3.2Unit 2 · Physical Chemistry
Reversible reactions & equilibria — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 7 parts Lesson 2 of 2 1 quiz
Lesson summary
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 7Reversible reactions & equilibria
1What 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.
2Example – copper(II) sulfate
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.
3Example – ammonium chloride
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.
4Energy in reversible reactions
Energy in reversible reactions if one direction is exothermic, the other is endothermic, and the same amount of energy is transferred.
5What 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'.
6Position of equilibrium – temperature
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.
7Position of equilibrium – pressure
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.
8Catalysts
Catalysts a catalyst speeds up the forward and backward reactions equally, so equilibrium is reached more quickly, but the position and yield are unchanged.
9Water of crystallisation
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.
10Le Chatelier's principle
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.
11Colour-change questions
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.
12Reversible vs complete
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 7Process & sequence
1
Reactants are mixed in a closed container; the forward reaction starts fast.
2
As reactants are used up, the forward rate falls.
3
Products build up, so the backward reaction starts and speeds up.
4
Eventually forward and backward rates become equal.
5
Concentrations of reactants and products now stay constant: dynamic equilibrium.
6
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.
1
Reactants in closed system
2
Forward rate high, then falls
3
Backward rate rises
4
Rates become equal
5
Concentrations stay constant
Section 3 of 7Compare
| 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 7Worked examples
Worked 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.
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.)
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Reversible reaction · Forward reaction · Backward reaction · Dynamic equilibrium · Closed system · Position of equilibrium · Exothermic · Endothermic · Catalyst
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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