In this lesson, you will learn about how a system at equilibrium responds to changes
in temperature.
LeChatelier's Principle states that a chemical system at equilibrium always works to restore
equilibrium when it is stressed. To consider what happens to a system at equilibrium
when temperature is changed, you must first consider the energetics of the reaction in
question. If the forward reaction is exothermic, then
the reverse reaction must be endothermic. Let's examine this hypothetical reaction
A + B C + D ΔH = -75kJ This means that 75kJ of energy is released
when the forward reaction occurs, and 75kJ is absorbed when the reverse reaction occurs.
So an increase in temperature would mean that the endothermic reaction would be favoured,
to remove the excess heat, therefore counteracting the imposed stress.
Decreasing the temperature would cause the system to produce more energy; therefore the
exothermic reaction would be favoured. An increase in temperature favours the endothermic
reaction. A decrease in temperature favours the exothermic
reaction. Challenge:
The dimerization of nitrogen dioxide to dinitrogen tetroxide is exothermic.
NO2(g) N2O4(g) ΔH = -57.2kJ brown colourless
Nitrogen dioxide is a brown gas, whereas dinitrogen tetroxide is colourless.
What observations do you think can be made when temperature is decreased?
How about when temperature is increased? Please pause the lesson to think about this,
and resume once you are done. A decrease in temperature favours the exothermic
reaction, so more dinitrogen tetroxide is produced.
Since it is a colourless gas, the mixture should appear paler.
An increase in temperature favours the endothermic reaction, so more nitrogen dioxide is produced.
The mixture should therefore appear darker brown.
Addition of a catalyst does not affect the position of equilibrium as it increases the
rate of both the forward and reverse reactions. It only quickens the attainment of equilibrium.
Let's return to the example of you digging a hole and your friend refilling it while
you dig. Imagine that you are both given much larger
shovels. The size of the hole still remains constant,
but with each dig or fill, more soil is removed or filled.
In conclusion, when the temperature of a system at equilibrium is increased, the endothermic
reaction is favoured. When the temperature of a system at equilibrium
is decreased, the exothermic reaction is favoured. Adding a catalyst has no effect on the position
of equilibrium.