A-Level Chemistry AQA 7405

Rate equations

The rate of a chemical reaction is related to the concentration of reactants by a rate equation of the form:

Rate = kA^m Bⁿ

where m and n are the orders of reaction with respect to reactants A and B and k is the rate constant.

The orders m and n are restricted to the values 0, 1, and 2.

The rate constant k varies with temperature as shown by the Arrhenius equation: $k = Ae^{-\dfrac{E_a}{RT}}$ where A is a constant, known as the Arrhenius constant, E_a is the activation energy and T is the temperature in K.

Students should be able to:

• define the terms order of reaction and rate constant
• perform calculations using the rate equation
• explain the qualitative effect of changes in temperature on the rate constant k
• perform calculations using the Arrhenius equation
• understand that the Arrhenius equation can be rearranged into the form $\ln{k} = -\dfrac{E_a}{RT} + \ln{A}$ and know how to use this rearranged equation with experimental data to plot a straight line graph with slope $-\dfrac{E_a}{R}$

These equations and the gas constant, R, will be given when required.

Determination of rate equation

The rate equation is an experimentally determined relationship.

The orders with respect to reactants can provide information about the mechanism of a reaction.

Students should be able to:

• use concentration–time graphs to deduce the rate of a reaction
• use initial concentration–time data to deduce the initial rate of a reaction
• use rate–concentration data or graphs to deduce the order (0, 1 or 2) with respect to a reactant
• derive the rate equation for a reaction from the orders with respect to each of the reactants
• use the orders with respect to reactants to provide information about the rate determining/limiting step of a reaction.

Required practical 7

Measuring the rate of reaction:

• by an initial rate method
• by a continuous monitoring method.