Chemical Kinetics (JEE Exam) - Detailed Notes

Collision Theory

• Collision theory provides a quantitative description of chemical reactions.
• Reaction rate depends on:
• Temperature: Higher temperature increases the average kinetic energy of molecules, leading to more collisions.
• Concentration: Higher concentration increases the chances of collision between reactant molecules.
• Activation energy (Ea): Minimum energy required for a reaction to occur.

Reference: NCERT Class 12, Chapter 4: Chemical Kinetics

Arrhenius Equation and Activation Energy

• Arrhenius equation: $$k = Ae^{-Ea/RT}$$

• k: Rate constant

• A: Pre-exponential factor

• Ea: Activation energy

• R: Gas constant

• T: Temperature in Kelvin.

• Factors affecting activation energy:

• Bond strength: Stronger bonds require higher activation energy to break.

• Catalysts: Catalysts lower the activation energy, allowing reactions to occur faster.

Reference: NCERT Class 12, Chapter 4: Chemical Kinetics

Rate Laws and Reaction Orders

• Rate law: Expression that shows how the reaction rate depends on the concentration of reactants.
• Reaction order: Sum of the exponents of the concentration terms in the rate law.
• Determining rate law:
• Method of initial rates: Measuring the initial rate of reaction at different reactant concentrations.
• Graphical methods (e.g., logarithmic plots).

Reference: NCERT Class 11, Chapter 15: Equilibrium; NCERT Class 12, Chapter 4: Chemical Kinetics

Integrated Rate Laws

• Integrated rate laws express the concentration of a reactant or product as a function of time.
• First-order integrated rate law: $$[A]_t = [A]_0 e^{-kt}$$
• [A]_0: Initial concentration
• [A]_t: Concentration at time t
• k: Rate constant.
• Half-life (t1/2): Time taken for the concentration to drop to half of its initial value.
• Integrated rate laws for second-order and zero-order reactions.

Reference: NCERT Class 12, Chapter 4: Chemical Kinetics

Temperature Dependence

• Increasing temperature increases the reaction rate.
• Arrhenius equation relates the rate constant to temperature: $$ k=Ae^{−Ea/RT} $$
• Temperature coefficient (Q10): Factor by which the rate constant increases when the temperature is raised by 10°C.

Reference: NCERT Class 12, Chapter 4: Chemical Kinetics

Catalysts

• Catalysts are substances that enhance the rate of a reaction without being consumed in the reaction.
• Homogeneous catalysts: Exist in the same phase as reactants.
• Heterogeneous catalysts: Exist in a different phase than reactants (e.g., solid catalysts).
• Mechanisms of catalysis:
• Surface catalysis: Reactants adsorb onto the catalyst’s surface, lowering the activation energy.
• Enzyme catalysis: Enzymes bind to specific substrates, forming enzyme-substrate complexes.

Reference: NCERT Class 12, Chapter 4: Chemical Kinetics

Chain Reactions and Free Radicals

• Chain reactions involve a sequence of steps, each generating highly reactive intermediates called free radicals.
• Mechanism of free radical chain reactions:
• Initiation: Formation of free radicals (e.g., by homolytic bond cleavage).
• Propagation: Free radicals react with other molecules, generating new free radicals.
• Termination: Free radicals react with each other or with inhibitors to stop the chain reaction.

Reference: NCERT Class 12, Chapter 16: Chemistry in Everyday Life

Complex Reactions and Steady State

• Complex reactions: Involve multiple steps and intermediates.
• Reaction mechanism: Step-by-step description of the reaction pathway.
• Steady state approximation: Assumes that the concentration of intermediates remains constant during the reaction.
• Rate-determining step: Slowest step in the reaction mechanism, which determines the overall rate of the reaction.

Reference: NCERT Class 12, Chapter 4: Chemical Kinetics