Chemistry, Third edition

15 · DYNAMIC CHEMICAL EQUILIBRIA

Kc(T)is practically important because it does not depend upon the concentrations of

the reactants at the start of the reaction, and for a named reaction varies only with

temperature of the equilibrium mixture. Remember,

The value of an equilibrium constant for a particular reaction changes only

with temperature.

The equilibrium constants of reactions involving gases may also be expressed in

terms of the partial pressures (in atm) of the reactants and products. The equilib-

rium constant is then symbolized Kp(T). For example, for reaction (15.5),

(pSO 3 )^2

Kp(T)

(pSO 2 )^2 (pO 2 )^2

3.0 104 atm^1 at 700 K

(15.6)

(The conversion of Kc(T)toKp(T)is described in the website, Appendix 15.)

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Reactions that go to completion

Study the equations

CO(g)H 2 (g)\===\H 2 O(g)C(s)

and

NaOH(aq)HCl(aq) H 2 O(l)NaCl(aq)

Which of the following statements are correct?

(a)If we started with 0.1 mol of H 2 and excess CO, we would expect to produce 0.1 mol of H 2 O

and 0.1 mol of C.

(b)If we started with 0.1 mol of NaOH and excess HCl, we would expect to produce 0.1 mol of

H 2 O and 0.1 mol of NaCl.

Exercise 15A

Equilibrium expressions

Write down expressions for the equilibrium constants of the following reactions:

(i) Oxidation of iron(II) ions by chlorine gas

2Fe^2 (aq)Cl 2 (g)\===\2Fe^3 (aq)2Cl(aq)

(ii)Formation of chlorine nitrate gas

ClO(g)NO 2 (g)\===\ClONO 2 (g)

(iii)Formation of nitrogen dioxide gas from dinitrogen tetroxide

N 2 O 4 (g)\===\2NO 2 (g)

What are the units of Kc(T)in each case?

Exercise 15B

Equilibrium constants and rate constants

First, a reminder – ‘big K’ symbolizes an equilibrium constant, whereas ‘little k’

symbolizes a rate constant.

Experiments show that the rate of reaction (15.1) follows the rate expression

rate of reaction= kf[H 2 (g)] [I 2 (g)]

wherekf(subscript f for ‘forward’) is the rate constant (see Chapter 14) and the

brackets indicate the concentrations at that instant. Immediately after starting the

reaction, the concentrations of hydrogen and iodine begin to fall and so the rate of

the forward reaction falls.