Chemistry, Third edition

15 · DYNAMIC CHEMICAL EQUILIBRIA

Suppose that for a different reaction, say C\===\D, there were 15 molecules of C and 1

of D at equilibrium at a particular temperature. Then,

Kc(T)

[D]



1

0.066

[C] 15

We see that the smaller the equilibrium constant, the smaller the relative number of

molecules of product to reactant in the equilibrium mixture, and vice versa.

Where chemical equations contain more than one reactant or product molecule,

the equilibrium constant for the reaction is not simply the ratio of a single product

and single reactant concentration. (Equation (15.4), for example, contains a concen-

tration which is squared.) Nevertheless, large equilibrium constants always mean that

the product molecules dominate the equilibrium mixture, whereas small equilibrium

constants show that reactant molecules dominate the equilibrium mixture.

As a rough guide, ifKc(T)is greater than 1000, the product dominates the equi-

librium mixture and we would say that (for most purposes) the reaction goes to

completion at that temperature. If Kc(T)is less than 0.001 we would say that (for

most purposes) a negligible amount of product has been made and that ‘no reaction

has occurred’ at that temperature. Summarising,

ifKc(T) 1000 reaction goes to completion

ifKc(T)0.001 no reaction occurs

IfKc(T)is in the range 1000–0.001, the number of concentrations being multiplied

together in the equilibrium expression need to be carefully considered in deciding

whether or not a Kc(T)value implies a high or low concentration of products in the

equilibrium mixture, or whether or not the reactants and product concentrations

are roughly equal.

Reactions which go to completion include many of the reactions of inorganic ions

we have already studied, for example

Ag(aq)Cl(aq) AgCl(s) Kc(25 °C) 1000

Most of the mole calculations discussed in previous units (including those involving

titrations) have assumed that the reactions go to completion.

A list of Kc(T)values for selected reactions is given in Table 15.1.

270

Table 15.1Equilibrium constants

No Reaction Kc(T) Temperature/K Units of Kc(T)

1Cl 2 (g)\===\Cl(g)Cl(g) 1.2  10 ^71000 mol dm^3

2(a) H 2 (g)Cl 2 (g)\===\2HCl(g) 4.0  1031 300 –

2(b) H 2 (g)Cl 2 (g)\===\2HCl(g) 4.0  1018 500 –

3 2BrCl(g)\===\Br 2 (g)Cl 2 (g) 377 300 –

4CO 2 (g)H 2 (g)\===\CO(g)H 2 O(g) 1.56 1073 –

5 HF(g) HCN(g)\===\HCN···HF(g) 1.04 298 mol^1 dm^3

6I(aq)I 2 (l)\===\I 3 (aq) 7.1  10 ^2298 mol^1 dm^3

7Cu^2 (aq)4NH 3 (aq)\===\Cu[(NH 3 ) 4 ]^2 (aq) 1.4  1013 298 mol^4 dm^12

8(a) N 2 (g)3H 2 (g)\===\2NH 3 (g) 4  108 298 mol^2 dm^6

8(b) N 2 (g)3H 2 (g)\===\2NH 3 (g) 2.2 623 mol^2 dm^6

9 2NH 3 (g)\===\N 2 (g)3H 2 (g) 0.46 623 mol^2 dm^6

10 Cu^2 (aq)Zn(s)\===\Zn^2 (aq)Cu(s) 1  1037 298 –