The Rate of Chemical Reactions
Subject :Chemistry
Term : Second Term
Week :Week 1
Topic : The Rate of Chemical Reactions
Behavioural Objectives :By the end of the lesson, students should be able to
- Define rate of reaction
- Explain the collision theory and how it relates to the factors affecting rate of reaction
- Identify and describe the factors affecting rate of reaction
- Understand the effects of each factor on the rate of reaction
- Apply knowledge of the factors affecting rate of reaction to solve problems
Materials Needed:
- Whiteboard and markers
- PowerPoint presentation
- Handouts with practice problems
- Chemicals and apparatus for demonstration experiments (optional)
Content
The rate of a chemical reaction refers to how quickly the reactants (the substances that are reacting with each other) are being converted into products (the new substances that are formed as a result of the reaction). It’s essentially a measure of how fast a reaction is happening.
Here are a few examples to help illustrate this concept:
- Rusting of Iron: When iron is exposed to oxygen and water, it will eventually start to rust. However, the rate at which this happens can vary depending on different factors. For example, if the iron is left in a dry environment, it will rust more slowly than if it’s exposed to a lot of moisture. Similarly, if the iron is coated with a protective layer (like paint or oil), the rusting process will be slowed down or even prevented altogether.
- Baking a Cake: When you mix together the ingredients for a cake and put it in the oven, a chemical reaction takes place that causes the batter to turn into a delicious cake. The rate at which this reaction occurs can be influenced by a number of factors, such as the temperature of the oven, the amount of baking powder or baking soda used, and the type of flour used in the recipe. If you don’t bake the cake long enough or don’t use enough baking powder, the reaction won’t occur as quickly and the cake won’t rise properly.
- Digesting Food: When you eat food, your body breaks it down into smaller molecules that can be absorbed into your bloodstream and used for energy. This process of digestion is a chemical reaction, and the rate at which it occurs can be influenced by factors like the type of food you’re eating, the amount of stomach acid you produce, and the enzymes in your digestive system. If you eat a meal that’s high in fiber, for example, it will take longer to digest than a meal that’s mostly simple carbohydrates
It’s important to note that the units may vary depending on the specific reaction being studied and the units used to measure the concentrations or masses involved
Worked Examples
EXAMPLE: When 0.5g of calcium trioxocarbonate (IV) was added to excess dilute hydrochloric acid, carbon (IV) oxide was evolved. The complete reaction took 5 minutes. What was the rate of reaction? SOLUTION: Rate of reaction = change in mass of reactant Time taken for the change = (0.5 – 0)g = 0.5 5×60 300 = 1.67 x 10-3 gs-1
Example 1: When 2.0 moles of hydrogen gas (H2) are mixed with 1.0 mole of oxygen gas (O2) at a temperature of 298 K and a pressure of 1 atm, they react to form water vapor (H2O). The reaction takes 10 seconds to complete. What is the rate of reaction?
SOLUTION: The balanced chemical equation for the reaction is: 2H2(g) + O2(g) -> 2H2O(g)
Since the reaction uses up both hydrogen and oxygen, we can measure the rate of reaction by tracking the change in concentration of either of these reactants over time. Let’s choose to measure the change in concentration of hydrogen gas:
Rate of reaction = change in concentration of H2 Time taken for the change
At the beginning of the reaction, the concentration of hydrogen is 2.0 moles / (total volume of the gas mixture). At the end of the reaction, all the hydrogen is used up and the concentration is 0 moles / (total volume of the gas mixture). So the change in concentration is 2.0 moles / (total volume of the gas mixture).
Time taken for the change is 10 seconds.
Therefore, the rate of reaction is: Rate of reaction = (2.0 moles / total volume of the gas mixture) / 10 seconds = 0.2 moles / (total volume of the gas mixture * seconds)
Example 2: When 5.0 grams of magnesium metal (Mg) is added to 100.0 mL of 0.5 M hydrochloric acid (HCl), hydrogen gas (H2) is evolved. The complete reaction takes 2.0 minutes. What is the rate of reaction?
SOLUTION: The balanced chemical equation for the reaction is: Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)
Since the reaction uses up magnesium, we can measure the rate of reaction by tracking the change in mass of magnesium over time:
Rate of reaction = change in mass of Mg Time taken for the change
At the beginning of the reaction, the mass of magnesium is 5.0 grams. At the end of the reaction, all the magnesium is used up and the mass is 0 grams. So the change in mass is 5.0 grams.
Time taken for the change is 2.0 minutes = 120 seconds.
Therefore, the rate of reaction is: Rate of reaction = 5.0 grams / 120 seconds = 0.042 g/s or 42 mg/s
Evaluation
- Which of the following factors does NOT affect the rate of a chemical reaction? A) Temperature B) Concentration of reactants C) Presence of a catalyst D) pH of the solution E) Surface area of the reactants
- Which of the following statements is true about the rate of a chemical reaction? A) The rate is always constant throughout the reaction. B) The rate decreases as the reaction progresses. C) The rate increases as the reaction progresses. D) The rate remains the same whether the reaction is exothermic or endothermic. E) The rate depends on the number of products formed.
- Which of the following is a unit of measurement for the rate of a chemical reaction? A) Joules B) Newtons C) Coulombs D) Meters per second E) Moles per liter per second
- Which of the following is an example of a homogeneous reaction? A) Combustion of coal B) Rusting of iron C) Digestion of food D) Reaction between hydrochloric acid and sodium hydroxide E) None of the above
- Which of the following statements is true about a catalyst? A) It decreases the activation energy of a reaction. B) It increases the rate of a reaction. C) It is used up in the reaction. D) It changes the equilibrium constant of the reaction. E) It increases the amount of products formed
- Which of the following factors affects the rate of a gas-phase reaction? A) Concentration of the reactants B) Temperature C) Pressure D) Surface area of the reactants E) pH of the solution
- Which of the following is an example of a first-order reaction? A) 2A + B -> C B) A + B -> C + D C) A -> products D) A + B -> products E) A + B + C -> products
- Which of the following statements is true about activation energy? A) It is the energy released when a reaction occurs. B) It is the energy required to start a reaction. C) It is the total energy change in a reaction. D) It is the energy required to break a bond. E) It is the energy required to form a bond.
- Which of the following statements is true about the rate law for a reaction? A) It can be determined from the balanced chemical equation. B) It depends on the concentrations of the products. C) It is the same as the equilibrium constant. D) It is independent of temperature and pressure. E) It can only be determined experimentally.
- Which of the following statements is true about the rate of a zero-order reaction? A) It is proportional to the concentration of the reactants. B) It is proportional to the square of the concentration of the reactants. C) It is independent of the concentration of the reactants. D) It is proportional to the inverse of the concentration of the reactants. E) It is proportional to the temperature of the reaction
Rate Curve
A rate curve is a graphical representation of the rate of a chemical reaction over time. It shows how the rate of the reaction changes as the reaction progresses.
To create a rate curve, the concentration of a reactant or product is measured at various time intervals during the reaction. The rate of the reaction can then be calculated by dividing the change in concentration by the change in time.
The resulting data can be plotted on a graph with time on the x-axis and the rate of the reaction on the y-axis. The curve can then be analyzed to determine the order of the reaction, the rate constant, and other important information about the reaction.
A typical rate curve for a first-order reaction would show an exponential decay, with the rate decreasing over time as the concentration of the reactant decreases. For a second-order reaction, the rate curve would show a more gradual decay.
Overall, a rate curve is a useful tool for understanding the kinetics of a chemical reaction and for predicting how the reaction will behave over time
A typical rate curve graph will have time on the x-axis and the rate of the reaction on the y-axis. The graph may show a curve that starts at a high rate and gradually decreases over time, or it may show a curve that starts low and increases over time, depending on the order of the reaction.
For a first-order reaction, the curve will be exponential and decrease over time. This is because the rate of the reaction is proportional to the concentration of the reactant, which decreases over time as the reaction progresses.
For a second-order reaction, the curve will also decrease over time, but the rate of the decrease will be more gradual compared to a first-order reaction. This is because the rate of a second-order reaction is proportional to the square of the concentration of the reactant.
Overall, a rate curve graph can provide valuable information about the kinetics of a chemical reaction and can be used to determine important parameters such as the rate constant and the order of the reaction
COLLISION THEORY
Collision theory is a concept in chemistry that explains how chemical reactions occur at the molecular level. It suggests that for a chemical reaction to occur, the reactant particles must collide with each other with sufficient energy and the correct orientation.
The basic principles of collision theory can be summarized as follows:
- Collision Frequency: The rate of a chemical reaction is proportional to the frequency of collisions between the reactant particles. The more often the particles collide, the greater the chances of a reaction occurring.
- Collision Energy: Not all collisions between reactant particles will lead to a reaction. The particles must collide with enough kinetic energy to overcome the activation energy barrier of the reaction. The activation energy is the minimum amount of energy required for a reaction to occur.
- Correct Orientation: The reactant particles must collide in the correct orientation for a reaction to occur. This means that the particles must align themselves in a certain way during the collision to allow the necessary bonds to form or break.
Based on these principles, collision theory suggests that increasing the temperature, pressure, or concentration of the reactants can increase the rate of a reaction by increasing the frequency of collisions and/or the kinetic energy of the particles. Adding a catalyst can also increase the rate of a reaction by lowering the activation energy barrier and increasing the number of successful collisions.
Overall, collision theory helps explain why certain reactions occur and others do not, and it provides a framework for predicting and controlling the rates of chemical reactions
1. State collision theory.
2. Graphically differentiate exothermic reaction from endothermic reaction.
- Collision Theory:
- Collision theory explains how chemical reactions occur at the molecular level.
- For a reaction to occur, the reactant particles must collide with sufficient energy and in the correct orientation.
- Increasing the frequency of collisions, increasing the kinetic energy of the particles, or decreasing the activation energy barrier can all increase the rate of a reaction.
- Collision theory helps explain why some reactions occur and others do not, and it provides a framework for predicting and controlling reaction rates.
- Graphically Differentiating Exothermic and Endothermic Reactions:
- Exothermic reactions release heat energy to the surroundings and have a negative change in enthalpy (ΔH < 0).
- Endothermic reactions absorb heat energy from the surroundings and have a positive change in enthalpy (ΔH > 0).
- The graph for an exothermic reaction shows the energy of the products being lower than the energy of the reactants. The difference between the energies represents the energy released to the surroundings.
- The graph for an endothermic reaction shows the energy of the products being higher than the energy of the reactants. The difference between the energies represents the energy absorbed from the surroundings.
- The activation energy is the energy required to start the reaction, and it is represented by the difference in energy between the reactants and the highest point on the curve.
Overall, understanding collision theory and how to differentiate between exothermic and endothermic reactions graphically can help us better understand chemical reactions and their behavior
Evaluation
- Which of the following best describes collision theory? A) The theory that reactions occur when reactant particles collide with each other. B) The theory that reactions occur when reactant particles are close together. C) The theory that reactions occur when reactant particles are far apart. D) The theory that reactions occur when reactant particles are in a specific orientation. E) The theory that reactions occur spontaneously without any input of energy.
- Which of the following factors can increase the rate of a chemical reaction according to collision theory? A) Decreasing the frequency of collisions B) Decreasing the kinetic energy of the particles C) Increasing the activation energy barrier D) Increasing the concentration of the reactants E) Decreasing the temperature
- Which of the following is NOT required for a successful collision between reactant particles? A) Correct orientation B) Sufficient energy C) High pressure D) Close proximity E) None of the above
- Which of the following factors can increase the frequency of collisions between reactant particles? A) Decreasing the temperature B) Increasing the pressure C) Decreasing the concentration of the reactants D) Decreasing the surface area of the reactants E) None of the above
- Which of the following is an example of an exothermic reaction? A) Ice melting B) Water boiling C) Photosynthesis D) Combustion of wood E) None of the above
- Which of the following statements is true about exothermic reactions? A) They absorb heat energy from the surroundings. B) They have a positive change in enthalpy. C) They release heat energy to the surroundings. D) They have a negative change in enthalpy. E) They have no change in enthalpy.
- Which of the following is an example of an endothermic reaction? A) Burning of gasoline B) Formation of snowflakes C) Neutralization of an acid and a base D) Rusting of iron E) None of the above
- Which of the following statements is true about endothermic reactions? A) They release heat energy to the surroundings. B) They have a negative change in enthalpy. C) They absorb heat energy from the surroundings. D) They have a positive change in enthalpy. E) They have no change in enthalpy.
- What is the activation energy of a reaction? A) The energy released when the reaction occurs B) The energy required to break a bond C) The energy required to start the reaction D) The energy of the products minus the energy of the reactants E) None of the above
- Which of the following factors can increase the kinetic energy of reactant particles? A) Decreasing the temperature B) Increasing the pressure C) Increasing the concentration of the reactants D) Decreasing the surface area of the reactants E) None of the above
FACTORS AFFECTING RATE OF REACTION From the collision theory, it can be seen that rates of reaction depends on the following features. 1. The energy of the particle. 2. The frequency of collision of the reaction. 3. The activation energy of the reaction. These features of a chemical reaction are in turn affected by some factors, which can make them to change and consequently affect the rate of reaction. These are factors that affect the rate of reactions. Some important ones are: 1. Nature of reactants. 2. Concentration / pressure (for gases) of reactants. 3. Surface area of reactants 4. Temperature of reaction mixture 5. Presence of light 6. Presence of catalysts To study the effect of any one of these factors on rate of reaction all other factors must be kept constant.
EFFECT OF NATURE OF REACTANTS
If all other factors are kept constant, different substances will have different rate of reaction with dilute HCl, for example. When dilute HCl reacts with zinc, iron and gold under the same condition, hydrogen gas is evolved fast with zinc, slow with iron and no gas evolved with gold. The difference in rate of reaction is due to the chemical nature of the elements as they naturally posses different amount of energy content
EFFECT OF CONCENTRATION OF REACTANTS
The frequency of collision among particles is high when the particles are crowded in a small space, i.e high concentration. This leads to high effective collision and thus high rate of reaction. An increase or decrease in the concentration of the reactants will result in corresponding increase or decrease in effective collisions of the reactants and hence the reaction rate. EFFECT OF SURFACE AREA OF REACTANTS This is a very important factor to be considered when a solid is involved in a chemical reaction. Lumped solids offer small surface area of contact for reaction while powdered solids offer large surface area for reaction. Rate of reaction is slow with lumped solid but high with powdered solids.
EFFECT OF TEMPERATURE
Increasing the temperature of a system can lead to an increase in reaction rate in two ways. When heat is raised, energy in form of heat is supplied to the reactant particles, so that 1. The number of particles with energy equal to or greater than the activation energy increases. 2. The velocity of all the reactant particles increases due to the greater kinetic energy, leading to a higher frequency of collision. As a result, the number of effective collisions increases and the reaction proceeds at a faster rate. Decreases in temperature leads to decrease rate of reactions.
EFFECT OF LIGHT
Some reactions are influenced by light. The rate of reaction is high when the lights intensity is high, low when the intensity is low and does not proceed at all in the absence of light. Such reactions are known as photochemical reaction. Examples of photochemical reactions include. 1. Reaction between hydrogen and chlorine and 2. Decomposition of hydrogen peroxide 3. Reactions between methane and chlorine 4. Photosynthesis in plant 5. Conversion of silver halides to grey metallic silver.
EFFECT OF CATALYST
A Catalyst is a substance, which alters the rate of a reaction, but itself does not undergo any change at the end of the reaction. A positive catalyst increases the rate of reaction by lowering the activation energy of the reaction whereas, the one which increases the activation energy is known as a negative catalyst or an inhibitor. The diagram below illustrates the energy profile for catalyzed and uncatalyzed exothermic and endothermic reactions
EXOTHERMIC REACTION ENDOTHERMIC REACTION
GENERAL EVALUATION/REVISION
1. List and explain three factors that can affect the rate of reaction of the following reaction: CaCO3(s) + 2HCl(aq)→ CaCl2(aq) + H2O(l) + CO2(g)
2. In a chemical reaction, after 10seconds, 6moles from the initial concentration of 16moles of the reactant disappeared. Calculate the rate of the reaction.
3. State THREE characteristics of catalyst.
4. What is the volume in dm3 of 8g of oxygen gas at s.t.p?
5. Calculate the percentage of water in sodium trioxocarbonate (VI) heptahydrate
READING ASSIGNMENT: New School Chemistry for Senior Secondary School by O.Y.Ababio (6th edition) pages 250-261
WEEKEND ASSIGNMENT SECTION A: Write the correct option ONLY
1. The minimum amount of energy that colliding molecules must possess for their collisions to be effective is a. thermal energy b. collision energy c. activation energy d. kinetic energy
2.Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) The rate of evolution of hydrogen gas in the above reaction will be greatly increased if a. the zinc is in the form of pellets b. a smaller volume of H2SO4 is used c. the reaction flask is immersed in an ice bath d. the zinc is in powdered form
3. The units of rate of reactions is a. moldm-3s-1 b. mol-1s-1 c. mol-1 d. smol-1
4. If 2g of zinc granules was reacted with excess dilute HCl to evolve hydrogen gas which came to completion after 5min. Calculate the rate of the chemical reaction in ghr-1 a. 48ghr-1 b. 12ghr-1 c. 24ghr-d d 240ghr-1
5. What do we do to increase the surface area of solid reactants to high rate of reaction? a. grinding them into powder b. subjecting the reactants to high pressure c. altering the directing of the reaction d. using reactants of different densities.
SECTION B
1a. Define rate of reaction
b. State the collision theory
2a. Explain in terms of collision theory, how rate of gaseous reaction is affected by an increase in pressure
b. Give the reason why red-hot iron wool reacts more readily with oxygen than red-hot iron nail
SECTION A
- c. activation energy
- d. the zinc is in powdered form
- a. moldm-3s-1
- b. 12ghr-1
- a. grinding them into powder
SECTION B
1a. Rate of reaction refers to the change in concentration of a reactant or product per unit time. b. Collision theory suggests that for a reaction to occur, the reactant particles must collide with each other with sufficient energy and the correct orientation.
2a. An increase in pressure increases the frequency of collisions between the reactant particles, leading to a higher rate of reaction. b. Red-hot iron wool has a larger surface area compared to a red-hot iron nail, allowing for more effective collisions between iron and oxygen molecules, which leads to a faster reaction
Evaluation
- Which of the following factors does NOT affect the rate of a chemical reaction? A) Nature of reactants B) Concentration of reactants C) Surface area of reactants D) Pressure of reactants E) Time of reaction
- Which of the following best describes activation energy? A) The energy released when a reaction occurs B) The energy required to start a reaction C) The energy of the products minus the energy of the reactants D) The energy required to break a bond E) The energy required to sustain a reaction over time
- Which of the following factors can increase the rate of a chemical reaction according to collision theory? A) Decreasing the concentration of the reactants B) Decreasing the kinetic energy of the particles C) Increasing the activation energy barrier D) Increasing the surface area of the reactants E) Increasing the temperature of the reaction mixture
- Which of the following statements is true about catalysts? A) They increase the activation energy of a reaction. B) They slow down the rate of a reaction. C) They are consumed during the reaction. D) They alter the rate of a reaction without undergoing any change themselves. E) They always increase the rate of a reaction by the same amount.
- Which of the following factors can increase the frequency of collisions between reactant particles? A) Decreasing the temperature B) Decreasing the pressure C) Increasing the concentration of the reactants D) Decreasing the surface area of the reactants E) None of the above
- Which of the following factors can increase the rate of a reaction involving a solid reactant? A) Decreasing the surface area of the solid B) Increasing the pressure on the solid C) Decreasing the temperature of the reaction mixture D) Increasing the size of the solid particles E) None of the above
- Which of the following is an example of an endothermic reaction? A) Combustion of methane gas B) Formation of rust on iron C) Dissolution of sodium hydroxide in water D) Decomposition of hydrogen peroxide E) None of the above
- Which of the following statements is true about exothermic reactions? A) They absorb heat energy from the surroundings. B) They have a positive change in enthalpy. C) They release heat energy to the surroundings. D) They have a negative change in enthalpy. E) They have no change in enthalpy.
- Which of the following factors can increase the rate of a reaction involving gases? A) Decreasing the pressure on the gases B) Decreasing the temperature of the reaction mixture C) Increasing the volume of the container holding the gases D) Increasing the kinetic energy of the gas particles E) None of the above
- Which of the following is NOT a characteristic of catalysts? A) They increase the rate of a reaction. B) They alter the activation energy of a reaction. C) They are consumed during the reaction. D) They are specific to certain reactions. E) They undergo a permanent change during the reaction
Lesson Presentation
Lesson Plan Presentation: Factors Affecting Rate of Reaction
Grade Level: Senior Secondary School 2
Introduction (5 minutes):
- Begin by asking students if they know what determines the speed of a chemical reaction.
- Introduce the concept of rate of reaction and explain that it is a measure of how fast or slow a chemical reaction occurs.
- State the objectives of the lesson.
Body (40 minutes):
- Define the collision theory and explain how it relates to the factors affecting rate of reaction.
- List and describe the factors affecting rate of reaction: nature of reactants, concentration/pressure of reactants, surface area of reactants, temperature of reaction mixture, presence of light, and presence of catalysts.
- Demonstrate the effect of each factor using examples and experiments where applicable.
- Use graphs and diagrams to help students visualize the effect of each factor on the rate of reaction.
- Encourage students to ask questions and participate in class discussions.
Conclusion (10 minutes):
- Summarize the main points of the lesson and review the factors affecting rate of reaction.
- Distribute handouts with practice problems and allow students to work on them individually or in groups.
- Provide feedback on student performance and address any misconceptions.
- Encourage students to further research and explore the topic outside of class.
Assessment:
- Formative assessment: Monitoring student participation in class discussions and activities.
- Summative assessment: Grading student performance on the practice problems and their ability to apply the knowledge of the factors affecting rate of reaction to solve problems
Weekly Assessment /Test
- What is rate of reaction?
- What is the collision theory and how does it relate to the factors affecting rate of reaction?
- List three factors that affect the rate of reaction and briefly describe how they do so.
- What happens to the rate of reaction when the concentration of the reactants is increased?
- How does the surface area of a solid reactant affect the rate of reaction?
- What is the effect of increasing temperature on the rate of reaction?
- Name a photochemical reaction and explain how it is affected by light.
- What is a catalyst and how does it affect the rate of reaction?
- What is the difference between an exothermic reaction and an endothermic reaction?
- Which of the following is NOT a factor that affects the rate of reaction: a) Nature of reactants b) Color of reactants c) Temperature of reaction mixture d) Presence of catalysts
