Reversible reactions and dynamic equilibrium
A reversible reaction goes both ways, shown by ⇌, e.g. . (Extended) In a closed system it reaches dynamic equilibrium: the forward and reverse reactions occur simultaneously; their rates are EQUAL (not merely constant); and the concentrations of reactants and products stay CONSTANT (not equal to each other). It is dynamic — particles keep reacting both ways, with no net change.
Le Chatelier: shifting the equilibrium
(Extended) Le Chatelier's principle: the equilibrium shifts to oppose a change. A temperature rise shifts towards the ENDOTHERMIC direction, a fall towards the EXOTHERMIC — so for exothermic Haber, higher temperature gives less ammonia. Pressure (gases only): a rise shifts towards FEWER gaseous moles. Concentration: adding a reactant or removing a product shifts towards products. A catalyst shifts neither the position nor the yield — it only reaches equilibrium faster.
Haber and Contact process conditions
The Haber process makes ammonia: (exothermic, kJ/mol) at ~450 °C, ~200 atm with an iron catalyst. (Extended) The Contact process makes sulfur trioxide: (exothermic, kJ/mol) at ~450 °C, ~2 atm with a vanadium(V) oxide () catalyst.
Drawn from real examiner reports.
(Extended) Dynamic equilibrium: equal and constant
Three phrasings are flagged wrong for dynamic equilibrium. (1) The forward reaction equals the reverse — misses rate; it is the RATES that are equal. (2) The rates are constant — not enough; they must be EQUAL. (3) Concentrations are the same — they are CONSTANT, NOT equal to one another. For full marks state both: equal forward and reverse rates, and constant concentrations.
S22 P41 Q5(a); W22 P42 Q3(iii) — all three phrasings (reactions equal, rates constant, concentrations equal) flagged as gaining no credit.
(Extended) Say the equilibrium shifts right
Cambridge wants the word equilibrium and a direction. The reaction goes forwards, breaks, or is destroyed gains no credit — say the equilibrium shifts to the right (or left / towards products or reactants). Do not contradict yourself by naming both directions, which examiners flag. Pair the direction with a Le Chatelier reason for the explanation mark.
W22 P41 Q3(f); W22 P42 Q3(iv) — goes forwards / breaks language, and contradictory left/right statements in one answer.
(Extended) Pressure: count gaseous moles
For a pressure change, the equilibrium shifts towards the side with fewer GASEOUS moles — count only gases, ignoring solids and aqueous species. Writing fewer moles without gaseous risks error in mixed-phase equations. In the Haber process the left has 4 gaseous moles (1 N2 + 3 H2), the right 2 (2 NH3), so higher pressure shifts right and raises ammonia yield.
W22 P41 Q3(f) — the side with fewer moles given rather than fewer gaseous moles; pressure and concentration effects conflated.
Haber vs Contact conditions
Do not swap the two processes. Haber (ammonia) uses ~200 atm (HIGH) with an IRON catalyst. The Contact process (sulfur trioxide, (Extended)) uses ~2 atm with a VANADIUM(V) OXIDE (V2O5) catalyst; both run at ~450 °C. The most penalised errors: vanadium(V) oxide given for Haber, or 2 atm for ammonia. Hook: Haber = High pressure + Iron.
W22 P41 Q3(d) — candidates cite wrong pressures and catalysts, swapping the Haber and Contact conditions.
(Extended) Answer the change actually asked
When a Haber question asks about DECREASING the temperature, many candidates explain a temperature INCREASE instead — the change is misread. Lowering the temperature lowers the rate but raises the equilibrium yield of ammonia (the equilibrium shifts towards the exothermic direction). Check whether the change is up or down, and answer that one.
W22 P41 Q3(g) — candidates default to explaining a temperature increase even when asked about a decrease.
(Extended) Catalyst changes rate, not yield
A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but its position and yield are unchanged. A common error is writing that an iron catalyst increases the yield of ammonia; it does not. Yield is set only by temperature, pressure and concentrations. Keep RATE separate from YIELD.
(Extended) Predict-and-explain scaffold
A predict-and-explain equilibrium question (3 marks) has three parts: direction (the equilibrium shifts left/right or towards reactants/products); the Le Chatelier reason (the system opposes the change); the consequence (yield up or down). Miss a link, drop a mark.
Underline up or down first
Underline whether the condition is INCREASED or DECREASED, and whether the question wants the effect on rate or on yield. Many marks are lost explaining the opposite change — especially a temperature decrease answered as an increase. Answer the exact change and quantity asked.
Keep rate and yield separate
State whether a change affects the RATE (speed of reaching equilibrium) or the YIELD (product formed). A catalyst changes only the rate. Raising the temperature raises the rate but, for an exothermic reaction, lowers the yield — opposite directions, never merge them.
Reversible reaction — a reaction that can proceed in both the forward and reverse directions, shown using the ⇌ symbol.
(Extended) Dynamic equilibrium — the state reached in a closed system when:
Le Chatelier's principle (Extended) — if the conditions of a system at dynamic equilibrium are changed, the equilibrium shifts in the direction that opposes the change.
Catalyst — a substance that increases the rate of reaction and is chemically unchanged at the end. A catalyst speeds up both forward and reverse reactions equally: it does not change the position of equilibrium or the yield at equilibrium.
Define reversible reaction and state the symbol used to show it.
Balance the following equation for the Haber process and state the number of gaseous moles on each side.
How does the mole count on each side explain why high pressure increases the yield of ammonia?
