Reversible reactions and equilibrium
The ⇌ symbol, dynamic equilibrium, and the effect of changing conditions.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 6.3.1Describe a reversible reaction and the characteristics of dynamic equilibrium in a closed system.
- 6.3.2Predict the effect of changing temperature, pressure or concentration on the position of equilibrium.Supplement
7 minute read
Reversible reactions and equilibrium
A reversible reaction can go in either direction — the products can react to re-form the reactants. It is written with a double arrow, ⇌, instead of a single arrow.
Dynamic equilibrium
If a reversible reaction happens in a closed system (nothing can enter or leave), it eventually reaches dynamic equilibrium. At equilibrium:
- The forward and reverse reactions are both still happening — the reaction has not stopped.
- They occur at equal rates, so there is no overall change in the concentrations of reactants and products.
"Dynamic" is the key word: it is a state of continuous activity in balance, not a state of rest.
Shifting the position of equilibrium
Changing the conditions shifts the position of equilibrium, favouring either the forward or the reverse reaction:
- Increasing concentration of a reactant shifts equilibrium to produce more product (using up the extra reactant); increasing the concentration of a product shifts it back towards the reactants.
- Increasing pressure (for gas reactions) shifts equilibrium towards the side with fewer moles of gas, since that reduces the total number of gas particles and therefore the pressure.
- Increasing temperature shifts equilibrium in the endothermic direction, since that direction absorbs the extra energy; decreasing temperature shifts it in the exothermic direction.
These shifts are exploited industrially — for example, in the Haber process, conditions are chosen to maximise the yield of ammonia while keeping the reaction rate acceptably fast.
Think of it like this
Dynamic equilibrium is like a busy escalator with exactly as many people stepping on at the bottom as stepping off at the top at every moment — the total number of people on the escalator stays constant, but it is absolutely not because everyone has stopped moving.
Common misconceptions
- Thinking equilibrium means the reaction has stopped. Both the forward and reverse reactions are still occurring continuously, just at matched rates.
- Believing equilibrium means reactants and products are present in equal amounts. Their concentrations are constant, but not necessarily equal to each other.
- Forgetting that dynamic equilibrium can only be reached in a closed system — if substances can escape (an open system), equilibrium is never established.
In the exam
- Always state clearly which direction (forward/reverse, or towards products/reactants) a change shifts the equilibrium, not just that "the equilibrium changes".
- For pressure questions, count the number of moles of gas on each side of the equation first — the side with fewer gas moles is favoured by an increase in pressure.