Reversible reactions and equilibria

When asked about a pressure or temperature change, writing only "the yield decreases" scores half marks at best. The mark scheme needs a clear statement of which way the equilibrium shifts (left/right) and the reason (fewer moles of gas, or the endothermic/exothermic direction). A bare yield statement reads like a 50:50 guess and is not credited alone.

On a reversible reaction a catalyst increases the rate of both the forward and the backward reaction by the same amount, so the yield is unchanged — only the time to reach equilibrium falls. Many write at length on activation energy (not asked) but never state that both rates rise equally. Never write that a catalyst raises the yield.

At dynamic equilibrium the concentrations of reactants and products stay constant — this does not mean they are equal to each other. There may be far more product than reactant (or vice versa); what matters is that the amounts stop changing. "Equal concentrations" is a classic wrong answer.

It is the forward and reverse rates that are equal at equilibrium — not the amounts of reactants and products. Dropping the word "rate" (writing "the reactions are equal" or "the amounts are equal") loses the mark. The concentrations are merely constant; the two opposing rates are what balance so that nothing appears to change.

In 4CH1, naming "Le Chatelier's principle" earns nothing — the specification does not require it. You must explain a shift in spec terms: the endothermic/exothermic direction for a temperature change, or fewer/more moles of gas for a pressure change. A bare "by Le Chatelier's principle" with no mechanism scores zero.

Dynamic equilibrium is only reached in a sealed (closed) container. If the system is open so a gas can escape, the reaction goes to completion instead of reaching equilibrium — the escaping product cannot react back. Always check the container is closed before describing an equilibrium.