Chemistry, Third edition

15 · DYNAMIC CHEMICAL EQUILIBRIA

For example, Kc(T)for the reaction

Cu^2 (aq)Zn(s)\===\Zn^2 (aq)Cu(s)

is 1  1037 at room temperature (Table 15.1). Thus Kc(T)for the reversereaction is

1  10 ^37. The consequence of these values is clear cut: for all practical purposes the

reaction of zinc metal and copper ions goes to completion, whereas the

reaction of zinc ions with copper metal does not take place.

Effects of changing concentration,

pressure and temperature upon

equilibria

Detailed studies have been made of many different examples of equilibria. We look

at two cases. In case 1, the hydrogen–iodine reaction, we confirm that Kc(T)is inde-

pendent of reactant concentrations. In case 2, an esterification reaction, we confirm

that the equilibrium concentrations of reactants and products (the ‘equilibrium

composition’) depends upon their starting concentrations.

Case 1: Use of the reaction between hydrogen and

iodine gases to show that Kc(T )is independent of

concentration

The reaction is

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

Table 15.2 shows the initial (i.e. at t= 0) and equilibrium compositions of three

different mixtures at 490 °C.

We start by checking whether or not the results obey the reaction equation.

According to the equation, 1 mol of H 2 or I 2 produces 2 mol of HI. Therefore, the

15.4

272

Table 15.2The composition of mixtures hydrogen

containing hydrogen, iodine and iodide at 490 °C

Time Concentrations/mol dm^3

[H 2 ][I 2 ] [HI]

Experiment 1

t 0 20.00 20.00 0

At equilibrium 4.58 4.58 30.86

Experiment 2

t 0 8.19 14.31 0

At equilibrium 0.72 6.84 14.94

Experiment 3

t 0 0 0 40.00

At equilibrium 4.58 4.58 30.86