The big idea: Many reactions are reversible — the products can react to re-form the reactants. We show this with a double arrow (⇌).
In a closed container, a reversible reaction reaches dynamic equilibrium: the forward and reverse reactions happen at the same rate, so the amounts of everything stop changing — even though both reactions are still going on.
Animated graph
Watch the graph build step by step in study mode.
Two definitions to nail: - Reversible reaction — one that can go both ways: reactants ⇌ products. Written with the ⇌ symbol. - Dynamic equilibrium — the state in a closed system where the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products stay constant.
'Dynamic' is the key word. The reaction has not stopped — molecules are still turning into products and back into reactants all the time. It only looks still because the two opposite changes exactly cancel out.
A picture for it: Imagine an escalator you are walking down while it moves up at the same speed. You are working hard, the steps are moving — but your position never changes.
Equilibrium is like that: lots of activity, no net change.
Watch the concentrations instead of the rates. The reactant falls and the product rises, then both level off and stay flat — that flat region is equilibrium.
Animated graph
Watch the graph build step by step in study mode.
Constant does NOT mean equal: At equilibrium the concentrations are constant — they stop changing.
They are almost never equal to each other. The graph above levels off at two different heights. Saying 'the concentrations are equal' is a classic lost mark.
Practice with real exam questions
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Equilibrium is reached only in a closed system — nothing can be added and nothing can escape (or the gases/heat would leak away and the system could never settle). In that closed system, four things are true at once:
The four features
- The rate of the forward reaction equals the rate of the reverse reaction.
- The concentrations of reactants and products are constant (but not equal).
- Macroscopic properties (colour, pressure, pH, density) are constant — there is no visible change.
- Both reactions are still occurring — equilibrium is dynamic, not stopped.
| Feature of a system at equilibrium | What you observe / what it means |
|---|---|
| Forward and reverse rates | equal (this is the defining feature) |
| Concentrations of reactants and products | constant — but not equal to each other |
| Macroscopic properties (colour, pressure, pH) | constant — no visible change |
| The reactions themselves | still happening in both directions ('dynamic') |
| The system | must be closed (nothing added or escapes) |
Equilibrium position: The equilibrium position describes how far the reaction has gone — i.e. the relative amounts of reactants and products once equilibrium is reached.
- Position to the right → mostly products. - Position to the left → mostly reactants.
It does not mean 50:50.
How this is tested: Dynamic equilibrium is usually an explain or describe question — often set in a practical, such as following a sealed reaction over time.
- Paper 1A (MCQ): pick the correct statement about what is (and is not) constant at equilibrium. - Paper 2 / 1B: 'state the features of dynamic equilibrium', or 'suggest a property you could measure to tell when equilibrium is reached' — the answer is any macroscopic property that becomes constant (colour/absorbance, pressure, pH).
The markers want rates equal and concentrations constant — and they penalise 'the reaction stops' or 'the concentrations are equal'.
Score it cleanly: Quote the two defining points in the right words: forward rate = reverse rate and concentrations remain constant. Add closed system and both reactions still occur for full marks. Never write 'the reaction has stopped'.
IB-style question — a sealed coloured equilibrium (a)
A reversible reaction A(g) ⇌ B(g), in which B is coloured, is set up in a sealed tube and left at constant temperature. (a) State two features that show the system has reached dynamic equilibrium. [2]
How to score the marks
- Mark 1 — the rates. The rate of the forward reaction equals the rate of the reverse reaction (the reactions have not stopped — both still occur).
- Mark 2 — what stays constant. The concentrations of A and B (and so the colour / a macroscopic property) become constant and no longer change with time.
Final answer
(1) forward rate = reverse rate (both reactions still occurring); (2) the concentrations — and therefore the colour — stay constant. (Closed system, constant temperature.)
IB-style question — a sealed coloured equilibrium (b)
(b) Suggest one measurable property the student could record over time to show when equilibrium has been reached, and state what its graph would look like at equilibrium. [2]
How to score the marks
- Mark 1 — the property. Measure the colour intensity (absorbance) of the mixture with a colorimeter — it depends on the concentration of coloured B. (Pressure would also be acceptable for a gas reaction with a change in the number of gas molecules.)
- Mark 2 — the graph. The absorbance would rise (or fall) at first, then become constant (level off) — the flat region shows equilibrium has been reached.
Final answer
Record the absorbance/colour intensity over time; it changes at first then levels off to a constant value, and that constant region indicates equilibrium.