Chemical Kinetics

Introduction to Reaction Rates

Chemical kinetics is the study of the speed or rate at which chemical reactions occur and the factors that affect these rates. Understanding reaction rates is essential for controlling chemical processes in laboratory and industrial settings.

• Reaction rate: The change in concentration of a reactant or product per unit time.

• Rate law: An equation that relates the rate of a reaction to the concentration of reactants.

• Example: For the reaction , the rate law is .

Order of Reaction

The order of a reaction describes how the rate depends on the concentration of reactants.

• Zero order: Rate is independent of reactant concentration.

• First order: Rate is directly proportional to the concentration of one reactant.

• Second order: Rate depends on the concentration of one reactant squared or two reactants multiplied together.

Practice Problem Example

For a first-order reaction with and initial concentration , how long does it take for the concentration to decrease by a factor of 50?

• Use the first-order integrated rate law:

• Plug in values:

• Solve for :

Integrated Rate Laws

Change of Concentrations with Time

Integrated rate laws allow us to calculate the concentration of reactants at any time during the reaction.

Half-life ()

The half-life of a reaction is the time required for the concentration of a reactant to decrease to half its initial value. The half-life depends on the order of the reaction.

• Example: The half-life of radioactive decay is 5730 years.

Reaction Mechanisms

Elementary Steps and Molecularity

Reactions may occur in a single step or through a series of elementary steps. The molecularity of a step refers to the number of molecules involved.

• Intermediate: A species formed in one step and consumed in another; does not appear in the overall reaction.

• Rate-determining step: The slowest step in a multistep mechanism, which controls the overall rate.

Reaction Coordinate Diagrams

Reaction coordinate diagrams illustrate the energy changes during a reaction, showing reactants, products, and the transition state.

• Activation energy (): The minimum energy required for a reaction to occur.

• Transition state: The highest energy point along the reaction path.

• Activated complex: The species present at the transition state.

• Endothermic vs. Exothermic: Endothermic reactions absorb energy; exothermic reactions release energy.

Factors Affecting Reaction Rate

Concentration Effects

Changing the concentration of reactants can affect the rate of reaction, depending on the order with respect to each reactant.

• For the reaction with rate law , changing changes the rate, but changing does not.

Temperature Effects and the Arrhenius Equation

Increasing temperature generally increases reaction rate by providing more molecules with sufficient energy to overcome the activation energy barrier.

• Arrhenius equation:

• is the frequency factor, is activation energy, is the gas constant, and is temperature in Kelvin.

• Plotting vs. yields a straight line with slope .

• To compare rate constants at two temperatures:

Catalysis

Role of Catalysts

Catalysts increase the rate of a reaction by lowering the activation energy and providing an alternative reaction pathway. They do not affect the equilibrium position.

• Heterogeneous catalysis: Catalyst is in a different phase than reactants (e.g., solid catalyst with gaseous reactants).

• Homogeneous catalysis: Catalyst is in the same phase as reactants (e.g., all dissolved in solution).

• Most industrial catalysts are heterogeneous.

Summary Table: Key Concepts in Chemical Kinetics

Additional info: These notes are based on class slides and handwritten notes for CHEM 1220 (General Chemistry II), focusing on chemical kinetics, reaction mechanisms, and catalysis. All equations are provided in LaTeX format for clarity.