3.3.1Physical Chemistry
Reversible Reactions — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
15 min read 12 parts Lesson 17 of 19
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 8Key ideas
Key ideas
Reversible reactions use ⇌; a forward reaction that is exothermic has an endothermic reverse reaction with the same magnitude of ΔH.
Hydrated CuSO4·5H2O (blue) ⇌ anhydrous CuSO4 (white) + 5H2O.
Hydrated CoCl2·6H2O (pink) ⇌ anhydrous CoCl2 (blue) + 6H2O.
Haber
N2(g) + 3H2(g) ⇌ 2NH3(g); 4 mol of gas → 2 mol of gas; forward exothermic.
Contact
2SO2(g) + O2(g) ⇌ 2SO3(g); 3 mol of gas → 2 mol of gas; forward exothermic.
Low temperature would give a higher yield for both processes but the rate would be too slow; 450 °C is a compromise.
High pressure favours NH3 but is expensive and dangerous; 200 atm is a compromise. Contact uses only 2 atm because the yield is already high.
Unreacted N2 and H2 are recycled; ammonia is removed by cooling to liquefy it.
Moles of gas ratio is used to predict the pressure effect: count coefficients of gaseous species only.
Part 2 of 8Learning objectives
1
Describe reversible reactions including hydrated/anhydrous salts.
2
State the features of dynamic equilibrium.
3
Predict and explain the effect of temperature, pressure, concentration and catalysts on equilibrium position.
4
Describe and explain the conditions used in the Haber and Contact processes.
Part 3 of 8Essential knowledge
1
Some reactions are reversible
Some reactions are reversible the direction can be changed by changing conditions, e.g. heating hydrated copper(II) sulfate (blue → white) and adding water (white → blue).
2
Anhydrous copper(II) sulfate and cobalt(II) chloride are used as chemical tests for water.
3
Supplement
Supplement in a closed system a reversible reaction reaches dynamic equilibrium: rates of forward and reverse reactions are equal and concentrations of reactants and products remain constant (they are not necessarily equal).
4
Supplement
Supplement raising temperature shifts equilibrium in the endothermic direction; for exothermic forward reactions this lowers the yield.
5
Supplement
Supplement increasing pressure shifts equilibrium to the side with fewer moles of gas; if both sides have equal moles of gas, pressure has no effect on position.
6
Supplement
Supplement increasing the concentration of a reactant shifts equilibrium to the right; removing a product also shifts it right.
7
Supplement
Supplement a catalyst increases the rates of forward and reverse reactions equally, so the position does not change but equilibrium is reached faster.
8
Haber process
N2 from fractional distillation of liquid air; H2 from methane reacting with steam. Conditions 450 °C, 20 000 kPa, iron catalyst.
9
Contact process
SO2 from burning sulfur or roasting sulfide ores; O2 from air. Conditions 450 °C, 200 kPa, V2O5 catalyst.
10
Supplement
Supplement the temperature is a compromise — lower temperature increases yield (exothermic forward) but rate too slow; higher temperature is fast but low yield.
11
Supplement
Haber pressure is a compromise — higher pressure increases yield and rate but costs more (equipment, energy) and is more hazardous.
12
Supplement
Supplement the equilibrium for the Contact process lies far to the right even at low pressure, so high pressure is not worth the extra cost.
13
Supplement
Contact process final stagesSO3 is not added directly to water (this forms a hazardous acid mist).
- Instead it is absorbed in concentrated (98%) H2SO4 to form oleum: SO3 + H2SO4 → H2S2O7.
- Oleum is then diluted with waterH2S2O7 + H2O → 2H2SO4.
14
Supplement
The iron catalyst in the Haber process lowers the activation energy, so an acceptable rate is reached at 450 °C. Without it a higher temperature would be needed, costing more energy and giving a lower yield because more NH3 would decompose.
Part 4 of 8Process & sequence
- Heat blue hydrated copper(II) sulfate crystals in an evaporating basin or boiling tube.
- Observe the colour change blue → white as water of crystallisation is driven off (endothermic).
- Allow the white anhydrous copper(II) sulfate to cool.
- Add a few drops of waterit turns blue again and becomes hot (exothermic).
- Conclude the reaction is reversible
- CuSO4·5H2O ⇌ CuSO4 + 5H2O;
- the same test confirms water is present.
- CuSO4·5H2O ⇌ CuSO4 + 5H2O;
- Similarly, hydrated cobalt(II) chloride is pink and anhydrous cobalt(II) chloride is blue (cobalt chloride paper turns blue → pink with water).
Predicting an equilibrium shiftwrite these steps in this order; each step is a marking point.
- Write the balanced equation with state symbols and the sign of ΔH.
- Identify the change made (temperature, pressure, concentration, catalyst).
- Apply Le Chatelierthe system opposes the change.
- For pressure, count moles of gas on each side;
- for temperature, identify exothermic and endothermic directions.
- State the direction of shift and the effect on the yield of product.
Part 5 of 8Worked examples
Yield increases
For N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92 kJ/mol, predict the effect on the yield of NH3 of (a) increasing pressure (b) increasing temperature.
- (a) Left side 1 + 3 = 4 mol gas; right side 2 mol gas.
- Increasing pressure shifts equilibrium to fewer moles of gas → yield increases.
- (b) Forward reaction exothermic;
- increasing temperature shifts equilibrium in the endothermic (reverse) direction → yield decreases.
No change in position
For H2(g) + I2(g) ⇌ 2HI(g), explain the effect of increasing pressure on the position of equilibrium.
- Moles of gas on left = 1 + 1 = 2;
- on right = 2.
- Equal numbers, so no change in position (the rate increases, as particles are closer together).
+197 kJ/mol
The reverse of an exothermic reaction with ΔH = −197 kJ/mol has what ΔH? For 2SO2 + O2 ⇌ 2SO3, state which direction is favoured by cooling.
- Reverse ΔH = +197 kJ/mol (same magnitude, opposite sign).
- Cooling favours the exothermic direction → forward, more SO3.
Part 6 of 8Summary
- Reversible reactions can go both ways (⇌).
- At dynamic equilibrium forward rate = reverse rate and concentrations stay constant (closed system).
- The equilibrium shifts to oppose the change made.
- Catalysts do not change the position of equilibrium, only how quickly it is reached.
- Haber and Contact use compromise conditions of 450 °C.
Part 7 of 8Keep going
Reversible Reactions & Equilibrium IGCSE Chemistry 0620: what to do next
These Reversible Reactions & Equilibrium IGCSE Chemistry 0620 notes covered the key ideas, worked examples and exam technique for this topic. Check your Reversible Reactions & Equilibrium IGCSE Chemistry 0620 understanding with the practice questions below, then reread any Reversible Reactions & Equilibrium IGCSE Chemistry 0620 point you were unsure about before moving on.
Reversible Reactions & Equilibrium IGCSE Chemistry 0620: revision checklist
- Revise the main Reversible Reactions & Equilibrium IGCSE Chemistry 0620 definitions until you can write them from memory.
- Practise Reversible Reactions & Equilibrium IGCSE Chemistry 0620 exam questions under timed conditions.
- Link each Reversible Reactions & Equilibrium IGCSE Chemistry 0620 idea to a diagram or a worked example.
- List the common Reversible Reactions & Equilibrium IGCSE Chemistry 0620 mistakes and how to avoid them.
- Explain one Reversible Reactions & Equilibrium IGCSE Chemistry 0620 idea out loud as if teaching a friend.
- Summarise Reversible Reactions & Equilibrium IGCSE Chemistry 0620 in five key points on a revision card.
- Practice: Reversible Reactions & Equilibrium IGCSE Chemistry 0620 practice questions
- Previous lesson: 5.2 Chemical Change & Rate of Reaction — 0620 Notes
- Next lesson: 5.4 Redox — 0620 Notes
- Course: Cambridge IGCSE Chemistry 0620 — all units
- Official syllabus: Cambridge IGCSE Chemistry 0620 syllabus
Part 8 of 8Compare: Compare
| Change | Shift | Reason |
|---|---|---|
| Increase temperature | Towards the endothermic direction | Opposes the increase by absorbing heat |
| Decrease temperature | Towards the exothermic direction | Opposes the decrease by releasing heat |
| Increase pressure (gases) | Towards the side with fewer moles of gas | Opposes the increase by reducing the number of gas molecules |
| Increase concentration of a reactant | To the right (more products) | Uses up some of the added reactant |
| Remove product | To the right | Replaces some of the removed product |
| Add a catalyst | No change | Speeds up forward and reverse reactions equally; equilibrium reached sooner |
| Feature | Haber (NH3) | Contact (SO3) |
|---|---|---|
| Equation | N2 + 3H2 ⇌ 2NH3 | 2SO2 + O2 ⇌ 2SO3 |
| Raw material sources | N2 from air; H2 from methane (natural gas) | S burned to SO2 (or roasting sulfide ores); O2 from air |
| Temperature | 450 °C | 450 °C |
| Pressure | 20 000 kPa (200 atm) | 200 kPa (2 atm) |
| Catalyst | Iron | Vanadium(V) oxide |
| Forward reaction | Exothermic | Exothermic |
Key terms
Command terms
Command terms
State / Name
Give a short answer only; no explanation is needed (1 mark = 1 fact).
Define
Give the precise meaning. Learn the syllabus definitions word-for-word: every keyword in the definition is a marking point.
Describe
Say what happens / what the data show, with key features and figures. Do not give reasons.
Explain
Give reasonswhat happens AND why/how, linked with “because”, “so”, “therefore”.
Suggest
Apply knowledge to an unfamiliar situation; any sensible, scientifically-based answer can score.
Compare
Give similarities AND differences, each as a direct comparison (“X is … whereas Y is …”).
Outline
Set out the main points briefly.
Calculate
Work out from given facts; show working, give the unit and the requested significant figures.
Identify
Name/select from given information.
Predict
Give an expected result without a full justification.
Evaluate
Judge the importance or quality of the evidence/method, with reasons.
Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Reversible reaction · Dynamic equilibrium · Closed system · Position of equilibrium · Le Chatelier’s principle · Hydrated salt · Anhydrous salt · Water of crystallisation · Haber process · Contact process · Yield · Compromise conditions
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)
1
Use the exact scientific term and learn syllabus definitions word-for-word.
2
Write “rate of … increases”, not “… increases”.
3
Say “concentration gradient”, never just “gradient”; name particles precisely (atoms, molecules, ions) — never “it”.
4
Give units with every numerical answer and use the significant figures asked for; show all working.
5
Compare = a similarity AND a difference; describe = what happens; explain = why it happens.
6
Repeats let you identify anomalies and calculate a mean — they do not make an experiment “valid”.
Words to avoid
1
at equilibrium the reaction has stoppedforward and reverse reactions continue at equal rates
2
at equilibrium the concentrations are equalat equilibrium the concentrations remain constant
3
the catalyst increases the yieldthe catalyst does not affect the position of equilibrium; equilibrium is reached faster
4
high pressure moves it to the side with fewer moleculesto the side with fewer moles of gas
5
450 °C is used because it is the best temperature450 °C is a compromise between an acceptable rate and an acceptable yield
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