Activation Energy and Enthalpy

Many of you may think that activation energy can be determined by the enthalpy change of a reaction.

Well, actually there is no way you can even predict the activation energy of a reaction from its enthalpy change.

A exothermic reaction that releases a lot of energy for example, may take a long time to be completed. As well as a endothermic reaction is able to happen in the blink of a eye.

Enthalpy change is the difference in potential energy of the reactants and products and it is independent of the pathway.

The activation energy, however, is determined by the rate of temperature in a reaction. 

In general, the amount of energy required to start a reaction will determine if this reaction will take long or not to occur, independent of the type of reaction it is.

Let's answer that questions:

1. Activation energy for a reaction is the amount of energy required for a reaction occur. Without this energy, bonds cannot be broken nor formed.
2. They are two distinct concepts. However, enthalpy change is only possible to happen if the activation energy is reached.
3. The spark will provide enough energy for combustion to happen.