Reversible Reactions and Equilibrium

Paper 4 reserves a 4-6 mark block most series for an equilibrium prediction: an unfamiliar ⇌ equation with stated conditions, then “explain the effect of increasing the temperature/pressure”. The prediction rules are short, but examiner reports show candidates losing the marks to incomplete definitions of dynamic equilibrium and to crediting catalysts with yield changes they do not cause.

Reversible reactions (Core)

A reversible reaction can go in both directions, shown by the ⇌ symbol. The Core set-piece is hydrated copper(II) sulfate:

CuSO4·5H2O ⇌ CuSO4 + 5H2O

Heat the blue hydrated crystals and they turn white as water is driven off (endothermic direction). Add water to the white anhydrous solid and it turns blue again and gets hot (exothermic direction). One fact generalises from this example: if the forward reaction is endothermic, the reverse reaction is exothermic, with the same magnitude of energy transfer. The energy vocabulary comes from Chemical Energetics.

The same pair of colour changes doubles as the chemical test for water, which is why this example keeps appearing in qualitative analysis questions.

Dynamic equilibrium (Supplement)

Seal a reversible reaction in a closed system (nothing enters or leaves) and it reaches dynamic equilibrium. The 2-mark definition has two compulsory parts:

1. The rate of the forward reaction equals the rate of the reverse reaction.
2. The concentrations of reactants and products remain constant.

“Dynamic” is doing real work: both reactions are still running, continuously, at equal rates. The concentrations are constant, not equal: reactants and products do not have to be present in equal amounts, and writing “equal amounts” instead of “constant concentrations” is a recurring penalised slip.

Shifting the position of equilibrium (Supplement)

Changing the conditions changes the position of equilibrium: the proportions of reactants and products at equilibrium. 0620 requires predictions for three changes, applied to equations like the ammonia synthesis N2(g) + 3H2(g) ⇌ 2NH3(g), forward reaction exothermic.

Change	Equilibrium shifts…	For N2 + 3H2 ⇌ 2NH3 (forward exothermic)
Increase temperature	In the endothermic direction	Left: yield of NH3 decreases
Decrease temperature	In the exothermic direction	Right: yield increases (but rate falls)
Increase pressure	Towards fewer gas molecules	Right (4 molecules → 2): yield increases
Decrease pressure	Towards more gas molecules	Left: yield decreases
Increase a reactant’s concentration	Away from that substance	Right: more NH3 formed
Add a catalyst	No shift	Equilibrium reached faster; yield unchanged

Count the gas molecules from the balanced-equation coefficients before answering any pressure question. That count is the entire argument. If both sides have equal gas molecules, pressure has no effect on position.

The answer template Cambridge credits: state the shift direction, then the reason, then the consequence. “Increasing the temperature shifts the equilibrium in the endothermic direction, which is the reverse reaction, so the yield of ammonia decreases.” Three clauses, up to three marks.

The industrial pay-off (why ammonia plants run at a compromise 450 °C and 200 atm with an iron catalyst) sits where this topic meets Rate of Reaction: low temperature gives the best yield but an uneconomically slow rate, so industry trades yield for speed.

Worked exam question

Sulfur dioxide reacts with oxygen: 2SO2(g) + O2(g) ⇌ 2SO3(g). The forward reaction is exothermic. (a) State what is meant by dynamic equilibrium. [2] (b) Explain the effect on the equilibrium yield of SO3 of (i) increasing the pressure [2] and (ii) increasing the temperature. [2]

Model answer: (a) The forward and reverse reactions occur at equal rates (1) and the concentrations of reactants and products remain constant (1). (b)(i) There are 3 gas molecules on the left and 2 on the right, so increasing the pressure shifts the equilibrium to the right (1); the yield of SO3 increases (1). (ii) Increasing the temperature shifts the equilibrium in the endothermic (reverse) direction (1); the yield of SO3 decreases (1).

Mark-by-mark: (a) is two distinct statements; either alone scores one. In (b)(i) the molecule count is the explaining mark: a bare “shifts right” without the 3-vs-2 comparison usually drops it. In (b)(ii) the shift must be tied to the endothermic direction; “heat breaks it down” earns nothing.

The mistakes that cost marks

1. Half a definition. Equal rates without constant concentrations (or vice versa) is one mark, not two.
2. “Equal concentrations” at equilibrium. Constant, not equal: the mixture can be 90% reactant and still be at equilibrium.
3. Giving a catalyst a yield effect. Catalysts change the time to reach equilibrium, never its position.
4. Pressure arguments without counting molecules. Quote the numbers from the coefficients: “4 molecules of gas on the left, 2 on the right”.

How examiners want it phrased

Student wording	Mark-scheme wording
”The reaction stops in the middle"	"Forward and reverse reactions continue at equal rates; concentrations remain constant"
"Pressure squashes it to the small side"	"Increasing pressure shifts the equilibrium towards the side with fewer gas molecules"
"Heat pushes it backwards"	"Increasing temperature shifts the equilibrium in the endothermic direction"
"The catalyst gets you more, faster"	"The catalyst increases the rate of both reactions equally; the position of equilibrium is unchanged”

Equilibrium is rule-application, not intuition: direction, reason, consequence, every time. The rest of Section 6 is mapped on the Chemical Reactions pillar, and if your predictions are right but your reasons keep scoring zero, a free 1-hour trial lesson with a Chemistry specialist will rebuild the three-clause answer on past-paper equilibria.

Test yourself

Predict first, then click: direction, reason, consequence every time.

Q1 (2 marks). Water is added to anhydrous cobalt(II) chloride. State the colour change observed and what this experiment shows about the reaction between cobalt(II) chloride and water.

Show answer

• blue to pink [1] • the reaction is reversible (heating the pink hydrated solid re-forms the blue anhydrous solid); the colour change is the test for water [1]

Q2 (3 marks). (Extended) Dinitrogen tetroxide decomposes: N2O4(g) ⇌ 2NO2(g). The forward reaction is endothermic. Explain the effect on the equilibrium yield of NO2 of (a) increasing the temperature and (b) increasing the pressure.

Show answer

• (a) increasing temperature shifts the equilibrium in the endothermic (forward) direction [1], so the yield of NO2 increases [1] • (b) there is 1 gas molecule on the left and 2 on the right, so increasing pressure shifts the equilibrium to the left and the yield of NO2 decreases [1]

Q3 (2 marks). (Extended) An iron catalyst is used in the synthesis of ammonia. Explain why the catalyst does not change the equilibrium yield of ammonia.

Show answer

• the catalyst increases the rate of the forward and reverse reactions equally [1] • so equilibrium is reached sooner but the position of equilibrium, and therefore the yield, is unchanged [1]