joules

Entropy increases when a substance is broken up into multiple parts. The process of dissolving increases entropy because the solute particles become separated from one another when a solution is formed. Entropy increases as temperature increases.

Entropy is considered to be an extensive property of matter that is expressed in terms of energy divided by temperature. The SI units of entropy are J/K (joules/degrees Kelvin).

∆S is the change in entropy (disorder) from reactants to products. R is the gas constant (always positive) T is the absolute temperature (Kelvin, always positive) What it means: If ∆H is negative, this means that the reaction gives off heat from reactants to products.

Standard molar entropy is defined as the entropy or degree of randomness of one mole of a sample under standard state conditions. A positive value indicates an increase in entropy, while a negative value denotes a decrease in the entropy of a system.

When ΔS > 0 and ΔH < 0, the process is always spontaneous as written. When ΔS < 0 and ΔH > 0, the process is never spontaneous, but the reverse process is always spontaneous. When ΔS > 0 and ΔH > 0, the process will be spontaneous at high temperatures and non-spontaneous at low temperatures.

An endothermic reaction can be spontaneous if entropy increases by more than the change in enthalpy.

If there’s an endothermic reaction that is spontaneous, then the reverse reaction is an exothermic reaction that’s non-spontaneous. Melting ice is spontaneous at room temperature, but absorbs heat so it’s endothermic. Therefore freezing of water at room temperature is an example, if you consider that a “reaction”.

These results lead to a profound statement regarding the relation between entropy and spontaneity known as the second law of thermodynamics: all spontaneous changes cause an increase in the entropy of the universe. All spontaneous changes cause an increase in the entropy of the universe, i.e., ΔSuniv>0.