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
- For an exothermic reaction, the value of
- 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, showing the activation energy, Ea, and the enthalpy change, 
H
Catalysts and Reaction Profiles
- Catalysts provide the reactants with another pathway which has a lower activation energy
- By lowering Ea, 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 that a catalyst increases the rate of a reaction by providing an alternative pathway which has a lower activation energy
Exam 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.
