The Foundations of Chemistry

Solution

(a) Increasing the pressure favors the reaction that produces the smaller number of moles of
gas (forward in this case).

More NH 3 is formed.

(b) This reaction involves the same number of moles of gas on both sides, so a pressure (volume)
change does not disturb the equilibrium. There is no effect on the position of equilibrium.

Changes in Temperature

Consider the following exothermic reaction at equilibrium:

AB 34 CDheat (
His negative)

Heat is produced by the forward (exothermic) reaction. Suppose we increase the temper-
ature at constant pressure by adding heat to the system. This favors the reaction to the
left, removing some of the extra heat. Lowering the temperature favors the reaction to
the right as the system replaces some of the heat that was removed.
By contrast, for an endothermic reaction at equilibrium,

WXheat 34 YZ (
His positive)

17-6 Factors that Affect Equilibria 725

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 16.12, Disturbing a Chemical

Equilibrium (2): Temperature Changes.

The gas-phase equilibrium for the exothermicreaction

2NO 2 (g) 34 N 2 O 4 (g) 
H^0 rxn57.2 kJ/mol rxn

The two flasks contain the same totalamounts of gas. NO 2 is brown, whereas N 2 O 4 is

colorless. The higher temperature (50°C) of the flask on the right favors the reverse

reaction; this mixture is more highly colored because it contains more NO 2. The flask

on the left, at the temperature of ice water, contains less brown NO 2 gas.