Entropy and spontaneity

Understanding Entropy

Entropy is a measure of the dispersal or disorder in a system. It is a fundamental concept in the study of energetics and thermochemistry.
The more disordered a system, the higher its entropy. For example, gases have higher entropy than solids because gas particles can occupy a larger volume and have more disordered movement.
Entropy is symbolised by ‘S’, and changes in entropy are represented as ‘∆S’.
The units used for entropy are Joules per Kelvin per mole (J K^-1 mol^-1).

Entropy Changes in Reactions

A reaction where the products have a higher entropy than the reactants has a positive entropy change (∆S > 0). These are often reactions where a solid reactant forms gaseous products.
A reaction where the products have a lower entropy than the reactants has a negative entropy change (∆S < 0). These are often reactions where gaseous reactants form a solid product.
Total entropy change for a chemical reaction can be calculated by subtracting the total entropy of the reactants from the total entropy of the products: ∆S = Stotal(products) - Stotal(reactants).

Understanding Spontaneity

In thermochemistry, a reaction is said to be spontaneous if it proceeds on its own without any outside intervention once it has been initiated.
A reaction is more likely to be spontaneous if it leads to a increase in entropy (∆S > 0), because nature tends towards disorder, a principle known as the Second Law of Thermodynamics.

Gibbs Free Energy (G) and Spontaneity

The spontaneity of a reaction at constant temperature and pressure can be predicted by the change in Gibbs Free Energy (∆G).
∆G is calculated by the equation ∆G = ∆H - T∆S, where ∆H is the change in enthalpy, T is the absolute temperature in Kelvin, and ∆S is the change in entropy.
If ∆G is negative, the reaction is spontaneous. If ∆G is positive, the reaction is non-spontaneous. If ∆G is zero, the system is in equilibrium.

Events Affecting Entropy

Any process or event that leads to an increase in disorder will increase the entropy of a system. This could include volume increase, temperature increase, of phase changes from solid to liquid or liquid to gas.

Entropy and Energy Distribution

Entropy reflects the distribution of energy amongst the molecules in a system. The more ways the energy can be distributed, the higher the entropy.