Reversible reactions and equilibria
Aims of this section
After studying this section, you should be able to:
- describe and explain reversible reactions
- describe and explain how changing the conditions affect the position of equilibrium
- describe the Haber process for manufacturing ammonia
- explain the conditions used in industrial reactions.
Summaries
Reversible reactions
Some reactions are described as reversible:
A + B ⇌ C + D
If a reversible reaction is endothermic in one direction, it is exothermic in the other direction. The amount of energy involved is the same in both directions.
Equilibrium
Reversible reactions that happen in closed systems eventually reach a dynamic equilibrium. At this point:
- the forward and reverse reactions still carry on (dynamic), but
- they happen at an equal rate, and so
- the concentrations of all reacting substances do not change (equilibrium).
Position of equilibrium
| Change | Position of equilibrium |
|---|---|
| Add more reactant | Moves to the right |
| Add more product | Moves to the left |
| Increase the temperature | Moves in the direction of the endothermic reaction |
| Increase the pressure | Moves in the direction of the fewest molecules of reacting gas |
| Add a catalyst | No change |
The Haber process
The Haber process is used to manufacture ammonia. It involves a reversible reaction:
nitrogen + hydrogen ⇌ ammonia
N2(g) + 3H2(g) ⇌ 2NH3(g)
The reaction conditions are chosen to give a reasonable yield at a reasonable rate:
- temperature = 450 °C
- pressure = 200 atmospheres (20 MPa)
- iron catalyst
