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QUESTION

# Why is an exothermic reaction spontaneous?

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:

(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.