What is Activation Energy? (Edexcel IGCSE Chemistry (Modular))
Revision Note
Reaction profiles
Reaction profiles showing ∆H and Ea
Reaction profiles are similar to energy level diagrams seen in a previous topic, but in addition to showing the relative energies of the reactants and products in chemical reactions, they also show how the energy changes as the reaction progresses
The difference in height between the energy of reactants and products represents the overall enthalpy change of a reaction
For an exothermic reaction, the value of H is negative and the arrow point downwards
For an endothermic reaction, the value of H is positive and the arrow point upwards
The initial increase in energy, from the reactants to the peak of the curve, represents the activation energy, Ea, required to start the reaction
The greater the initial rise then the more energy that is required to get the reaction going e.g., more heat needed
Reaction profiles of exothermic and endothermic reactions
Reaction profiles show enthalpy change and activation energy for the reaction
Catalysts and reaction profiles
Catalysts provide the reactants with an alternative pathway for the reaction which has a lower activation energy
By lowering the activation energy a greater proportion of molecules in the reaction mixture have sufficient energy for an effective collision
As a result of this, the rate of the catalysed reaction is increased compared to the uncatalysed reaction
Diagram showing the effect of a catalyst on activation energy
Catalysts lower the activation energy
Examiner Tip
The activation energy is constant for a particular reaction. Reactions with a low activation energy occur readily as little energy is needed to break the bonds and initiate the reaction.
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