1. Concentration and rate graphs
Read the axes before explaining a graph. Horizontal concentration curves mean concentrations are constant; the concentrations need not be equal. At dynamic equilibrium the forward and reverse rates are equal and non-zero. A concentration graph and a rate graph therefore have different shapes.
| Change imposed | Immediately | As equilibrium is restored |
|---|---|---|
| Add one reactant at fixed volume and temperature | Only the added species has a concentration jump | Added reactant is partly used up; other reactants fall and products rise continuously |
| Compress a gas mixture at constant temperature | All gas concentrations jump upwards | If gas coefficients differ, reaction towards fewer gas moles changes the concentrations further |
| Change temperature at fixed volume | No concentration jump from temperature alone; reaction rates change | Concentrations move continuously to new equilibrium values |
| Add catalyst to a mixture already at equilibrium | Both reaction rates increase equally | Concentrations stay constant; no shift |
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Concentration–time graph after adding A
Labels to include:
- Time on x-axis; concentration / mol dm⁻³ on y-axis
- Three labelled initial plateaux: A 0.40, B 0.30, C 0.20
- Dashed vertical line at the addition time
- A jumps to 0.60, then falls to a new plateau above 0.40
- B falls continuously from 0.30; C rises continuously from 0.20
- No instantaneous jump for B or C
The vertical step identifies the imposed change; the curved adjustments identify the subsequent reaction. All concentrations become constant again.
When comparing rates before and after a disturbance, explain which direction initially becomes faster and why. Do not assume that every new equilibrium has the same forward and reverse rate as the original equilibrium; the two rates must equal each other at each equilibrium.
2. Yield graphs
Compare points with just one variable changed. If the equilibrium product yield falls as temperature rises at fixed pressure, the forward reaction is exothermic: heating favours the endothermic reverse direction. If product yield rises with pressure at fixed temperature, the forward direction has fewer gas moles. These deductions concern equilibrium yield, not reaction speed.
| Temperature / °C | Yield at 100 atm / % | Yield at 200 atm / % |
|---|---|---|
| 350 | 48 | 62 |
| 450 | 30 | 43 |
| 550 | 18 | 28 |
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Equilibrium yield against temperature at two pressures
Labels to include:
- Temperature / °C on x-axis; equilibrium yield / % on y-axis
- Plot the six illustrative table points
- Two smooth decreasing curves labelled 100 atm and 200 atm
- 200 atm curve above 100 atm curve
- Compare vertically at the same temperature or along one constant-pressure curve
Higher pressure favours products in this example, while higher temperature favours reactants. The data remain usable without the drawing.
3. Industrial conditions
Industry needs an economical amount of product per unit time. For an exothermic reaction, low temperature favours equilibrium yield but may make the reaction too slow. A moderate temperature and catalyst can give a useful rate while retaining a useful yield. Higher pressure can increase both gas reaction rate and equilibrium yield when products have fewer gas moles, but compression needs energy and strong, costly equipment.
| Process and gas-phase reaction | Typical conditions | Reasoning |
|---|---|---|
| Haber: N₂ + 3H₂ ⇌ 2NH₃; exothermic | About 450 °C, 200 atm; iron catalyst | Four gas moles become two; pressure favours ammonia. Moderate temperature balances rate and yield |
| Contact: 2SO₂ + O₂ ⇌ 2SO₃; exothermic | About 450 °C, near 1–2 atm; V₂O₅ catalyst | Three gas moles become two, but conversion is already high at low pressure; extra compression may not justify its cost |
| Ethene hydration: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g); exothermic | About 300 °C, 60–70 atm; supported phosphoric acid | Two gas moles become one; higher pressure favours ethanol. Ethanol is a gas under these reactor conditions |
| Methanol: CO(g) + 2H₂(g) ⇌ CH₃OH(g); exothermic | Use the catalyst and conditions supplied in the question | Higher pressure favours methanol; lower temperature favours equilibrium yield but reduces rate |
Give the reason behind a compromise, rather than writing only “a compromise temperature”. A catalyst changes the rate, not the equilibrium constant or equilibrium yield at the same temperature. It can make a lower operating temperature economically practical; the temperature change then changes the equilibrium yield.
4. Separation and recycling
In processes such as ammonia manufacture, cool the gas mixture downstream to condense and separate the product, then return unreacted gases to the reactor. Recycling increases the overall conversion of fresh feed over repeated passes; it does not increase Kc or the single-pass equilibrium yield at unchanged reactor conditions. Real plants may also use a purge to prevent inert gases accumulating.
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Industrial separation and recycle loop
Labels to include:
- Fresh reactant feed
- Reactor labelled catalyst, temperature and pressure
- Downstream cooler and product separator
- Product outlet
- Unreacted-gas recycle arrow from separator to reactor inlet
- Optional purge on recycle line, labelled inert-gas control
The product leaves after separation; unreacted feed returns for another pass. The cooler is downstream, so reactor and separator conditions are different.
5. Carbon accounting
Calling a fuel carbon neutral requires considering the whole life cycle: where its carbon came from, how hydrogen and energy were produced, processing, transport and combustion. Capturing carbon for methanol production does not by itself prove zero net atmospheric CO₂ emissions.
Quick checks
Original Finesse questions; these stepwise solutions are indicative, not official AQA mark allocations.
Q1. Only [B] jumps upwards on a concentration–time graph at fixed temperature and volume. Afterwards [A] falls and [C] rises for A + B ⇌ C. Explain.Show answer
B was added. The subsequent forward reaction consumes A and B and produces C. At the new equilibrium all three concentrations become constant; they need not be equal.
Q2. Using the illustrative yield table, deduce the forward enthalpy sign and whether products have more or fewer gas moles.Show answer
At a fixed pressure, heating reduces yield, so the reverse direction is endothermic and the forward direction is exothermic (ΔH negative).
At a fixed temperature, compression raises yield, so products have fewer gas moles.
Q3. Why does an exothermic industrial process not simply use the lowest possible temperature?Show answer
Low temperature favours product equilibrium yield, but particles react more slowly, reducing production per unit time. A moderate temperature provides a useful rate while keeping a useful equilibrium yield; a catalyst helps the rate.
Q4. An ideal recycle model converts 40% of the remaining reactant each pass. What fraction of the original feed has converted after two passes?Show answer
After pass 1, 60% remains. Pass 2 converts 0.40 × 60% = 24% of the original feed. Total conversion = 40% + 24% = 64%. The single-pass conversion is still 40%.
Q5. A catalyst is added to a mixture already at equilibrium at unchanged temperature. Sketch descriptions are acceptable: what happens to the rate and concentration graphs?Show answer
Both forward and reverse rates rise equally and remain equal. Each concentration stays on its existing horizontal plateau. There is no equilibrium shift or change in Kc.
Sources
Sources and examiner guidance (reviewed 1 October 2026)
- Chemrevise — AQA 1.6 Equilibria revision guide (N. Goalby, Oct 2025 upload) — Industrial reasoning and yield graphs; gas-phase ethanol and life-cycle claims clarified.
- AQA 7405 specification — 3.1.6 Chemical equilibria, Le Chatelier's principle and Kc — 3.1.6.1: economic considerations and changing conditions.
- AQA 7404/1 mark scheme, June 2022 — Q07.1–Q07.3: yield trends and catalysts.
- AQA 7404/1 examiner report, June 2022 — Q07: distinguishing pressure, temperature and rate explanations.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.