METALS AND THEIR COMPOUNDS RELATIVE ABUNDANCE AND CHEMICAL CHARACTERISTICS OF METALS,

SECOND TERM E-LEARNING NOTES

SUBJECT: CHEMISTRY

CLASS: SS3

 

 

 

WEEK 5

TOPIC: METALS AND THEIR COMPOUNDS

CONTENTS:

1. RELATIVE ABUNDANCE AND CHEMICAL CHARACTERISTICS OF METALS,
2. COMPOUNDS OF METALS AND PRINCIPLES OF EXTRACTION OF METALS
3. SODIUM-EXTRACTION, PROPERTIES
4. COMPOUNDS OF SODIUM AND USES OF SODIUM

PERIOD 1: : RELATIVE ABUNDANCE OF METALS AND CHEMICAL CHARACTERISTICS

RELATIVE ABUNDANCE

Metals are widely distributed in the earth’s crust either as compounds or in the free metallic form. More than 80 of the known elements are metals except for metals like silver, gold, and platinum, most metals are not found free in nature. They are rather too reactive to exist as free elements. They are more often found as compounds in the form of ores. These ores can exist either as the oxides, hydroxides, trioxocarbonates, sulphates, sulphides, chlorides or nitrate of the metals. Some of the metals are also found in the form of solutions because their stable compounds are stable.

The very reactive metals for example sodium and potassium usually found combined with other elements. For example they are found as chlorides or carbonates which are very stable compounds. Moderately reactive metals, such as zinc and lead, are usually found as oxides or sulphide while the least reactive metals are usually found in un combined state, for example, gold which is found almost entirely as the free metals.

CHEMICAL CHARACTERISTICS OF METALS

1. When metals react, they ionize by loss of electrons to form cations such as Na+, Ca²+ and Al³⁺, hence they are reducing agents.
2. A metal reacts with oxygen to form one or more oxides e.g Na₂O, MgO, FeO, Fe₂O₃, Fe₃O₄, CuO, Cu₂O etc. Oxides of metals are generally basic.
3. Metals react with dilute mineral acids, such as Hcl and H₂SO₄, to liberate hydrogen (except metals that are below hydrogen in the reactivity series. Zn(s) + H₂SO₄ Zn SO_4(aq) + H₂
4. They form electrovalent (ionic) compounds with non-metals such as chlorine. Ca(s) + Cl₂(g) CaCl_2(s)

Activity Series of Metals

The reactivity of a metal depends on the ease with which its atoms lose electrons; the ease varies from one metal to another. When common metals are arranged in the order of their ability to lose electrons and act as reducing agents, the activity series is obtained.

K

Na

Ca Very Reactive

Mg

Al

Zn

Fe

Sn Moderately Reactive

Pb

(H)

Cu

Hg

Ag Least Reactive

Pt

Au Least reactive (weakest reducing agent)

Of all the metals listed above, potassium K is the most electro positive and hence the strongest reducing agent, white fold, an is the weakest.

The activity series arranges the metals in the order of their ability to react and displace the ions of one another from the solutions of their salts. Generally a metal higher up in the series will displace the ions of metals lower down the series from the solutions of their salts. For instance Zn metal will displace or replace ions of copper in a solution of a copper II salt.

Zn(s) + CuSO_{4 (aq)} Cu(s) + ZnSO_4(aq)

EVALUATION

1. What name is given to the arrangement of metals in order of their ability to lose electrons?
2. From the periodic table, write out 30 elements that are metals.
3. Arrange the following metals in order of increasing reactivity Pb, Al, Sn, Q, Au, and Mg.

PERIOD 2: COMPOUNDS OF METALS AND PRINCIPLES OF EXTRACTING OF METALS

Metals form different compounds such as oxides, hydroxides, trioxocarbonates(iv), trioxonitrates(v), chloride and sulphides.

Principles of Extraction of Metals

The method of extraction of metals from its ore depends on the metals reactivity. The reactivity of a metal depends on its readiness to lose electrons since they react by forming positivity charged ion. Very reactive metals such as sodium, calcium and aluminium are extracted by electrolysis.

Other metals down the activity series are not so strongly electropositive and their compounds are not stable. They can be extracted from their ores by reduction with coke or carbon few metals e.g. gold occurs freely in nature.

Various methods used in the extraction of metals electrolysis – this is a reduction and oxidation process that is often used for the extraction of very strongly electropositive metals (Na, K Q, Mg).

During electrolysis, metallic ions from the electrolytic solution are deposited as atoms on the cathode by accepting electrons from the cathode.

Reduction of oxides: This is basically a reduction process. These metals exist in their ores as metallic ions which must take up the required number of atoms. This method is used for the extraction of less electropositive metals occur as oxides (Fe₂O₃, Fe₃O₄, SnO₂) and are extracted by the reduction of these oxides with coke or carbon (II) oxides.

Reduction of Sulphides

Some of the less electropositive metals e.g. lead occurs as sulphides e.g Feline (Pbs), the sulphide is roasted in air to convert it to the oxide which is reduced to metallic lead by heating with coke in a blast furnace.

2Ps_(s) + 3O_2(s) 2PbO_(s) + 2SO_2(g)

2PbO(s) + C(s) 2P(s) + CO₂(g)

Reduction of Chlorides

Some metals especially alkaline metals are formed as chlorides with their ores (Na+CL-) and are extracted by electrolysis of the chlorides. Sodium collect at the cathode which supplies the electrons, chlorine is formed at the anode.

NaCl Na⁺ + Cl^–

At the Anode:

Na⁺ + e- Na

At the Cathode

Cl^– + e Cl₂

EVALUATION

1. Give two methods of extraction of metals.
2. Explain any of the methods.

PERIOD 3: (ALKALI METALS (GENERAL PROPERTIES)

There is group 1 element: All except Hydrogen are metals. The following are their general properties.

1. They have one electron on their valence shall, hence they ionize to form univalent position ion M M⁺ + e
2. They are reducing agents
3. They tend to be soft and light with relatively 100 melting points.[mediator_tech]
4. First ionization energy decreases down the group.
5. They all burn in air with characteristics flame colours which are used to identify them.
6. They react with water to form alkaline hence the name alkaline metals sodium react vigorously, potassium vigorously and lithium readily.

SODIUM (Na)

This is the second member of the group I metals. It is very reactive and hence not found free in nature. Sodium occurs in sea water, as chloride, bromide or iodide i.e NaCl, NaBr, NaI.

It occurs as rock salt, NaCl in deposits. Sodium trioxonitrate(v) NaNo₃ is found abundantly in Chile and often called chile salt. It is also found in clay soil as complex trioxosilicate(iv)

EXTRACTION OF SODIUM

Sodium is extracted (in commercial quantity) from fused sodiumchloride by electrolysis. The electrolytic cell used is called down cell as shown below:

The anode is a graphite rod, while cathode is a steel cylinder round the anode together with the electrolyte are contained in a regular steel tank lined inside with fine bricks. The electrolyte i.e. fused sodium chloride with melting point 80 1⁰C is kept melting in the cell by electrical heating. The melting point of sodium chloride can be reduced to about 600⁰C by adding calcium chloride. A cylindrical steel gauze separates the cathode and the anode so that sodium and chloride which are the products of electrolysis can be obtained separately.

At the Cathode

Na⁺ ions takes up an electron each and are reduced to metallic sodium

Na⁺ + e- Na

At the Anode

Cl- ions release an electron each and gets oxidized to atomic chlorine which pair up to form chlorine gas molecules.

Cl^– Cl + e-

Cl + Cl Cl₂

2Na + 2Cl^– 2Na_(s) + Cl_2(g)

Physical Properties of Sodium

1. It is a silvery metallic solid
2. Relative density is 0.98, it therefore floats on water.
3. It is soft and very malleable
4. Melting point is rather low for a metal (97⁰C)
5. It is a good heat and electricity.

Chemical Properties

1. With Air: Sodium tarnishes readily and gets oxide by air to form sodium oxide.

4Na_(s) + O₂ 2Na₂O

The sodium oxide reacts with water vapour in air to form sodium hydroxide

Na₂O_(s) + H₂O_(g) 2NaOH_(aq)

NaOH(aq) slowly absorbs carbon (iv) oxide (CO₂)

In the atmosphere to form crystals of hydrated sodium trioxocarbonate(iv)

2NaOH_(aq) + CO_2(g) Na₂CO₃ + H₂O_(l)

Due to this great affinity of sodium for atmospheric gases it is always stored in a paraffin oil, toluene or naphtha.

If heated in air, sodium burns with golden yellow flame to form Na₂CO₃ (sodium peroxide) and Na2O (sodium oxide) in limited supply of air and oxygen

2Na_(s) + O_2(g) Na₂O_2(s)

4Na_(s) + O_2(g) 2Na₂O₂

1. With Water:

Sodium reacts silently with cold water releasing a lot of heat. It darts in water and melts to a silvery bell because of the heat liberated.

2Na_(s) + 2H₂O_(l) 2NaOH_(aq) + H_2(g)

1. With non – Metals

Sodium reacts on heating with hydrogen, halogens, sulphur, phosphorous and most other non-metals except boron, carbon and nitrogen

2Na_(s) + H_(g) 2NaOH_(s)

3Na_(s) + P Na₃P_(s)

2Na_(s) + S_(s) Na₂S_(s)

2Na_(s) + Cl₂ 2NaCl_(s)

It dissolves in mercury to give sodium amalgam

Na_(s) + H_(g) NaH_(g)

1. With Acid

Sodium reacts explosively with dilute acids to form hydrogen and salt (dangerous reaction should not be carried out in the laboratory)

2Na_(s) + 2HCl(_aq) 2NaCl_(aq) + H_2(aq)

1. With Ammonia

Sodium reacts with ammonia to form sodium and hydrogen

2Na_(s) + 2NH_3(aq) 2NaNH₂(_s) + H_2(g)

EVALUATION:

1. Explain why sodium is stored in paraffin oil.
2. With chemical equation only describe the cemical properties of sodium

Test for Sodium

Flame test: compounds containing sodium ions burn with a golden yellow flame in non-luminous flame.

PERIOD 4: USES OF SODIUM AND ITS COMPOUNDS

1. Sodium is used in manufacturing compounds like sodium peroxide, sodium cyanide, sodium as well as lead (iv) tetraethyl which is an anti-knock agent in petrol.
2. Liquid sodium is used as coolant in nuclear reactors
3. Mixtures of sodium amalgam and water or sodium and ethanol are useful reducing agents in organic chemistry. Sodium is used in reducing titanium tetrachloride to a metal in the extraction of titanium.
4. Sodium vapour lamps are used in lighting highways and airports because they give bright orange yellow light.

EVALUATION

1. Enumerate the general properties of alkali metals
2. Briefly explain the extraction of sodium metal.

GENERAL EVALUATION

OBJECTIVE TEST:

1. Most reactive metals are extracted from their ore through electrolysis except (a) Na (b) Cu (c) Al (d) Mg.
2. Group 1 metals are good reducing agents becomes (a) they are soft (b) they are light (c) they easily form unipositive ions (d) they have low melting points.
3. In down’s cell used for the extraction of sodium metal, the anode and the cathode are screened with gauze diagram in order to: (a) obtain screened products (b) obtain cheap products (c) separate the products from reacting (d) obey conventional rule electrolysis
4. Which of these compounds is not an alkali? (a) sodium hydroxide (b) aqueous ammonia (c) magnesium hydroxide (d) potassium hydroxide.
5. Metals react with oxygen to form any of these oxide except (a) acidic oxides (b) basic oxides (c) peroxides (d) amphoteric oxides

ESSAY TEST:

1. (a) Discuss the reasons why some metals occur as free elements in nature while others occur in combined forms. Give two examples each type.
2. Draw the electrolytic cell for the extraction of sodium from sodium chloride
3. Describe and explain the reactions take place at the electrodes.
4. How is metallic sodium usually stored? Explain why it is thus store.
5. Outline the uses of sodium.

WEEKEND ASSIGNMENT

Read about compounds of sodium

PRE-READING ASSIGNMENT

Read about Alkali earth metals.

WEEKEND ACTIVITY

Highlight all the compounds of sodium and discuss the uses of any two of them.