Chemistry, Third edition

15 · DYNAMIC CHEMICAL EQUILIBRIA

Catalysts and equilibria

A catalyst increases the rate constant for the forward and back reaction by the same

factor. For example, it might double both rate constants. It follows from the equation

Kc(T)=kf/kb

thatKc(T)is unaffected:

Catalysts do not alter the equilibrium constant of a reaction. They do not alter

the equilibrium concentrations. They merely increase the speed at which equi-

librium is achieved.

Le Chatelier’s principle

The effects of changing conditions upon chemical equilibria are summarized in Table

15.4. One way of remembering (but not explaining) such effects is to think of the

chemical reaction as actively opposing (almost as if it were alive!) any changes in

conditions that have been made. This is the basis of Le Chatelier’s principle:

The concentrations of reactants and products in an equilibrium mixture will alter

so as to counteract any changes in pressure, temperature or concentration.

Predictions made using Le Chatelier’s principle

1.If we add more reactant to an equilibrium mixture, the equilibrium composition

shifts so that more reactants are used up, i.e. so that the equilibrium mixture contains

a higher concentration of product. (We arrived at the same conclusion by looking at

experimental data for the ethanoic acid–ethanol reaction.)

2.The effect of temperature can be predicted by thinking of the heat evolved as a

special kind of product. Suppose we increase the temperature of an equilibrium mix-

ture in which the reaction is exothermic:

aAbB\===\cCdDheat

Le Chatelier’s principle predicts that the equilibrium composition alters so that less

‘heat product’ will be made, i.e. it alters in the direction in which the temperature will

be lowered. This is, of course, in the direction of the endothermic reaction. This

means that the equilibrium concentration of reactants increases and that of the prod-

ucts falls. As we have seen, this is indeed what happens.

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BOX 15.1

Explaining the variation of Kc(T)with temperature

Why do the equilibrium constants of reactions

vary with temperature? The answer lies in the

enthalpy–progress of reaction diagram

discussed in Box 14.1 (page 249).

For a reaction that is exothermicfrom left to

right, the activation energy of the forward

reaction is lessthan the activation energy of

the reverse reaction. The smaller the

activation energy, the less sensitive is the

associated rate constant to changes in

temperature. Thus, for an exothermic

reaction,kfchanges more slowly with

temperature changes than kb. Put another

way, for a particular increasein temperature

(say 50 °C), kfincreases by a smaller factor

thankb. Since Kc(T)=kf/kb, this causes Kc(T)

to decrease with temperature. For reactions

which are endothermic, EA(f)EA(b)and so kf

changes faster with temperature than kb. This

causesKc(T)to increase with increasing

temperature.