Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3cPhysical Chemistry

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

5 min read 7 parts Lesson 3 of 3 2 quizzes

📚 Edexcel Modular - OL (Chemistry) Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

Reversible reactions and equilibria Edexcel O Level Modular 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 aci…

  • Reversible reactions and equilibria Edexcel O Level Modular Chemistry: what to do next
  • Dynamic equilibrium
  • Closed system
  • Le Chatelier principle
  • Bronsted–Lowry acid
  • Bronsted–Lowry base

Section 1 of 7Process & 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 7Keep going

Reversible reactions and equilibria Edexcel O Level Modular Chemistry: what to do next

These Reversible reactions and equilibria Edexcel O Level Modular Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Reversible reactions and equilibria Edexcel O Level Modular Chemistry understanding with the practice questions below, then reread any Reversible reactions and equilibria Edexcel O Level Modular Chemistry point you were unsure about before moving on.

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

Reversible reactions and equilibria Edexcel O Level Modular Chemistry revision notes

Section 3 of 7Compare

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

Section 4 of 7Key terms

Section 5 of 7Syllabus 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 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
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.

Last updated: