Activation Energy and Heat of Reaction
Activation Energy
Activation energy is the minimum energy required for reactants to successfully react and turn in to products. In equilibrium reactions there is an activation energy for both the forward and reverse reaction. Exothermic reactions have lower activation energies than endothermic reactions:
Activation Energy and Equilibrium Reactions
In equilibrium reactions, the activation energies of the forward and reverse reactions must not be too great so that it can be overcome by the reacting particles. This is most common with the endothermic reaction in an equilibrium system.
An exothermic reaction is one that releases energy from the system and this could be observed as an increase in temperature to the environment. As the exothermic reaction proceeds the increase in temperature will favour the endothermic reaction and a new equilibrium will be established. The opposite is observed for an endothermic reaction. Energy is absorbed by the system and this could be observed as a decrease in temperature to the environment. As the endothermic reaction proceeds the decrease in temperature will favour the exothermic reaction and a new equilibrium will be established.
The Impact of Temperature Change on Exothermic and Endothermic Reactions
When the temperature of an equilibrium system is changed it impacts both the forward and reverse reactions in the same way. That is, an increase in temperature will mean all particles will have more energy and a greater chance of overcoming the activation energy required for a successful reaction. The opposite is also true: a decrease in temperature will mean all particles will have less energy and a less chance of overcoming the activation energy required for a successful reaction. The difference is that the impact on the forward and reverse reaction is not equal – one direction will be favoured over the other.
- When the temperature of an equilibrium system is increased, the impact on the endothermic reaction is greater. This is because a greater proportion of particles in the endothermic reaction can now achieve the activation energy required to react.
- When the temperature of an equilibrium system is decreased, the impact on the exothermic reaction is greater. This is because a greater proportion of particles in the exothermic reaction can now achieve the activation energy required to react.
This concept is illustrated below. The first diagram illustrates the activation energy and the molecular energy distribution for a reaction when an initial equilibrium is established. Both the exothermic and endothermic reactions are shown. *note that a higher proportion of molecules have the necessary energy required for a successful reaction in the exothermic reaction:
The second diagram represents the molecular energy distributions for the same reaction at a higher temperature. Whilst both the exothermic and endothermic reaction rates are increased as a result of the higher temperature, a higher proportion of molecules are impacted by this increased temperature in the endothermic reaction, hence the equilibrium favours and shifts to the endothermic side.
The third diagram represents the molecular energy distributions for the same reaction at a lower temperature. Whilst both the exothermic and endothermic reaction rates are decreased as a result of the lower temperature, a higher proportion of molecules are impacted by this decreased temperature in the endothermic reaction, hence the equilibrium favours and shifts to the exothermic side. (Less particles can successfully react in the endothermic reaction)
The Addition of Catalysts.
The addition of a catalyst to an equilibrium system lowers the activation energy required for the reaction to proceed. This results in more particles being able to react and the reaction rate increases.
The addition of a catalyst does not change the position of the equilibrium, it only helps to achieve that equilibrium position faster: