Chemistry Lesson Note On Chemical Kinetics – Rate Laws, Order of Reaction

Topic: Chemical Kinetics – Rate Laws, Order of Reaction

Class: SS1

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Specific Objectives:

By the end of the lesson, students should be able to:

1. Define chemical kinetics and reaction rate.
2. State the factors affecting the rate of chemical reactions.
3. Write and interpret rate laws.
4. Define and determine the order of reaction.
5. Perform simple calculations involving rate expressions.

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Instructional Materials:

• Charts showing reaction rate vs. time graphs
• Videos or animations of slow vs. fast reactions
• Stopwatch, beakers, and reagents for demonstration
• Sample rate law expressions
• Graph paper

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Lesson Content:

Step 1: Meaning of Chemical Kinetics and Reaction Rate

Chemical kinetics is the branch of chemistry that studies the speed (rate) of chemical reactions and the factors that influence this speed.

Rate of reaction is defined as the change in concentration of a reactant or product per unit time.

\text{Rate} = \frac{\text{Change in concentration}}{\text{Time taken}}

Example:
If the concentration of H₂ decreases from 0.2 mol/dm³ to 0.1 mol/dm³ in 10 seconds, the rate =

\frac{0.2 - 0.1}{10} = 0.01 \, mol\,dm^{-3}s^{-1}

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Step 2: Factors Affecting Reaction Rate

1. Concentration: Increasing concentration increases collision frequency.
2. Temperature: Higher temperature increases kinetic energy and reaction rate.
3. Surface Area: More exposed surface = faster reaction (especially for solids).
4. Catalyst: Lowers activation energy, speeding up reaction without being used up.
5. Nature of Reactants: Ionic compounds react faster than covalent ones.
6. Pressure: Affects reactions involving gases – higher pressure increases rate.

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Step 3: Rate Laws and Rate Expression

A rate law expresses the relationship between the rate of a reaction and the concentration of reactants.

For a reaction:

aA + bB \rightarrow \text{products}

The rate law can be:

\text{Rate} = k[A]^m[B]^n

Where:

• k = rate constant
• m and n = orders of reaction with respect to A and B
• [A] and [B] = concentrations

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Step 4: Order of Reaction

The order of a reaction is the sum of the powers (exponents) of the concentration terms in the rate law.

• Zero-order reaction: Rate is independent of concentration

  \text{Rate} = k

  \text{Rate} = k[A]

  \text{Rate} = k[A]^2 \quad \text{or} \quad k[A][B]

Overall order = m + n

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Step 5: Sample Calculation

Given:
Rate = k[A]²[B]

If [A] = 0.2 mol/dm³, [B] = 0.1 mol/dm³, and rate = 0.004 mol/dm³s, find the rate constant, k.

0.004 = k \times (0.2)^2 \times (0.1)

0.004 = k \times 0.004 \Rightarrow k = \frac{0.004}{0.004} = 1 , mol^{-2}dm^6s^{-1} ]

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Evaluation:

1. Define rate of reaction.
2. State four factors that affect the rate of chemical reactions.
3. For the reaction:

   \text{A} + \text{B} \rightarrow \text{C},\quad \text{Rate} = k[A]^2[B]

b. What is the overall order of the reaction?

4. If a reaction rate doubles when the concentration of a reactant is doubled, what is the order with respect to that reactant?

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Homework:

1. A reaction is found to follow the rate law:

   \text{Rate} = k[X][Y]^2

b. What is the total order of the reaction?

2. A student observes that increasing temperature increases the rate of a reaction. Explain why.

3. Define activation energy and explain its role in chemical reactions.

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Conclusion:

Chemical kinetics helps us understand how fast reactions occur and how to control them. By studying rate laws and orders of reaction, chemists can design better industrial and laboratory processes.