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.


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.


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

The position of equilibrium (also called equilibrium position) describes the relative concentrations of reactants and products at equilibrium.

The position of equilibrium can be changed by changing the reaction conditions.

ChangePosition of equilibrium
Add more reactantMoves to the right
Add more productMoves to the left
Increase the temperatureMoves in the direction of the endothermic reaction
Increase the pressureMoves in the direction of the fewest molecules of reacting gas
Add a catalystNo 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