Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Position of Equilibrium
What is Position of Equilibrium?
A description of how far a reversible reaction has proceeded when equilibrium is reached, in other words the relative amounts of reactants and products in the equilibrium mixture. It moves to the right when more product is present at equilibrium and to the left when more reactant is, and it is changed by temperature, pressure and concentration but never by a catalyst.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Position of Equilibrium in context
A reversible reaction, shown with the symbol \(\rightleftharpoons\), can run in both directions at once, and in a closed system it reaches dynamic equilibrium when the forward and reverse rates are equal, so the concentrations of reactants and products stop changing. Changing temperature, pressure or concentration shifts the position of equilibrium, but a catalyst never does — it speeds up both directions equally, reaching the same equilibrium sooner rather than reaching a different one. The Haber and Contact processes each pick operating conditions that trade off rate, yield, safety and cost rather than chasing the highest possible yield alone. In redox terms, oxidation is loss of electrons and reduction is gain of electrons, so an oxidising agent is itself reduced and a reducing agent is itself oxidised.
Common mistakes with Position of Equilibrium
- 16. “Heat acts as a catalyst.” Why temptingBoth speed reactions up. RepairA catalyst lowers \(E_\mathrm{a}\) and leaves the position of equilibrium alone. Heat leaves \(E_\mathrm{a}\) alone and does move the position. They are opposites in both respects. TransferWhich of the two changes the yield? Heat.
Examiner tips on Position of Equilibrium
- The two-sentence rule for “explain” in this topic. Sentence one names the particle-level change. Sentence two names the consequence for successful collisions or for the position of equilibrium. An answer with only the first sentence has described rather than explained, and an “explain” question is asking for the second one.
- The row worth memorising is the third. Whenever the gas counts are equal, the correct answer to “what happens to the position of equilibrium?” is nothing. Students who have learned “pressure pushes it right” as a slogan get this wrong every time. Pressure still speeds the reaction up — but it does not move the position.
Questions students ask about Position of Equilibrium
What is the difference between rate of reaction and position of equilibrium?
Rate measures how fast a reaction happens; position of equilibrium measures how far it goes. A catalyst is the clearest proof they are separate: it raises the rate by lowering the activation energy for both directions, but it never moves the position of equilibrium, because it speeds up the forward and reverse reactions by the same factor.