Cambridge IGCSE Chemistry · Syllabus 0620 · Chemical Energetics
Activation Energy
What is Activation Energy?
The minimum energy that colliding particles must have in order to react; on a reaction pathway diagram it is the vertical difference between the reactants level and the peak of the curve.
This definition is part of the Chemical Energetics chapter in Cambridge IGCSE Chemistry.
Common mistakes with Activation Energy
- S2. “The \(E_\mathrm{a}\) arrow goes from the bottom of the axis up to the peak.” Repair \(E_\mathrm{a}\) is measured from the reactants level to the peak, never from zero and never from the products level. Drawn from zero it is not activation energy at all; drawn from the products level it is not the forward \(E_\mathrm{a}\) either — that arrow measures the barrier for the reverse reaction, which statement 5.1.6 does not ask you for.
- S4. “This reaction has a big activation energy, so \(\Delta H\) must be large and positive.” Repair The two are independent. The barrier height says how much energy colliding particles need; \(\Delta H\) says where the products ended up relative to the reactants. A strongly exothermic reaction can have a very large barrier, and a barely endothermic one can have a small barrier. You cannot infer either from the other.