6.2.6Chemical Change: Rate and Extent
The Effect of Temperature Changes on Equilibrium — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 15 of 16
Part 1 of 5Heat and reversible reactions (HT only)
One direction each way
- One direction each way In a reversible reaction one direction is exothermic and the other is endothermic.
Temperature as a condition
- Temperature as a condition Changing the temperature of a system at equilibrium alters the relative rates of the two directions, so the position moves.
Part 2 of 5Raising the temperature (HT only)
1Heat the mixture
- Heat the mixture The system now contains extra thermal energy.
2Opposing the change
- Opposing the change The system favours the direction that absorbs heat, the endothermic direction.
3Position shifts
- Position shifts More of the substances formed in the endothermic direction appear.
4Yield changes
- Yield changes The yield of the product from the exothermic direction falls;
- the yield from the endothermic direction rises.
Part 3 of 5Lowering the temperature (HT only)
1Cool the mixture
- Cool the mixture Heat is removed from the system.
2Opposing the change
- Opposing the change The system favours the exothermic direction, which releases heat.
3Position shifts
- Position shifts More of the substances formed in the exothermic direction appear.
4Rate falls
- Rate falls Particles move more slowly, so equilibrium is reached more slowly.
Part 4 of 5Summary table (HT only)
| Forward reaction is... | Temperature raised | Temperature lowered |
|---|---|---|
| Exothermic | Position moves left; less product | Position moves right; more product |
| Endothermic | Position moves right; more product | Position moves left; less product |
Part 5 of 5Example: Haber process and compromise
N2 + 3H2 ⇌ 2NH3
Forward reaction
- Forward reaction Making ammonia is exothermic, so a lower temperature gives a higher percentage yield.
Rate problem
- Rate problem At low temperatures the reaction is too slow to be economic.
Compromise
- Compromise A moderate temperature of about 450 °C, with an iron catalyst, gives an acceptable yield at a reasonable rate.
Key terms
| Term | Meaning |
|---|---|
| Exothermic direction | The direction of a reversible reaction that releases heat. |
| Endothermic direction | The direction of a reversible reaction that absorbs heat. |
| Position of equilibrium | How much product there is compared with reactants at equilibrium. |
| Compromise conditions | Settings chosen to balance yield, rate and cost. |
| Haber process | The industrial manufacture of ammonia from nitrogen and hydrogen. |
Avoid these mistakes
1
Saying that heating always increases the yield; it depends on whether the forward reaction is endothermic or exothermic.
2
Confusing rate and yielda higher temperature speeds up both directions even if it reduces the yield.
3
Stating that the system 'absorbs the cold' rather than releasing heat when cooled.
4
Forgetting that the endothermic and exothermic changes are equal in size but opposite in sign.
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