arsenic and antimony as volatile oxides. The product is mixed with powdered charcoal and heated to 1300^oC. to reduce the oxide. Molten tin is tapped off. Iron compounds, which might be present as impurities are removed by electromagnetic separation.

The metal is extracted from its ore by carbon reduction. The concentrated ore is mixed with coke and heated in a furnace.

SnO_2(s) + 2C_(s) → Sn_(l) + 2CO_(g)

The tin obtained is purified. It is separated from copper, iron and any other element present as impurities by either thermal -heating beyond it melting point of 232 K, and running off the molten tin, leaving behind any less fusible impurities – or by electrolytic means.

Much purer tin is obtained by the electrolysis of aqueous solution of tin(II) chloride, SnCl₂ – the impure tin is made anode, while the cathode is pure tin.

Test for Tin ions

Hydrogen Sulphide – Pass hydrogen sulphide into a solution of the unknown salt acidified with dilute hydrochloric acid. Tin(ii) ions are present if a brown precipitate which dissolves in yellow ammonium sulphide and in hot concentrated hydrochloric acid is obtained

Mercury (ii) chloride- The formation of a white precipitate of mercury(i) chloride would indicate the presence of tin(ii) ions.

Physical Properties of Tin

1. Tin is solid, with silvery white appearance with lustre.
2. It’s melting point is 232^oC.
3. It is malleable and soft (enough to be cut with a knife).
4. It is a good conductor of heat and electricity.
5. It exists in three different forms. These are grey tin of density 5.76 g/cm³; white tin of density 7.28 g/cm³; and rhombic tin of density 6.6 g/cm³. These allotropes (the different forms) can be converted from one to another by changes in temperature.

grey white rhombic

Grey

13.2oC

White

161oC

Rhombic

1. Tin is not ductile enough to be drawn into wires.

Chemical properties of Tin

1. Reaction with Oxygen – it does not react with oxygen, except at temperature above 1300^oC.

Sn_(s) + O_2(g) → SnO_2(s)  at temperatures greater than 1300^oC

Hence, It does not corrode or explode in air.

1. With nitrogen and carbon – no reaction occurs.
2. With non metals, example, chlorine – it reacts when heated with chlorine to form tin(IV) chloride.

Sn_(s) + 2Cl_2(g) → SnCl_4(s)

1. With acids – it reacts with acids to different degrees at different concentrations and temperatures. With dilute HCl – no reaction occurs.

With Concentrated HCl the reaction is rapid, producing tin(II) chloride.

Sn_(s) + 2HCl_(aq) → SnCl_2(s) + H_2(g)

With dilute H₂SO₄ – no reaction occurs. With hot concentrated H₂SO₄ a reaction occurs to release SO₂.

Sn_(s) + 2H₂SO₂ → SnSO_4(aq) + SO_2(g) + 2H₂O_(g)

With dilute HNO₃ – tin reacts with dilute HNO₃, forming Sn(NO₃)₂ and hydrogen.

With conc. HNO₃ – tin reacts with conc. HNO₃ , forming SnO₂.

1. With alkalis – it forms trioxostannate(IV) salts and hydrogen with conc. solutions of alkalis.

Sn_(s) + 2NaOH_(aq) + H₂O_(l) → Na₂SnO_3(aq) + 2H_2(g)

Uses of Tin  [mediator_tech]

There are a number of ways tin can be used. These include:

1. For coating steel – the coating is done by electrolytic method. Tin prevents corrosion in steel, such may be used to can food and drinks.
2. For making alloys together with lead, antimony and copper. Alloys of tin are important, such as soft solder, pewter, bronze and phosphor bronze. A niobium-tin alloy is used for superconducting magnets.
3. Due to its resistance to atmospheric corrosion and low melting point, it can be used to make sheet glass.
4. The most important tin salt used is tin(II) chloride, which is used as a reducing agent and as a mordant for dyeing calico and silk. Tin(IV) oxide is used for ceramics and gas sensors. Zinc stannate (Zn₂SnO₄) is a fire-retardant used in plastics.
5. Some tin compounds have been used as anti-fouling paint for ships and boats, to prevent barnacles. However, even at low levels these compounds are deadly to marine life, especially oysters. Its use has now been banned in most countries.
6. A very important application of tin is tin-plating. Tin-plating is the process by which a thin coat of tin is placed on the surface of steel, iron, or another metal. Tin is not affected by air, oxygen, water, acids, and bases to the extent that steel, iron, and other metals are. So the tin coating acts as a protective layer.
7. Another tin alloy is Babbitt metal. Babbitt metal is a soft alloy made of any number of metals, including arsenic, cadmium, lead, or tin. Babbitt metal is used to make ball bearings for large industrial machinery. The Babbitt metal is laid down as a thin coating on heavier metal, such as iron or steel. The Babbitt metal retains a thin layer of lubricating oil more efficiently than iron or steel.

Copper

Copper was one of the first metals discovered and used by man. It is a stable metal readily obtained from its compounds. Copper ores are widely found around the world. The main ores are copper pyrites (CuFeS2), malachite (CuCO3.Cu (OH)2), chalcocite (CuS2) and cuprite (CuO).

Extracting copper from its ores

The method used to extract copper from its ores depends on the nature of the ore. Sulphide ores such as chalcopyrite (copper pyrites) are converted to copper by a different method from silicate, carbonate or sulphate ores.

The process:

The concentrated ore is heated strongly with silicon dioxide (silica) and air or oxygen in a furnace or series of furnaces.

• The copper(II) ions in the chalcopyrite are reduced to copper(I) sulphide (which is reduced further to copper metal in the final stage).
• The iron in the chalcopyrite ends up converted into an iron(II) silicate slag which is removed.
• Most of the sulphur in the chalcopyrite turns into sulphur dioxide gas. This is used to make sulphuric acid via the Contact Process.

An overall equation for this series of steps is:

The end product of this is called blister copper – a porous brittle form of copper, about 98 – 99.5% pure.

Purification of copper

When copper is made from sulphide ores by the first method above, it is impure. The blister copper is first treated to remove any remaining sulphur (trapped as bubbles of sulphur dioxide in the copper – hence “blister copper”) and then cast into anodes for refining using electrolysis.

Electrolytic refining

The purification uses an electrolyte of copper(II) sulphate solution, impure copper anodes, and strips of high purity copper for the cathodes.

The diagram shows a very simplified view of a cell.

For every copper ion that is deposited at the cathode, in principle another one goes into solution at the anode. The concentration of the solution should stay the same.

All that happens is that there is a transfer of copper from the anode to the cathode. The cathode gets bigger as more and more pure copper is deposited; the anode gradually disappears.

In practice, it isn’t quite as simple as that because of the impurities involved.

Physical properties of copper

1. Copper is a heavy, reddish-brown metal
2. It is very malleable and ductile
3. It has a density of 8.95 g cm^-1
4. It is a good conductor of heat and electricity
5. It has a high melting point of 1083^oC
6. It has a boiling point of 2300^oC
7. It also forms alloys very readily

Chemical properties of Copper

• Reaction with Air: It is resistant to pure dry air, but over a long period of time in a moist, impure atmosphere, it becomes coated with green, basic copper(II) tetraoxosulphate (VI) CuSO₄.3Cu(OH)₂ and trioxocarbonate (IV).

On heating in air or oxygen, copper is readily oxidized to give black copper (II)oxide

2Cu(s) + O₂(g) ——–> 2CuO(s)

• Effect of Acids: Copper is lower than hydrogen in the electrochemical series, hence, it is not capable of displacing hydrogen from dilute acids. It is however attacked by oxidizing acids like trioxonitrate (V) acid and tetraoxosulphate (VI) acids

3Cu(s) + 8HNO₃(aq) ——> 3Cu(NO₃)₂(s) + 4H₂O(l) 2NO(g)

• Displacement Reaction: Because of its low position in the activity and electrochemical series, copper is easily displaced from its compound

CuSO₄(aq) + Fe(s) ——-> FeSO₄(aq) + Cu(s)

• Hydrogen gas reduces copper oxides to the metal

CuO(s) + H₂(g) ——> Cu(s) + H₂O(l)

Test For Copper(II) Ions

With sodium hydroxide: Add a few drops of sodium hydroxide solution to a solution of copper salt. The formation of a blue precipitate which is insoluble in excess sodium hydroxide confirms the presence of copper(II) ions

Cu²⁺(aq) + 2NaOH(aq) ——-> Cu(OH)₂(s) + Na⁺(aq)

Uses of Copper

Amongst other things copper is used for:

1. Electrical wiring. It is a very good conductor of electricity and is easily drawn out into wires.
2. Domestic plumbing. It doesn’t react with water, and is easily bent into shape.
3. Boilers and heat exchangers. It is a good conductor of heat and doesn’t react with water.
4. Baking brass. Brass is a copper-zinc alloy. Alloying produces a metal harder than either copper or zinc individually. Bronze is another copper alloy – this time with tin.
5. Coinage. In the UK, as well as the more obvious copper-coloured coins, “silver” coins are also copper alloys – this time with nickel. These are known as cupronickel alloys. UK pound coins and the gold-coloured bits of euro coins are copper-zinc-nickel alloys.

Assessment

1. Tin exists in 3 different forms. Mention them?
2. How can Tin be extracted?
3. The method of extraction of copper depends on what?
4. Mention 4 uses of copper

Week 10

Topic: Iron 

Iron

Iron is the most important element in the industry. It is the second most abundant element on the earth crust after aluminium, but often occurs as a free metal.

The common ores are haematite found in united states, Australia and USSR. It can also occur as impure iron (III) oxide (Fe₂O₃), Magnetite or magnetic iron ore (Fe₃O₄) is found in Sweden and in North America.

Siderite or spathic iron ore, (FeCO₃), found in Great Britain. Iron also occurs as iron pyrites (FeS₂) and limonite (Fe₂O₃.3H₂O).

Iron is widely present as trioxosilicate (IV) in clay soils. Iron ore is available in Itakpe, Ajaokuta, Jebba and Lokoja all in Kwara State (Nigeria).

Extraction of Iron

The extraction of iron from iron ore (haematite), using coke, limestone and air in a blast furnace

Haematite is basically iron oxide, and the oxygen must be removed to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions. Since carbon is more reactive than iron, it can displace the iron from its oxide. Hence the method for extraction of iron is called ‘reduction by carbon’.

Coke is impure carbon, and it burns in the hot air blast to form carbon dioxide. This is a strongly exothermic reaction which makes it an important reaction, as it helps heat up the blast furnace. The iron ore, coke and limestone enter the blast furnace at the top. The hot waste gases at the top of the furnace are piped away and used to heat the air blast at the bottom.

C (s) + O₂ (g) ——> CO₂ (g)

At high temperatures in the furnace, the carbon dioxide is reduced by more carbon to give carbon monoxide.

CO₂ (g) + C (s) ——> 2CO (g)

It is the carbon monoxide which is the main reducing agent in the furnace-especially in the cooler parts.

Assuming that the iron ore is haematite, Fe₂O₃:

Fe₂O₃ (s) + 3CO (g) ——> 2Fe (l) + 3CO₂ (g)

Due to the high temperatures, the iron produced melts and flows to the bottom of the furnace, where it can be tapped off.

In the hotter parts of the furnace, some of the iron oxide is also reduced by carbon itself.

Fe₂O₃ (s) + 3C (s) ——> 2Fe (l) + 3CO (g)

Notice that carbon monoxide is formed, rather than carbon dioxide, at these temperatures.

However some use this equation instead:

iron oxide + carbon    →    iron + carbon dioxide

2Fe₂O₃ + 3C    →    4Fe + 3CO₂

The limestone is added to the furnace to remove impurities in the ore which would otherwise clog the furnace with solid material.

The furnace is hot enough for the limestone (calcium carbonate) to undergo thermal decomposition. It splits up into calcium oxide and carbon dioxide. This is an endothermic reaction (it absorbs heat) and it is important not to add too much limestone to avoid cooling the furnace.

CaCO₃ (s) ——> CaO (s) + CO₂ (g)

Calcium oxide is a basic oxide, and its function is to react with acidic oxides such as silicon dioxide, SiO₂. Silicon dioxide is the main constituent of sand, and is typical of the sort of impurities that need to be removed from the furnace.

CaO (s) + SiO₂ (s) —–> CaSiO₃ (l)

The product is calcium silicate. This melts and trickles to the bottom of the furnace as a molten slag, which floats on top of the molten iron as it is less dense, and can be tapped off separately. Slag is used

Uses of iron

• Cast iron

Molten iron straight from the furnace can be cooled rapidly and solidified by running it into sand moulds. This is known as pig iron. If the pig iron is remelted and cooled under controlled conditions, cast iron is formed. This is very impure iron, containing about 4% carbon as its main impurity. Although cast iron is very hard, it is also very brittle and tends to shatter if it is hit hard. It is used for things like manhole covers, gutterings and drainpipes, and cylinder blocks in car engines.

• Mild steel

Mild steel is iron containing up to about 0.25% carbon. This small amount of carbon increases the hardness and strength of the iron. It is used for (among other things) wire, nails, car bodies, ship building, girders and bridges.

• Wrought iron

This is pure iron. It was once used to make decorative gates and railings but has now been largely replaced by mild steel. The purity of the iron makes it very easy to work because it is fairly soft, but the softness and lack of strength mean that it isn’t useful for structural purposes.

• High-carbon steel

High carbon steel is iron containing up to 1.5% carbon. Increases the carbon content makes the iron harder, but at the same time it gets more brittle. High-carbon steel is used for cutting tools and masonry nails. Masonry nails are designed to be hammered into concrete blocks or brickwork where a mild steel nail would bend. If you miss-hit a masonry nail, it tends to break into two bits because of its increased brittleness.

• Stainless steel

Stainless steel is an alloy of iron with chromium and nickel. Chromium and nickel form strong oxide layers in the same way as aluminium, and these oxide layers protect the iron as well. Stainless steel is therefore very resistance to corrosion.

Obvious uses include kitchen sinks, saucepans, knives and forks, and gardening tools. But there are also major uses for it in the brewing, dairy and chemical industries where corrosion- resistant vessels are essential

Types of iron	Iron mixed with	Some uses
Wrought iron	(pure iron)	Decorative work such as gates and railings
Mild steel	Up to 0.25% carbon	Nails, car bodies, ship building, girders
High-carbon steel	0.25-1.5% carbon	Cutting tools, masonry nails
Cast iron	About 4% carbon	Manhole covers, guttering, engine blocks
Stainless steel	Chromium and nickel	Cutlery, cooking utensils, kitchen sinks

Physical Properties of Iron

1. Pure iron is a grey metal with density 7.8g cm^-3
2. It melts at 1535^oC and boils at 2800^oC
3. It is very malleable and ductile
4. It is a good conductor of heat and electricity  [mediator_tech]
5. It can easily be magnetized

Chemical Properties of Iron

• Reaction With Air: It burns in air to form reddish hydrated iron (III) oxide of variable composition

4Fe(s) + 3O₂(g) + 2xH₂O(l) ——-> 2Fe₂O₃.xH₂O

• When finely divided iron is heated in air it burns at high temperature to form magnetic iron oxide, which behaves like a compound oxide

3Fe(s) + 2O₂ (g) ——> Fe₃O₄ (s)

• Reaction With Steam: When steam is passed over red-hot iron filings, tri iron tetraoxide and hydrogen are produced and the reaction is reversible

Fe(s) + 4H₂0 <——> Fe₃O₄(s) + 4H₂ (g)

Order of reactivity	Symbol	Method of Extraction
Potassium	K	Electrolysis

The metal compound is:

1.           Melted, then
2.           Has electricity passed through it

Assessment

1. What are the uses of the following Iron
   – Cast Iron
   – Stainless steel
   – Wrought Iron
2. Stainless steel is an alloy of iron with ………. and ………
3. Mention 2 common ores of iron ……… and ………