Activation Energy (Edexcel IGCSE Chemistry)

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

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