Why is an exothermic reaction spontaneous?

1 Answer
May 17, 2015

Exothermic reactions are not necessarily spontaneous.

Take the combustion of magnesium for example:

#2Mg_((s))+ O_(2(g))rarr2MgO_((s))#

#DeltaH# is negative.

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:

Exothermic reaction of energy from www.docbrown.info


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.

The #color(red)("red")# arrow relates to thermodynamics and is concerned with initial and final states.

The #color(purple)("purple")# arrow relates to kinetics and is concerned with how quickly the change from initial to final state is achieved.