Why is an exothermic reaction spontaneous?
Exothermic reactions are not necessarily spontaneous.
Take the combustion of magnesium for example:
Yet a piece of magnesium is quite safe to handle at room temperature.
This is because a very high temperature is needed to make the magnesium burn. The reaction has a very high activation energy.
This is shown in the diagram:
A low activation energy can result in the reaction being spontaneous.
A good example is sodium reacting with water.
The diagram shows two important areas of physical chemistry.