Activation Energy

The activation energy, E_a, is the minimum energy that colliding particles must have for a reaction to occur.Even exothermic reactions need an input of energy to get started, because bonds in the reactants must begin to break before new bonds can form. This is why methane does not burn in air until a spark or flame is applied.Reaction profilesA reaction profile shows the enthalpy of the reactants and products and the energy barrier between them. The highest point is the transition state, where old bonds are partly broken and new bonds partly formed.E_a is measured from the reactants up to the top of the barrier. ΔH is measured from the reactants to the products.In an exothermic reaction the products are lower than the reactants, so ΔH is negative. In an endothermic reaction the products are higher, so ΔH is positive, and E_a must be at least as large as ΔH.Worked exampleFor a reaction, E_a (forward) = +75 kJ mol^-1 and ΔH = -40 kJ mol^-1. In the reverse reaction the particles must climb from the products to the same peak, which is 40 kJ further:E_a(reverse) = 75 + 40 = +115 kJ mol^-1Effect on rateThe larger the activation energy, the smaller the proportion of collisions with enough energy to react, so the slower the reaction at a given temperature.