Chemistry, Third edition

15 · DYNAMIC CHEMICAL EQUILIBRIA

sides of the equation, altering the total pressure of the equilibrium mixture will not

affect the equilibrium concentration of reactants or products.

Finally, imagine that an equilibrium mixture is maintained at constant volume

(for example, the reaction is carried out in a sealed and pistonless steel can) but that

we subsequently increase the mixture pressure by injecting in an unreactive gas

(such as argon). Because the partial pressures of the reactantsandproductsare

unchanged, Le Chatelier’s principle successfully predicts that the equilibrium com-

position will be unaffected.

280

Example 15.2

Predict how the equilibrium yield of product(s) in the following

reactions will be affected by (i) allowing the mixture to expand

so that its pressure falls, and (ii) compressing the mixture so

that its pressure increases:

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

2NH 3 (g)\===\3H 2 (g)N 2 (g) (B)

Answer

We take ‘equilibrium yield’ to mean the concentration of products in the

equilibrium mixture. Applying the rules discussed above:

Reaction Pressure decreases Pressure increases

(A) no change no change

(B) [H2(g)] and [N2(g)] increases [H2(g)] and [N2(g)] decreases

Reaction (A) is unaffected because there is no change in the number of molecules

going from left to right.

Predicting the effects of changes on the equilibrium

composition of a gaseous reaction

Ethene and hydrogen react in the presence of a catalyst, making ethane:

(Ni catalyst)

C 2 H 4 (g)H 2 (g)\== =======\C 2 H 6 (g) H^ —=137 kJ mol^1

An equilibrium mixture at 300 K was subjected to the following changes in separate

experiments:

(i) The mixture was compressed into a smaller volume.

(ii)Hydrogen gas was pumped into the mixture while keeping the volume constant.

(iii)The mixture was maintained at constant volume and nitrogen gas (which does not react

with any of the reactants or products) was pumped into the mixture.

(iv)The mixture was cooled.

How will these changes affect (a) the equilibrium yield of ethane and (b) the equilibrium

constant?

Exercise 15J