Overview
Chemical kinetics is the study of reaction rates—how fast chemical reactions occur—and the factors that influence them. This topic helps students understand how to determine rate laws, calculate rate constants, and interpret energy diagrams to explain how reactions proceed.
Key Concepts and Structures
- Reaction Rate: The change in concentration of a reactant or product over time. Typically expressed in M/s (molarity per second).
- Factors Affecting Rate: Temperature, concentration, surface area, catalysts, and the nature of the reactants all influence reaction speed.
- Rate Law: An equation that relates reaction rate to the concentrations of reactants. General form:
rate = k[A]^m[B]^n
- Order of Reaction: The exponents m and n represent the order with respect to each reactant. The overall order is the sum of the exponents.
- Rate Constant (k): A proportionality constant that depends on temperature and reaction conditions. Units vary based on the overall order.
- Determining Rate Laws: Use experimental data to compare initial rates and determine how changes in concentration affect the rate.
- Integrated Rate Laws: Used to determine concentration at any time. Each order has a unique equation and graph shape (zero, first, second order).
- Half-Life (t1/2): The time required for half the reactant to be consumed. For first-order reactions, it is constant:
t1/2 = 0.693/k
- Activation Energy (Ea): The minimum energy needed for a reaction to occur. Shown in energy profile diagrams.
- Transition State and Reaction Pathway: The highest energy point between reactants and products. Catalysts lower the activation energy but do not affect ΔG or equilibrium.
Quick Tip
To determine a rate law from data, find two trials where only one reactant changes. The rate change reveals the order with respect to that reactant. Always check units on k—they change based on the overall reaction order.