Chemistry, Third edition

284 15 · DYNAMIC CHEMICAL EQUILIBRIA

(ii) In an experiment, ethanol and ethanoic acid were mixed

and allowed to equilibrate at 25 °C. The equilibrium con-

centrations of ethanol, ethanoic acid and ester were found

to be 2.67, 1.93 and 4.48 mol dm^3 , respectively. Calculate

(a) the equilibrium constant for this reaction, and (b) the

initial concentrations of ethanol and ethanoic acid.

(iii)The rate constant for the formation of ethyl ethanoate

from ethanol at 25 °C is approximately 2.4 

10 ^4 mol^1 dm^3 s^1. Estimate the rate constant for the

reverse reaction.

15.4.The formation of nitrosyl chloride from nitric oxide,

2NO(g)O 2 (g)\===\2NOCl(g)

was investigated at a particular temperature. The pressures of

gases at equilibrium were pNO= 0.22 atm, pO 2 = 0.11 atm, and

pNOC1) = 0.32 atm. Calculate Kp(T)and state its units.

15.5.Kcfor the reaction of tin(II) ions with iron(III) ions,

Sn^2 (aq)2Fe^3 (aq)\===\Sn^4 (aq)2Fe^2 (aq)

is 1.0  1010 at 298 K. In an experiment at room temperature

in which solutions of Sn^2 (aq) and Fe^3 (aq) were mixed, the

equilibrium concentrations of tin ions were found to be

[Sn^2 (aq)] = 0.050 mol dm^3

[Sn^4 (aq)] = 0.040 mol dm^3

Calculate the concentrations of iron(III) and iron(II) ions at

equilibrium.

15.6.The equilibrium constant for the decomposition of

phosgene (a World War I poison gas) at 100 °C is 2.2 

10 ^10 mol dm^3 :

COCl 2 (g)\===\CO(g)Cl 2 (g)

(Note how toxic the products are!) Phosgene is contained in a

metal drum, whose outside surfaces are heated with steam.

Would you expect all the phosgene to have decomposed once

equilibrium had been achieved?

15.7.The exothermic reaction

CO(g)2H 2 (g)\===\CH 3 OH(g)

is allowed to reach equilibrium at 1000 K. How would the yield

of methanol, and the value of the equilibrium constant, be

affected by the following separate changes?

(i) Increasing the partial pressure of methanol by injecting

some methanol into the mixture.

(ii) Compressing the mixture so that its pressure increases.

(iii)Allowing the mixture to expand so that its pressure

decreases.

(iv)Pumping in an inert gas (such as argon) which increases

the total pressure without a change in volume.

(v) Lowering the temperature of the equilibrium mixture.

15.8.The formation of brown nitrogen dioxide gas (NO 2 )

from nitrogen and oxygen, involves two stages:

N 2 (g)O 2 (g)\===\2NO(g) (1)

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

The net reaction is

N 2 (g)2O 2 (g)\===\2NO 2 (g) (3)

Show that Kc(3) = Kc(1)Kc(2).

15.9.Show that:

(i)Henry’s law, such as for the dissolution of oxygen in water,

oxygen in air \===\oxygen in water

and(ii)and the distribution law, such as for the solubility of I 2

between two solvents,

I 2 in H 2 O\===\I 2 in CCl 4

both follow from an application of the equilibrium law.

Extension material to support this unit is available on our website. More advanced calculations involving

equilibrium constants, and an introduction to ‘free energy’ changes (G) are to be found in Appendix 15

on the website.