The Foundations of Chemistry

Addition of a Catalyst

Adding a catalyst to a system changes the rate of the reaction (Section 16-9), but this
cannotshift the equilibrium in favor of either products or reactants. Because a catalyst
affects the activation energy of bothforward and reverse reactions equally, it changes both
rate constants by the same factor, so their ratio, Kc, does not change.

Adding a catalyst to a reaction at equilibrium has no effect; it changes neither Qnor

Kc.

The same equilibrium mixture is achieved with or without the catalyst, but the equilib-
rium is established more quickly in the presence of a catalyst.

Not all reactions attain equilibrium; they may occur too slowly, or else products or
reactants may be continually added or removed. Such is the case with most reactions in
biological systems. On the other hand, some reactions, such as typical acid–base neutral-
izations, achieve equilibrium very rapidly.

EXAMPLE 17-10 Applying a Stress to a System at Equilibrium

Given the following reaction at equilibrium in a closed container at 500°C, predict the effect
of each of the following changes on the amount of NH 3 present at equilibrium: (a) increasing
the temperature; (b) lowering the temperature; (c) introducing some platinum catalyst.

N 2 (g)3H 2 (g) 34 2NH 3 (g) 
H^0 92 kJ/mol rxn

Plan

We apply LeChatelier’s Principle to each part of the question individually.

Solution

(a) The negative value for
Htells us that the forward reaction is exothermic. Increasing the
temperature favors the endothermic reaction (reverse in this case).

Some NH 3 is used up.

(b) Lowering the temperature favors the exothermic reaction (forward in this case).

More NH 3 is formed.

(c) A catalyst does not favor either reaction.

It would have no effect on the amount of NH 3.

You should now work Exercises 54, 57, and 58.

Now we shall illustrate the commercial importance of these changes.

Can you use the Arrhenius equation

(Section 16-8) to show that lowering

the activation energy barrier increases

forward and reverse rates by the same

factor?

17-6 Factors that Affect Equilibria 727