Chemistry, Third edition

15 · DYNAMIC CHEMICAL EQUILIBRIA

for experiment 1 Kc(T)= 4.1

for experiment 2 Kc(T)= 4.0

for experiment 3 Kc(T)= 4.0

Three features need commenting upon:

1.Constancy ofKc(T)

Within experimental error all three values of Kc(298 K)are the same.

2.Effect of higher concentration of reactant upon product concentration at equilibrium

In experiment 2 the initial mixture contains a higher concentration of acid than was

used in 1. This causes the equilibrium concentration of ester and water to be greater

than that observed in experiment 1. This effect is observed in other equilibria:

Increasing the concentration of reactant(s) increases the equilibrium

concentration of products.

3.Effect of the presence of product upon the gain in product concentration in the

equilibrium mixture

Although the equilibrium concentration of ester in experiment 3 is greater than

in experiment 2, we need take into account that the initial reaction mixture in

experiment 3 contained some ester. To do this, we define the gainin ester con-

centration (conc.):

gain in ester conc.equilibrium ester conc. initial ester conc.

The final concentration of ester in experiment 3 (1.87 mol dm^3 ) represents a

gain of 1.87 1.00 = 0.87 mol dm^3 ester. This is lessthan that obtained with

experiment 2, where the gain was 0.93 mol dm^3. The same effect is observed in

other equilibria.

Generalizing,

The presence of some product at the start of a reaction reduces the gain of

product in the equilibrium mixture.

In principle, the effects of increased reactant concentration, and of the presence of

product in the initial mixture, are applicable to all equilibria. However, if the equilib-

274

Table 15.3Equilibrium composition (at 25 °C) of mixtures

containing ethanoic acid, ethanol, water and ethyl

ethanoate

Time Concentrations/mol dm^3

[CH 3 COOH] [C 2 H 5 OH] [CH 3 COOC 2 H 5 ][H 2 O]

Experiment 1

t 0 1.00 1.00 0 0

At equilibrium 0.33 0.33 0.67 0.67

Experiment 2

t 0 4.00 1.00 0 0

At equilibrium 3.07 0.07 0.93 0.93

Experiment 3

t 0 4.00 1.00 1.00 0

At equilibrium 3.13 0.13 1.87 0.87