Reversible Reactions & EquilibriaEdexcel GCSE Chemistry: Revision notes
Section 1
What is a reversible reaction?
Many chemical reactions are reversible, meaning the products can react together to re-form the original reactants. Reversible reactions are shown using the symbol ⇌ in place of a normal arrow, e.g. A + B ⇌ C + D.
The direction in which a reversible reaction proceeds (forward or backward) can be altered by changing the reaction conditions, such as temperature, pressure or concentration.
Section 2
What is dynamic equilibrium?
When a reversible reaction takes place in a closed system, a dynamic equilibrium is eventually reached. At dynamic equilibrium:
- The forward and backward reactions are both still happening (the reaction hasn't stopped)
- They occur at exactly the same rate
- The concentrations of reactants and products therefore remain constant (though not necessarily equal to each other)
A common error is thinking equilibrium means the reaction has stopped — it hasn't; both reactions continue but their effects cancel out.
Section 3
How is ammonia made? (The Haber process)
The formation of ammonia (NH3) is a reversible reaction between nitrogen (extracted from the air) and hydrogen (obtained from natural gas):
N2 + 3H2 ⇌ 2NH3
This reaction can reach a dynamic equilibrium, since ammonia can also decompose back into nitrogen and hydrogen.
The conditions used industrially for the Haber process are:
- Temperature: 450 °C
- Pressure: 200 atmospheres
- Catalyst: iron
Learn the exact Haber process conditions (450 °C, 200 atm, iron catalyst) precisely — recall questions often ask for these exact numbers and the catalyst.
Section 4
How do conditions affect the position of equilibrium? (Higher tier)
Changing the reaction conditions of a system at dynamic equilibrium shifts the position of equilibrium — meaning it favours either the forward or backward reaction, changing the relative amounts of reactants and products:
- Temperature: increasing temperature shifts equilibrium in the endothermic direction; decreasing temperature shifts it in the exothermic direction
- Pressure (for gas reactions): increasing pressure shifts equilibrium towards the side with fewer gas molecules; decreasing pressure shifts it towards the side with more gas molecules
- Concentration: increasing the concentration of a reactant shifts equilibrium towards the products (to use up the extra reactant); increasing the concentration of a product shifts equilibrium back towards the reactants
In the Haber process, N2 + 3H2 ⇌ 2NH3 has 4 moles of gas on the left and 2 on the right, so increasing pressure shifts equilibrium towards ammonia (fewer gas molecules).
Must Know
- Reversible reactions use the ⇌ symbol; direction can be changed by altering conditions
- Dynamic equilibrium (closed system): forward and backward reactions continue at equal rates, so concentrations stay constant
- Haber process: N2 + 3H2 ⇌ 2NH3, at 450 °C, 200 atmospheres, with an iron catalyst, reaches dynamic equilibrium
- (Higher) Increasing temperature favours the endothermic direction; increasing pressure favours the side with fewer gas molecules; increasing reactant concentration favours the product side
That's the notes covered.
Carry on to the next subtopic.