The Effect of Changing TemperatureOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the general rule for temperature and equilibrium?
Every reversible reaction has one direction that is exothermic (releases energy) and one direction that is endothermic (takes in energy). Changing the temperature shifts the position of equilibrium:
- If temperature is raised, the yield from the endothermic reaction increases, and the yield from the exothermic reaction decreases
- If temperature is lowered, the reverse applies: the yield from the exothermic reaction increases, and the yield from the endothermic reaction decreases
Think of it as the system 'using up' extra heat by favouring the endothermic direction, and 'making up' for lost heat by favouring the exothermic direction — this is the reasoning examiners look for.
Section 2
Why does the system respond this way?
When temperature is raised, the system responds in the way that absorbs some of that extra energy — this is the endothermic direction, so its yield increases. When temperature is lowered, the system responds by releasing more energy to compensate — this is the exothermic direction, so its yield increases. This can be thought of as the equilibrium shifting to oppose the change that has been made.
Section 3
How does this apply to industrial processes?
In the Haber process, N2 + 3H2 ⇌ 2NH3, the forward reaction (forming ammonia) is exothermic. Therefore:
- Lowering temperature would increase ammonia yield
- However, a very low temperature makes the reaction rate far too slow
This is why the Haber process uses a moderate, compromise temperature of about 450 °C, rather than the lower temperature that would maximise yield.
In the Contact process, 2SO2 + O2 ⇌ 2SO3 is also exothermic in the forward direction, so a moderate temperature of ~450 °C is again used as a compromise between yield and rate.
Section 4
How is this different from the effect on rate of reaction?
It is important not to confuse the effect of temperature on equilibrium yield with its effect on reaction rate. Raising temperature always increases the rate at which equilibrium is reached (particles collide more frequently and energetically), regardless of which direction is exothermic or endothermic. But it only increases the yield of the endothermic direction.
Do not assume raising temperature always increases yield — it increases rate, but only increases yield of whichever direction is endothermic.
Must Know
- Raising temperature increases the yield of the endothermic direction and decreases the yield of the exothermic direction
- Lowering temperature increases the yield of the exothermic direction and decreases the yield of the endothermic direction
- The system shifts in the direction that opposes the temperature change
- In the Haber and Contact processes (both exothermic forward reactions), a compromise, moderate temperature is used to balance yield against rate
- Raising temperature always increases the rate of reaction, but only increases the yield of the endothermic direction
That's the notes covered.
Carry on to the next subtopic.