Multiple choiceQuestion 1 · Apply
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By the end you can predict how a change in temperature moves an equilibrium, explain why a catalyst does not move it at all, and say which of the two changes the value of the equilibrium constant.
Pressure and concentration change how much of each substance is present. Temperature changes the energy the particles have, and that is a different kind of lever.
Heating a mixture speeds up both the forward and the reverse reaction. The question is which one gains more.
Temperature rule
Raising the temperature always favours the endothermic direction.
Lowering the temperature always favours the exothermic direction.
You can remember it from the principle itself: heating adds energy, and the system minimises that change by absorbing energy — which is exactly what an endothermic reaction does.
Both reactions speed up when heated, because more particles have enough energy to react. But the endothermic reaction needs a bigger share of that extra energy, so its rate grows by the larger factor.
With the endothermic reaction now faster than the exothermic one, the position of equilibrium moves in the endothermic direction.
This is why temperature is the only change that alters the value of or .
Worked example: Worked example: heating nitrogen dioxide
Dinitrogen tetroxide dissociates endothermically:
A sealed tube of the equilibrium mixture is placed in a beaker of hot water. Predict the colour change, and state what happens to the value of .
A heated mixture reaches its new equilibrium faster than a cold one, because every reaction is faster when hot.
But the final mixture is different too. Rate tells you how quickly equilibrium arrives; temperature changes where that equilibrium sits.
A catalyst provides an alternative pathway with a lower activation energy. Both the forward and the reverse reaction use the same pathway, so both speed up by the same factor.
Consequences, and every one of them is examinable:
A catalyst is a time-saver, not a yield-improver. If a reaction has a poor equilibrium yield, a catalyst will not fix it.
A catalyst lowers the energy barrier between reactants and products; it does not change the energies of the reactants or products themselves.
Since the barrier is lowered by the same amount in both directions, a molecule crossing from left to right is helped exactly as much as one crossing back. The balance point cannot move.
A catalyst increases the equilibrium yield because it speeds up the forward reaction.
It speeds up the forward and reverse reactions equally, so the position of equilibrium is unchanged.
Raising the temperature always increases the yield, because reactions go faster when hot.
Heating favours the endothermic direction. For an exothermic forward reaction, the yield of product falls.
A catalyst increases the value of because more product forms.
depends only on temperature. A catalyst leaves it exactly where it was.
Lowering the temperature increases the rate of the exothermic reaction.
Lowering the temperature slows both reactions; it merely slows the endothermic one more, so the equilibrium shifts towards the exothermic products.
If the forward reaction is exothermic, heating shifts the equilibrium right because heat is a product.
Treating heat as a product gives the right answer only if you apply it correctly: extra heat shifts the equilibrium away from the side that produces heat, i.e. left.