# How is entropy related to the spontaneity of a reaction?

Dec 23, 2016

Generally a reaction will occur spontaneously if the entropy increases, but the enthalpy is also negative, or small and positive, according to Gibbs free energy equation.

#### Explanation:

Entropy ($S$) is a measure of the disorder in a system.

In a closed system, entropy always increases over time. In an open system, energy can be added to a system to cause a decrease in entropy, but this is not necessarily a spontaneous reaction.

If entropy (disorder) increases, and the reaction enthalpy is exothermic ($\Delta H < 0$) or weakly endothermic ($\Delta H > 0$ & small), the reaction is generally spontaneous.

The Gibbs free energy equation is:

$\Delta G = \Delta H - T \Delta S$

A reaction will be spontaneous if the change in $G$, $\Delta G$, is negative.

For the product of temperature times $\Delta S$, where $\Delta S$ is the change in entropy, if the change in entropy is positive (disorder increases), then $T \Delta S$, when subtracted, becomes negative.

Since the cosmos (observable universe) is essentially a closed system, entropy will continue to increase until the universe essentially ceases to exist.