Chemistry, Third edition

EFFECTS OF CHANGING CONCENTRATION 279

3.Consider the gaseous reaction

H 2 SO 4 (g)\===\H 2 O(g)SO 3 (g)

which we shall assume is occurring in a cylinder fitted with a piston heated to a con-

stant temperature. There is an increase in the number of molecules going from left

(reactants) to right (products) in this reaction, and therefore (at fixed volume) the

pressure of the mixture increases as reaction takes place.

Imagine that the reaction mixture is allowed to reach equilibrium. Suppose we

now increase the pressure of the equilibrium mixture by forcing the piston handle

inwards. Le Chatelier’s principle predicts that the reaction will attempt to resist such

changes by shifting the equilibrium composition so that the pressure of the mixture

decreases, i.e. so that fewer molecules are formed. Fewer molecules can only be made

if the equilibrium concentration of product decreases. This is achieved by the com-

bination of product molecules to form reactants – here H 2 SO 4 (g).

If the reaction involves a reduction in the number of molecules going from left to

right, Le Chatelier’s principle correctly predicts that increasing the pressure of the

mixture will cause the equilibrium concentration of products to increase.

If the reaction involves an equal number of molecules on the reactant and product

Table 15.4Summary of the effect of varying conditions on the equilibrium

aAbB\==\cCdD

Change Effect on percentage of Effect on
product at equilibrium equilibrium
constant

Increasingthe concentration* increases no change
of one or both reactants

Decreasingthe concentration* decreases no change
of one or both reactants

For gas reactions, increasing 1.decreasesif there are fewer no change
the pressure of the reactants molecules on the left-hand side
andproducts(e.g. if the mixture of the reaction equation, i.e. if
is in a piston, we apply extra (ab)(cd)
force to the piston handle)

2.increasesif there are more molecules no change

on the left-hand side of the reaction

equation, i.e. if (ab)(cd)

For gas reactions, allowing the 1.increasesif (ab) < (cd) no change
mixture to expand so reducing
the pressure of the reactants 2.Decreasesif (ab)(cd) no change
andproducts(e.g. if the mixture
is in a piston, we reduce the
force on the piston handle)

For gas reactions, injecting no change. (This is true whatever the no change
an inert gas into the mixture values of a,b,candd).
while keeping the total gas
volume constant

Increased temperature 1.increasesfor an endothermic increases
reaction
2.decreasesfor an exothermic decreases
reaction

Catalyst no change no change

*For gases, the partial pressure.