Chemistry, Third edition

REVISION QUESTIONS 283

Since [C(s)] is constant,

[CO(g)]^2

Kc(T)

[CO 2 (g)]

whereKc(T)=Kc(T)[C(s)].

Application of the equilibrium law to physical equilibria

The equilibrium law also applies to physical processes. For example, the equilibrium

law expression for the dissolution of silver chloride,

AgCl(s)\===\Ag(aq)Cl(aq)

is

K(AgCl)

[Ag(aq)][Cl(aq)]

(15.8)

[AgCl(s)]

However, since the concentration of a pure solid is constant

K(AgCl)[AgCl(s)][Ag(aq)][Cl(aq)]constant

This explains the expression

[Ag(aq)][Cl(aq)] = constant at that temperature = Ks

discussed in Unit 11. The common ion effect (page 179) may now be explained using

equation (15.8). If chloride ions are added to a saturated solution of AgCl, this equa-

tion predicts that [Ag(aq)] must be reduced in order to keep [Ag(aq)][Cl(aq)]

equal to K(AgCl). The reduction is achieved by precipitation of AgCl(s).

Application of the

equilibrium law to

vaporization

Show that application of

the equilibrium law to the

process

H 2 O(l)\===\H 2 O(g)

leads to the conclusion

that the equilibrium vapour

pressure of water is fixed

at a particular

temperature.

Exercise 15K

15.1.What is missing from the statement ‘Kc= 3.5  104 for

the reaction of hydrogen and bromine gases at 2040 °C’?

15.2.The table below shows the variation in the concentra-

tions of H 2 (g), I 2 (g) and HI(g) with time when the following

reaction was carried out at a fixed temperature:

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

(Concentrations/mol dm^3 ) 103

Time/s H 2 (g) I 2 (g) HI(g) Q

0 1.000 1.000 0.000

50 0.897 0.897 0.206

100 0.813 0.813 0.374

200 0.685 0.685 0.630

300 0.592 0.592 0.817

400 0.521 0.521 1.040

500 0.465 0.465 1.069

600 0.420 0.420 1.160

700 0.383 0.383 1.234

800 0.352 0.352 1.296

850 0.333 0.333 1.334

900 0.332 0.332 1.336

950 0.331 0.331 1.338

(Notice that the concentrations have been multiplied by 1000.

For example, the concentration of HI at t= 300 s is 0.817 

10 ^3 = 8.17  10 ^4 mol dm^3 .)

(i) Calculate the value of the expression

[HI(g)]^2

Q

[H 2 (g)][I 2 (g)]

at each time. (If possible, do this using a suitable computer

spreadsheet such as Microsoft Excel.) Enter these values in

the column labelled Q.

(ii) Plot [H 2 (g)] and [HI(g)] against time. Estimate the initial

rateat which (a) H 2 is used up and (b) at which HI is

produced. Comment upon your answers.

(iii)PlotQversus t. At what time would you say that

equilibrium has been reached? What is the value of

the equilibrium constant for the reaction at this tempera-

ture?

(iv)Use Fig. 15.6 to estimate the temperature at which the

experiments were carried out.

15.3.(i)Explain why the presence of water reduces the equi-

librium yield of ester in the reaction

C 2 H 5 OH(l)CH 3 COOH(l)\===\CH 3 COOC 2 H 5 (l)H 2 O(l)

Revision questions