Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level Chemistry.
Activation Energy in context
Every chemical reaction both breaks bonds and makes bonds. Bond breaking is endothermic, because separating bonded atoms works against the attraction holding them together and needs an energy input; bond making is exothermic, because forming a bond releases energy. The overall enthalpy change is the balance: \(\Delta H\) equals the total energy taken in to break bonds minus the total energy given out when bonds form. If more energy is given out than taken in, the reaction is exothermic and \(\Delta H\) is negative; if less, it is endothermic and \(\Delta H\) is positive. Activation energy, \(E_\mathrm{a}\), is the minimum energy colliding particles must have to react.
Common mistakes with Activation Energy
- 5. “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 it is the activation energy of the reverse reaction.
- 8. “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.
Questions students ask about Activation Energy
What is activation energy, and how is it shown on a reaction pathway diagram?
Activation energy, \(E_\mathrm{a}\), is the minimum energy that colliding particles must have to react. Every reaction, exothermic or endothermic, has this barrier. On a reaction pathway diagram it is the arrow drawn from the reactants level up to the peak of the curve, never from zero and never from the products level. The enthalpy change, \(\Delta H\), is a different arrow: it runs between the reactants level and the products level, and the peak plays no part in it. Same start, different finish.
Does a large activation energy mean a large enthalpy change?
No. The two are independent. The activation energy says how much energy colliding particles need to cross the barrier; the enthalpy change 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, so two reactions with very different peaks can have exactly the same \(\Delta H\). You cannot infer either quantity from the other, and a question that gives only one does not let you conclude the other.