Chapter 3: Chemical Kinetics - Comprehensive NEET Chemistry Notes

1. Introduction to Chemical Kinetics

Chemical kinetics is the branch of chemistry that deals with the study of reaction rates and their mechanisms. Understanding the speed of a reaction and the factors controlling it is essential for predicting how a reaction proceeds over time.

2. Rate of a Chemical Reaction

2.1 Average and Instantaneous Rate

The rate of a reaction can be defined as the change in concentration of reactants or products per unit time. It can be expressed as: Rate of disappearance of

$Rate of disappearance of R = - \frac{Δ [ R ]}{Δ t}$

$Rate of appearance of P = \frac{Δ [ P ]}{Δ t}$

2.2 Units of Rate

The units of the rate of a reaction depend on the units of concentration and time. For instance, if concentration is in mol $L^{- 1}$ and time is in seconds, the rate units will be mol $L^{- 1} S^{- 1}$ .

NEET Tip:

Always ensure to convert time units appropriately when solving rate problems.

3. Factors Influencing the Rate of Reaction

3.1 Concentration

The rate of a reaction generally increases with the concentration of reactants. This relationship is often described by the rate law: $Rate = k [A]^{x} [B]^{y}$

3.2 Temperature

Increasing the temperature usually increases the reaction rate. According to the Arrhenius equation: $k = A e^{- \frac{E _{a}}{RT}}$ where $E_{a}$ is the activation energy, $R$ is the gas constant, and $T$ is the temperature in Kelvin.

Real-life Application:

The spoilage of food is faster at higher temperatures due to increased reaction rates of decomposition processes.

3.3 Catalyst

A catalyst increases the rate of a reaction without being consumed. It works by lowering the activation energy.

3.4 Surface Area

For reactions involving solids, an increased surface area leads to a higher reaction rate due to more available reaction sites.

Mnemonic:

FACT: Factors Affecting Chemical kinetics: Frequency of collisions, Activation energy, Catalyst, Temperature.

4. Rate Laws and Reaction Order

4.1 Differential and Integrated Rate Laws

The differential rate law expresses the rate as a function of concentration, while the integrated rate law links concentration with time.

4.2 Zero Order Reactions

For a zero-order reaction: $Rate = k$ The concentration vs. time graph is linear with a slope of $(- k)$ .

4.3 First Order Reactions

For a first-order reaction: $Rate = k [A]$ The integrated rate law is: $ln [A] = - k t + ln [A]_{0}$

Did You Know?

Radioactive decay follows first-order kinetics, where the rate is proportional to the amount of undecayed nuclei.

5. Half-life of a Reaction

The half-life $t_{1/2}$ is the time required for the concentration of a reactant to decrease to half its initial value. For first-order reactions: $t_{1/2} = \frac{0.693}{k}$

Common Misconception:

The half-life of a first-order reaction is independent of the initial concentration, unlike zero-order reactions.

6. Temperature Dependence of Reaction Rates

6.1 Arrhenius Equation

The Arrhenius equation relates the rate constant ( $k$ ) to temperature ( $T$ ): $ln k = ln A - \frac{E _{a}}{RT}$

6.2 Activation Energy

The activation energy ( $E_{a}$ ) is the minimum energy required for a reaction to occur. Lowering $E_{a}$ increases the reaction rate.

NEET Problem-Solving Strategy:

When solving Arrhenius equation problems, ensure to use consistent units for energy, temperature, and the gas constant.

7. Catalysts and Reaction Mechanisms

A catalyst provides an alternative pathway with a lower activation energy. It does not alter the equilibrium position but helps in reaching equilibrium faster.

Concept Connection:

In biology, enzymes act as biological catalysts, speeding up biochemical reactions without being consumed.

Quick Recap

Rate of Reaction: Change in concentration per unit time.
Factors Influencing Rate: Concentration, temperature, catalyst, and surface area.
Rate Laws: Mathematical relationship between rate and concentration.
Reaction Order: Sum of the powers of concentration terms in the rate law.
Half-life: Time for reactant concentration to reduce to half.
Arrhenius Equation: Relates rate constant to temperature and activation energy.
Catalysts: Increase reaction rate by lowering activation energy.

Practice Questions

Calculate the average rate of reaction for the disappearance of reactant R from 0.50 M to 0.20 M in 10 minutes.
For the reaction 2A + B → Products, the rate law is Rate = k[A][B]. If the concentration of A is doubled, what happens to the rate of the reaction?
Given the reaction: $2 H I \to H (_{2}) + I (_{2})$ , with a rate constant $k = 3.5 \times 1 0^{- 4} s^{- 1}$ at 600 K, calculate the half-life.
Explain the effect of a catalyst on the activation energy and reaction rate.
Using the Arrhenius equation, calculate the activation energy if the rate constant increases from $0.1 s^{- 1}$ to $0.4 s^{- 1}$ when the temperature increases from 300 K to 310 K.
What is the order of a reaction with the rate law: Rate = k[A]^2[B]?
Determine the rate constant for a first-order reaction if the concentration of the reactant decreases from 0.8 M to 0.2 M in 15 minutes.
Describe how temperature affects the Maxwell-Boltzmann distribution curve.
Explain why the half-life of a first-order reaction is constant.
Identify the molecularity and order of the reaction: $2 NO + O_{2} \to 2 N O_{2}$