3cPhysical Chemistry
Reversible reactions and equilibria — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 11 parts Lesson 3 of 9 1 quiz
Lesson summary
Reversible reactions and equilibria Edexcel O Level Linear Chemistry notes: know that some reactions are reversible and this is indicated by the symbol in equationsOnly temperature changes the value of Kc.Contact process2SO₂ + O₂ ⇌ 2SO₃, V₂O₅ catalyst, 450 °C, 1–2 atm.Strong acid…
- Reversible reactions and equilibria Edexcel O Level Linear Chemistry: what to do next
- Dynamic equilibrium
- Closed system
- Le Chatelier principle
- Bronsted–Lowry acid
- Bronsted–Lowry base
Section 1 of 11Process & sequence
1
Write the balanced equation.
2
Write the Kc expression: products over reactants.
3
Use an initial / change / equilibrium table of moles.
4
Divide by volume to get concentrations.
5
Substitute and work out the units.
1
Change applied
2
System opposes it
3
New equilibrium position
Section 2 of 11Keep going
Reversible reactions and equilibria Edexcel O Level Linear Chemistry: what to do next
These Reversible reactions and equilibria Edexcel O Level Linear Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Reversible reactions and equilibria Edexcel O Level Linear Chemistry understanding with the practice questions below, then reread any Reversible reactions and equilibria Edexcel O Level Linear Chemistry point you were unsure about before moving on.
2
Previous lesson: 3b Rates of reaction — Notes
3
Next lesson: 4a Introduction — Notes
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Official syllabus: Pearson Edexcel International GCSEs
Section 3 of 11Dynamic equilibrium
Dynamic equilibrium
In a closed system, a reversible reaction may reach dynamic equilibrium: the forward and reverse reactions continue, but occur at equal rates. Concentrations then remain constant even though molecules are still reacting.
Changing a condition can shift the position of equilibrium. The system responds in a direction that partly opposes the imposed change. Predict the favoured side by looking at the balanced equation and the condition changed.
Section 4 of 11Pressure and gas molecules
Pressure and gas molecules
For gas equilibria, increasing pressure favours the side with fewer gas molecules. Decreasing pressure favours the side with more. Count the gaseous coefficients on each side of the balanced equation; solids and liquids are not included in this count.
For the brown nitrogen dioxide and colourless dinitrogen tetroxide system, 2NO2 ⇌ N2O4, two gas molecules become one. Higher pressure favours colourless N2O4, so the mixture becomes paler at the new equilibrium.
Section 5 of 11What a catalyst changes
1Both
A catalyst provides an alternative reaction pathway with lower activation energy. It increases the rates of both forward and reverse reactions, so equilibrium is reached sooner.
2Not change the equilibrium position
Because both directions are accelerated, the catalyst does not change the equilibrium position or the final equilibrium concentrations at the same temperature and pressure. It changes the time taken, not the equilibrium yield.
Section 6 of 11Apply the rule
For a pressure question, state the number of gas molecules on each side, identify the side favoured by the pressure change, and then relate that shift to the colour or yield observed. Saying only “equilibrium shifts right” is unreliable unless the written equation's orientation is clear.
Check:
Check Will adding a catalyst make more N2O4 at equilibrium? Answer: No. It helps the system reach the same equilibrium composition faster.
Section 7 of 11Compare
| Change | Shift | Effect on Kc | Reason | Haber process example (ΔH −92) | Industrial compromise |
|---|---|---|---|---|---|
| More reactant | Right | None | Uses up added reactant | More NH₃ | — |
| Higher pressure | To fewer gas moles | None | Reduces pressure | Right (4 mol → 2 mol) | About 200 atm – cost and safety |
| Higher temperature | Endothermic direction | Changes | Absorbs heat | Left – less NH₃ | About 450 °C – rate vs yield |
| Lower temperature | Exothermic direction | Changes | Releases heat | Right – more NH₃ but slow | — |
| Catalyst | No shift | None | Speeds both directions equally | Iron catalyst | Equilibrium reached faster |
Section 8 of 11Key terms
Section 9 of 11Syllabus checklist
Syllabus points
know that some reactions are reversible and this is indicated by the symbol in equations
describe reversible reactions such as the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride
- the forward and reverse reactions occur at the same rate
- the concentrations of reactants and products remain constant
- an increase (or decrease) in temperature shifts the position of equilibrium in the direction of the endothermic (or exothermic) reaction
- an increase (or decrease) in pressure shifts the position of equilibrium in the direction that produces fewer (or more) moles of gas
Section 10 of 11Command 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 11 of 11Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Dynamic equilibrium · Closed system · Le Chatelier principle · Kc · Kp · Bronsted–Lowry acid · Bronsted–Lowry base · Conjugate pair
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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