## Introduction

3.1.9.1 Rate equations (A-level only)

The rate of a chemical reaction is related to the concentration of reactants by a rate equation of the form: Rate = k[A]^{m} [B]^{n} 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.

Students should be able to:

• define the terms order of reaction and rate constant

• perform calculations using the rate equation

## Calculating order of reactions using initial rates

3.1.9.2 Determination of rate equation (A-level only)

The rate equation is an experimentally determined relationship.

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

## Effect of temperature on rate constants

The rate constant k varies with temperature as shown by the equation:

k = Ae^{–Ea/RT}

where A is a constant, known as the Arrhenius constant, Ea is the activation energy and T is the temperature in K.

Students should be able to:

• explain the qualitative effect of changes in temperature on the rate constant k

• perform calculations using the equation k =Ae^{–Ea/RT}

• understand that the equation k =Ae^{–Ea/RT} can be rearranged into the form ln k = –Ea /RT + ln A and know how to use this rearranged equation with experimental data to plot a straight line graph with slope –Ea /R These equations and the gas constant, R, will be given when required.

## Rate equations and reaction mechanisms

3.1.9.2 Determination of rate equation (A-level only)

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

Students should be able to:

use the orders with respect to reactants to provide information about the rate determining/limiting step of a reaction.

## Required Practical 7: Initial rate method

3.1.9.2 Determination of rate equation (A-level only)

Required practical 7

Measuring the rate of reaction:

• by an initial rate method

## Required Practical 7: Continuous monitoring method.

3.1.9.2 Determination of rate equation (A-level only)

Required practical 7

Measuring the rate of reaction:

• by a continuous monitoring method.

Credits: Neil Goalby