Untitled - Kelly Walsh High School

208 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Don’t Forget!
size is increased, the pressure decreases and the equilibrium will shift to the side
containing the most number of moles of gas in order to increase the pressure. If
the number of moles of gas is the same on both sides, then changing the pressure
will not have an effect on the equilibrium.
Once again consider the Haber reaction: N 2(g) + 3 H 2(g) K 2 NH 3(g). Note that
on the left side there are 4 mol of gas (1 of nitrogen and 3 of hydrogen) and
2 mol on the right. If the container is made smaller, the pressure increases
and the equilibrium will shift to the right because 4 mol of gas would be converted
to 2 mol. In doing so, the concentrations of nitrogen and hydrogen gases
would decrease and the concentration of ammonia would increase.
Pressure effects are only important for gases.
Changes in Temperature
Changing the temperature really changes the amount of heat in the system and
is similar to a concentration effect. In order to treat it in this fashion, one must
know which reaction, forward or reverse, is exothermic (releasing heat).
Once again, let’s consider the Haber reaction: N 2(g) 3 H 2(g) K 2 NH 3(g). The
formation of ammonia is exothermic (liberating heat), so that we could write
the reaction as:
N 2(g) 3 H 2(g) K 2 NH 3(g) heat
If the temperature of the reaction mixture is increased, the amount of heat
increases and the equilibrium would shift to the left in order to consume the
added heat. In doing so, the concentration of nitrogen and hydrogen gases
would increase and the concentration of ammonia gas would decrease.
A change in the temperature is the only way to change the value of K.