Cracking the SAT Chemistry Subject Test

H   +   I   +   Heat        J   +   K

The reaction consumes heat in the forward direction, and it produces heat in the
reverse direction. In a sense, you can think of heat as being one of the reactants
in this equation. So what happens if we increase the temperature of this reaction?
That is, what happens if we add heat to the system? According to Le Chatelier’s
principle, we drive the equilibrium to the right. What happens if we decrease the
temperature of the system? We drive the equilibrium to the left.

There is one important thing to know about temperature changes: This is the
only type of stress that causes an equilibrium shift and changes the value of Keq

for a given reaction. The others do not alter Keq.

So to summarize:

Le  Chatelier’s Principle

If  some    stress  is  placed  on  a   reaction    at  equilibrium,    then    the equilibrium

will    shift   in  a   direction   that    relieves    the stress.

The Effects of Pressure Changes on Equilibrium

If one or more of the species in the reaction are gaseous, then changing the
system’s pressure can affect equilibrium. Consider the important ammonia-
producing reaction that is at the heart of what is known as the Haber process.

N 2 (g) +   3H 2 (g)        2NH 3 (g)

If the above reaction at equilibrium is stressed by a reduction of its volume
(which would increase the pressure of the system), then the reaction will relieve
this stress by shifting equilibrium in the direction that produces fewer moles of