What is entropy and how is it used in chemistry?

Entropy (S) is a measure of disorder in a system. Any process, in this case a chemical reaction, is said to entropically favoured if the overall disorder in the system increases. This is usually when there is a net increase in the number of molecules, or the products of the reaction are in a more disordered state - for example, gas instead of solid. This can be used to find if a reaction is feasible at a given temperature, using Gibbs Free Energy (G), which is found by the equation G=dH-TdS, where dH, dS and T are the enthalpy change, entropy change, and temperature respectively. If the value for G is found to be negative, the reaction will occur spontaneously.