Overall

Entropy (S) is related to probability and is higher for substances that have a larger number of arrangements available to them.
Thus for the same substance

Thus the sign of the entropy change for a process depends on the physical states of the reactants and products.

• If more gases are formed as products, ΔS is positive.
• If gases or liquids are converted to solids, ΔS is negative.

The magnitude of ΔS for a process can be calculated using one of the relationships below. Entropies are usually reported in J K^–1 mol^–1.
 

• For any reaction:
  ΔS° = ΣΔS°(products) - ΣΔS°(reactants)
   
• For liquid to gas phase change with no change in temperature

  ΔS =	ΔvapH
  	T

• For solid to liquid phase change with no change in temperature

  ΔS =	ΔfusH
  	T

• For temperature change with no phase change where n is the amount in moles and C_p is the heat capacity

  ΔS =nCp × ln	T2
  	T1

The absolute entropy (S°) of a substance at temperature T is the sum of the entropy changes occurring on warming the substance from absolute zero to temperature T.