Question

How is entropy related to the spontaneity of a reaction?

Answer 1

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 (`DeltaH < 0`) or weakly endothermic (`DeltaH > 0` & small), the reaction is generally spontaneous.

The Gibbs free energy equation is:

`DeltaG = DeltaH - TDeltaS`

A reaction will be spontaneous if the change in `G`, `DeltaG`, is negative.

For the product of temperature times `DeltaS`, where `DeltaS` is the change in entropy, if the change in entropy is positive (disorder increases), then `TDeltaS`, 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.