3.2Unit 2 · Physical Chemistry
Reversible reactions & equilibria — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 2 of 2 1 quiz
Describe reversible reactions such as dehydration of hydrated copper(II) sulfate and decomposition of ammonium chloride.The symbol ⇌ shows a reversible reaction.CuSO4·5H2O (blue) ⇌ CuSO4 (white) + 5H2O.NH4Cl(s) ⇌ NH3(g) + HCl(g).If the forward reaction is exothermic, the backward…
- What are reversible reactions?
- Example – copper(II) sulfate
- Example – ammonium chloride
- Energy in reversible reactions
- What is a dynamic equilibrium?
- Position of equilibrium – temperature
Section 1 of 4Reversible reactions & equilibria
What are reversible reactions?
- Some reactions can go in both directions;
- the products can react to re-form the reactants.
- They are written with the ⇌ symbol.
- Changing conditions such as temperature can change the direction.
- Some reactions can go in both directions;
Example – copper(II) sulfate:
- heating blue hydrated copper(II) sulfate removes water, leaving white anhydrous copper(II) sulfate (endothermic).
- Adding water turns it blue again and releases heat (exothermic).
- This is used as a test for water.
Example – ammonium chloride:
- heating ammonium chloride decomposes it into ammonia and hydrogen chloride gases.
- At the cooler top of the tube the gases recombine to form white solid ammonium chloride again.
Energy in reversible reactions:
if one direction is exothermic, the other is endothermic, and the same amount of energy is transferred.What is a dynamic equilibrium?
In a closed system a reversible reaction reaches a point where the forward and backward reactions happen at the same rate.- The concentrations of reactants and products stay constant, but both reactions are still occurring – that is why it is called 'dynamic'.
Position of equilibrium – temperature:
- if temperature is increased, the equilibrium shifts in the endothermic direction to absorb the extra heat.
- If temperature is decreased, it shifts in the exothermic direction.
Position of equilibrium – pressure:
for reactions of gases, increasing pressure shifts the equilibrium to the side with fewer gas molecules (reducing pressure).- Decreasing pressure favours the side with more molecules.
- If both sides have equal numbers, pressure has no effect on position.
Catalysts:
a catalyst speeds up the forward and backward reactions equally, so equilibrium is reached more quickly, but the position and yield are unchanged.Water of crystallisation:
- is water held within the crystal structure of some salts.
- Copper(II) sulfate pentahydrate, CuSO4·5H2O, has five water molecules per formula unit;
- losing it changes the colour and shape of the crystals.
Le Chatelier's principle:
- if the conditions of a system at equilibrium are changed, the position of equilibrium moves to oppose the change. 'Shifts right' means more products form;
- 'shifts left' means more reactants form.
- if the conditions of a system at equilibrium are changed, the position of equilibrium moves to oppose the change. 'Shifts right' means more products form;
Colour-change questions:
in 2NO2(g, brown) ⇌ N2O4(g, colourless), the forward reaction is exothermic.- Higher pressure favours fewer molecules, so the mixture gets paler.
- Heating favours the endothermic backward reaction, so it gets darker brown.
Reversible vs complete:
- many reactions go to completion and stop when a reactant runs out.
- In a reversible reaction the products can change back, so the reactants are never fully used up.
Section 2 of 4Process & sequence
- Reactants are mixed in a closed container; the forward reaction starts fast.
- As reactants are used up, the forward rate falls.
- Products build up, so the backward reaction starts and speeds up.
- Eventually forward and backward rates become equal.
- Concentrations of reactants and products now stay constant: dynamic equilibrium.
- If conditions change, the equilibrium shifts to oppose the change until a new equilibrium forms.
Reaching dynamic equilibriumwrite these steps in this order; each step is a marking point.
- Reactants in closed system
- Forward rate high, then falls
- Backward rate rises
- Rates become equal
- Concentrations stay constant
Section 3 of 4Compare
| Change | Equilibrium shifts | Example: N2 + 3H2 ⇌ 2NH3 (exothermic forward) |
|---|---|---|
| Increase temperature | In the endothermic direction | Left – less NH3 |
| Decrease temperature | In the exothermic direction | Right – more NH3 |
| Increase pressure (gases) | To side with fewer gas molecules | Right (4 → 2 molecules) – more NH3 |
| Decrease pressure | To side with more gas molecules | Left – less NH3 |
| Add a catalyst | No shift; equilibrium reached faster | Same yield, sooner |
| Process | Colour change | Energy |
|---|---|---|
| Heat blue crystals | Blue → white | Endothermic |
| Add water to white powder | White → blue | Exothermic |
| Heat ammonium chloride | Solid decomposes to NH3 + HCl | Endothermic |
| Cool the gases | White solid re-forms | Exothermic |
Section 4 of 4Worked examples
For 2SO2(g) + O2(g) ⇌ 2SO3(g), the forward reaction is exothermic. Predict the effect on SO3 yield of (a) increasing pressure (b) increasing temperature.
- (a) Left side has 3 gas molecules, right side has 2.
- Increasing pressure shifts equilibrium to the side with fewer molecules, to the right, so the yield of SO3 increases.
- (b) Increasing temperature favours the endothermic backward reaction, so equilibrium shifts left and the yield of SO3 decreases.
- (a) Left side has 3 gas molecules, right side has 2.
For H2(g) + I2(g) ⇌ 2HI(g), explain the effect of increasing pressure.
- There are 2 gas molecules on each side, so changing pressure does not shift the position of equilibrium.
- (The rate increases, as molecules collide more often.)
- There are 2 gas molecules on each side, so changing pressure does not shift the position of equilibrium.
Marked practiceQuizzes for this lesson
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🌳 Student Search Tree & Frequently Asked Exam Questions: reversible reactions & equilibria
Step-by-step problem-solving guide, mark scheme criteria, and experimental error analysis curated by Dr. Khaled Dabour (DABOURHUB).
1. Identify all given values and convert non-standard units, 2. State the primary formula or physiological/chemical principle, 3. Calculate to 3 significant figures and clearly state units.
Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.
Where can I find reversible reactions & equilibria past paper questions by topic with the mark scheme?
On DABOURHUB every reversible reactions & equilibria question is grouped by syllabus point and shown with the mark-scheme wording, so you can see exactly which sentence earns the mark instead of scrolling through whole year papers.
What is the most common reversible reactions & equilibria exam question and why do students lose marks on it?
The repeated pattern on Edexcel papers is a short stem that asks you to apply reversible reactions & equilibria and then justify it with a command word (explain, suggest, compare). Marks are lost by stating the fact without linking it back to the wording of the question, so each reversible reactions & equilibria item here is written as a full-mark answer.
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Yes: the reversible reactions & equilibria notes, topical questions and MCQ quizzes on this site follow the same syllabus headings as the official specification, and they open straight from the search result with no download step.
How do I revise reversible reactions & equilibria in two weeks before the exam?
Three passes: read the reversible reactions & equilibria notes once, then do the topical questions and mark them against the mark scheme, then redo only the ones you missed. The quiz on the page gives instant feedback so the third pass takes minutes rather than an evening.