Activation Energy (Edexcel IGCSE Chemistry)

Reaction profiles

Reaction profiles showing ∆H and E_a

• 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, E_a,  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