Reversible Reactions and EquilibriumAQA GCSE Chemistry: Revision notes
Section 1
What Is a Reversible Reaction?
In a reversible reaction, the products of the reaction can themselves react to re-form the original reactants.
- Represented using the symbol ⇌ instead of a normal arrow
- The direction the reaction goes in (forward or reverse) can be changed by changing the conditions, such as temperature or pressure
Ammonium chloride ⇌ ammonia + hydrogen chloride is a reversible reaction — heating decomposes it, cooling reforms it.
Section 2
How Are Energy Changes Linked in a Reversible Reaction?
If a reversible reaction is exothermic in one direction, it must be endothermic in the reverse direction — and the amount of energy transferred is exactly the same in each direction, just in opposite senses.
If you're told a forward reaction releases 50 kJ, the reverse reaction must absorb exactly 50 kJ.
Section 3
What Is Dynamic Equilibrium?
When a reversible reaction takes place in a closed system (nothing can escape or be added), it can reach dynamic equilibrium:
- This is reached when the forward and reverse reactions occur at exactly the same rate
- Both reactions are still happening — it is dynamic, not static — but because the rates are equal, the overall amounts of reactants and products stay constant
- The relative amounts of reactants and products present at equilibrium depend on the specific conditions of the reaction (concentration, temperature, pressure)
Section 4
How Does Changing Conditions Shift the Equilibrium Position? (HT)
Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in conditions, the system responds in a way that counteracts that change, shifting the position of equilibrium.
This allows qualitative predictions to be made about how changes in concentration, temperature and pressure affect where the equilibrium lies (i.e. whether more reactants or more products are favoured), without needing to know the exact numerical effect.
Le Chatelier's Principle explains which way the equilibrium shifts — it does not mean the change has no effect at all, just that the system opposes it rather than reinforcing it.
Must Know
- A reversible reaction's products can react to re-form the reactants; shown with the ⇌ symbol
- Changing conditions can change the direction a reversible reaction favours
- If a reversible reaction is exothermic one way, it is endothermic the other way, transferring the same amount of energy
- Dynamic equilibrium in a closed system: forward and reverse reaction rates are equal, so amounts of reactants/products stay constant
- The relative amounts of reactants and products at equilibrium depend on the reaction conditions
- (HT) Le Chatelier's Principle: a system at equilibrium shifts to counteract any change in concentration, temperature or pressure
That's the notes covered.
Carry on to the next subtopic.