Define the term activation energy, \({E_{\text{a}}}\).

State two conditions necessary for a reaction to take place between two reactant particles.

Sketch an enthalpy level diagram to describe the effect of a catalyst on an exothermic reaction.

(minimum) energy needed for a reaction to occur / (minimum) energy difference between reactants and transition state;

particles must collide;

appropriate collision geometry/orientation;

\(E \geqslant {E_{\text{a}}}\);

Diagram showing:

correct labelling of axes (enthalpy/H/(potential) energy for y-axis and time/progress/course of reaction/reaction coordinate for x-axis) and H (products) line shown below H (reactants) line;

correct labelling of the two curves, catalysed and uncatalysed;

correct position of \({E_{\text{a}}}\) shown with lines for a catalysed and uncatalysed reaction;

the correct label \(\Delta H\) /change in enthalpy;

Do not penalize if reactants and products are not labelled.

If an endothermic reaction is shown, award [2 max] if all other parts are shown correctly.

Most candidates gave the correct definition of activation energy in (a).

The two conditions needed for a reaction to take place were given by the majority of candidates.

In (c) some of the enthalpy level diagrams had many labels missing. Axes weren‟t always labelled, one of them was wrongly labelled as delta H, and the curves of Ea with and without catalyst were not properly indicated. A few answers showed an endothermic reaction instead.