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Reversible reactions and equilibria — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 11 parts Lesson 3 of 9 1 quiz

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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

Part 1 of 6Key ideas

1
Only temperature changes the value of Kc.
2Contact process
2SO₂ + O₂ ⇌ 2SO₃, V₂O₅ catalyst, 450 °C, 1–2 atm.
3
Strong acids dissociate fully; weak acids only partially.

Part 2 of 6Syllabus points

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

Part 3 of 6Process & 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

Part 4 of 6Summary

Summary
Only temperature changes Kc.
A catalyst does not shift equilibrium.

Part 5 of 6Keep 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.

1
Practice: Reversible reactions and equilibria Edexcel O Level Linear Chemistry practice questions
2
Previous lesson: 3b Rates of reaction — Notes
3
Next lesson: 4a Introduction — Notes
4
Course: Edexcel Linear – OL (chemistry) — all units
5
Official syllabus: Pearson Edexcel International GCSEs

Reversible reactions and equilibria Edexcel O Level Linear Chemistry revision notes

Part 6 of 6Compare: Compare

Effect of changes on equilibrium
ChangeShiftEffect on KcReasonHaber process example (ΔH −92)Industrial compromise
More reactantRightNoneUses up added reactantMore NH₃—
Higher pressureTo fewer gas molesNoneReduces pressureRight (4 mol → 2 mol)About 200 atm – cost and safety
Higher temperatureEndothermic directionChangesAbsorbs heatLeft – less NH₃About 450 °C – rate vs yield
Lower temperatureExothermic directionChangesReleases heatRight – more NH₃ but slow—
CatalystNo shiftNoneSpeeds both directions equallyIron catalystEquilibrium reached faster

Key terms

Command 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.

Exam 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.

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