Chemistry, Third edition

Dynamic Chemical

Equilibria

Objectives

Explains what is meant by dynamic chemical equilibrium

Defines equilibrium constant

Discusses factors which affect equilibrium concentrations

Looks at calculations involving simple equilibria

Introduction

Chemical reactionsare examples of dynamic equilibria. Equilibrium reactions are

sometimes called reversible reactions. As an example of an equilibrium reaction,

consider the reaction of hydrogen and iodine to make hydrogen iodide in a sealed

container at high temperature.

If we heat pure hydrogen and iodine the forwardreaction takes place:

H 2 (g)I 2 (g) 2HI(g) (15.1)

Almost as soon as HI is formed, some HI molecules decompose in the back reaction:

2HI(g) H 2 (g)I 2 (g) (15.2)

Atequilibriumtheratesof both reactions are equal. This does not mean the concentra-

tionsof the reactants and products are equal, merely that the ‘status quo’ (the equi-

librium composition) is being maintained by two opposite processes which are

proceeding at the same rate (Fig. 15.1). We summarise the forward and reverse

reactions in one equation:

H 2 (g)I 2 (g)\===\2HI(g)

Figure 15.2 shows the general way that the concentrations of reactants and products

varies with time for an equilibrium reaction. Equilibrium is achieved tseconds after

the start of the reaction.

Picture of dynamic chemical equilibrium

The idea that although the ratesof the forward and back reactions are equal at equi-

librium, the concentrationsof the reactants and products are not necessarily equal, is

15.1

Contents

15.1Introduction 265

15.2Equilibrium law

and equilibrium

constant 267

15.3Meaning of

equilibrium

constants 269

15.4Effects of changing

concentration,

pressure and

temperature upon

equilibria 272

15.5Production of

ammonia by the

Haber–Bosch

process 281

15.6Heterogeneous

equilibria 282

15.6Revision questions 283

Fig. 15.1An illustration of a

dynamic equilibrium – an

athlete on a treadmill. The

athlete does not fall off

because he is moving at the

samerate as the treadmill but

in the oppositedirection.

15

UNIT