Dynamic equilibrium

Dynamic equilibrium is the state a reversible reaction reaches in a closed system when the forward and reverse reactions happen at exactly the same rate. Both reactions keep going, which is why it is called dynamic, but because they cancel out, the amounts of reactants and products stop changing. The mixture looks still even though particles are reacting all the time.

Reversible reactions

In a reversible reaction, the products can react to make the original reactants again. It is written with a double arrow, ⇌. Heating ammonium chloride is a classic example: it breaks down into ammonia and hydrogen chloride, and on cooling they join back together.

NH₄Cl ⇌ NH₃ + HCl

AQA's GCSE Chemistry specification adds that if a reversible reaction is exothermic in one direction, it is endothermic in the other, and by the same amount. Blue hydrated copper sulfate turns white when heated, as its water leaves (endothermic). Add the water back and it turns blue again and warms up (exothermic).

How dynamic equilibrium is reached

Start with only reactants in a sealed container. At first the forward reaction is fast, because there is lots of reactant, and the reverse reaction can't happen at all. As products build up, the forward reaction slows and the reverse reaction speeds up. Eventually the two rates are equal, and from then on the concentrations stay constant.

Here x is time and y is the concentration of product. The curve climbs quickly at first, then flattens out once equilibrium is reached. A flat line doesn't mean the reaction has stopped: product is still being made, but it is being used up by the reverse reaction just as fast.

The conditions for dynamic equilibrium

• A reversible reaction.
• A closed system: nothing can get in or out. AQA's definition says equilibrium is reached in apparatus that prevents the escape of reactants and products.
• Forward rate = reverse rate.
• Constant concentrations, though not usually equal ones.

Check you can spot when a system is at equilibrium, then press Mark to check in the course.

Le Chatelier's principle

When the conditions of a system at equilibrium change, the position of equilibrium shifts to counteract the change. That is Le Chatelier's principle, and AQA uses it to predict the effects of changing conditions. These effects are Higher tier at GCSE.

Example: making ammonia

The Haber process makes ammonia from nitrogen and hydrogen. The forward reaction is exothermic.

N₂ + 3H₂ ⇌ 2NH₃

• Pressure: there are 4 gas molecules on the left (1 + 3) and 2 on the right, so a higher pressure favours the right-hand side and more ammonia forms.
• Temperature: the forward reaction is exothermic, so raising the temperature shifts the equilibrium left, giving less ammonia. But a low temperature makes the reaction slow, so industry uses a compromise temperature.
• Catalyst: an iron catalyst helps the system reach equilibrium faster but doesn't change how much ammonia is there at equilibrium.

The equilibrium constant, Kc (A-level)

At A-level, the position of equilibrium is measured with the equilibrium constant. For aA + bB ⇌ cC + dD:

Square brackets mean concentration in mol dm⁻³ at equilibrium. For H₂ + I₂ ⇌ 2HI, if [H₂] = 0.020, [I₂] = 0.020 and [HI] = 0.14 mol dm⁻³, then Kc = 0.14² ÷ (0.020 × 0.020) = 49, with no units because they cancel. A large Kc means the equilibrium lies to the right, with mostly products. Only temperature changes the value of Kc; changing concentration or pressure moves the position but leaves Kc the same. The rates and equilibrium calculator works out Kc with a full ICE table.

Common mistakes

• Saying the reaction stops at equilibrium. Both reactions carry on at equal rates.
• Saying the concentrations are equal. They are constant, not equal.
• Saying a catalyst shifts the equilibrium. It only gets there faster.
• Counting solids or liquids for pressure. Only count gas molecules.
• Forgetting the closed system. In an open container, a gas product escapes and equilibrium is never reached.

To practise equilibrium questions under time pressure, try today's GCSE Chemistry Higher or A-level Chemistry: Year 1 short timed paper. Each paper mixes topics. For energy changes in each direction, see reaction profiles.

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Chemistry · Rates and equilibriumKc equation: writing Kc expressions and calculating KcThe Kc equation for A-level Chemistry: writing the expression from a balanced equation, units, calculating Kc from equilibrium amounts, what changes Kc.