Summary
The rate of a reaction is determined by the change in concentration of reactants or products over time. The rate equation, orders of reaction, and rate constants are key concepts in understanding reaction kinetics.
- Rate of Reaction — The change in concentration of a reactant or product per unit time. Example: If the concentration of a reactant decreases by 0.1 mol dm-3 in 10 seconds, the rate is 0.01 mol dm-3 s-1.
- Rate Equation — An expression that relates the rate of a reaction to the concentration of reactants. Example: For A + B → C, rate = k[A]^m[B]^n.
- Order of Reaction — The power to which the concentration of a reactant is raised in the rate equation. Example: If doubling [A] quadruples the rate, the reaction is second order with respect to A.
- Rate Constant (k) — A constant that relates the rate of reaction to the concentration of reactants. Example: In rate = k[A]^2[B], k is calculated using experimental data.
- Half-life — The time taken for the concentration of a reactant to reduce by half. Example: In a first order reaction, the half-life is constant.
- Rate-determining Step — The slowest step in a reaction mechanism that determines the overall rate. Example: In a multi-step reaction, the step with the highest activation energy is often the rate-determining step.
Exam Tips
Key Definitions to Remember
- Rate of Reaction: Change in concentration of a reactant or product per unit time.
- Rate Equation: An expression relating reaction rate to reactant concentrations.
- Order of Reaction: The power to which a reactant's concentration is raised in the rate equation.
- Rate Constant: A constant relating reaction rate to reactant concentration.
- Half-life: Time taken for the concentration of a reactant to reduce by half.
Common Confusions
- Confusing the order of reaction with the stoichiometric coefficients in the balanced equation.
- Assuming a curved rate-concentration graph is always second order.
Typical Exam Questions
- What is the rate equation for the reaction A + B → C? Rate = k[A]^m[B]^n
- How does doubling the concentration of a reactant affect the rate if the reaction is first order with respect to that reactant? The rate doubles.
- How do you determine the rate constant from experimental data? Use the rate equation and substitute known values to solve for k.
What Examiners Usually Test
- Ability to write and interpret rate equations.
- Understanding of how changes in concentration affect reaction rates.
- Calculation of rate constants and understanding their units.
- Identification of the rate-determining step in a reaction mechanism.