What is Spontaneity ? Give criteria forspontaneity of a reaction in terms of(i) entropy and (ii) Gibbs energy

Spontaneity refers to the tendency of a reaction to occur without any external influence. In other words, it is the natural tendency of a reaction to proceed in a particular direction.

(i) Entropy: The criterion for spontaneity in terms of entropy is based on the concept of disorder or randomness in a system. According to the second law of thermodynamics, the entropy of the universe tends to increase in a spontaneous process. Therefore, for a reaction to be spontaneous, the overall change in entropy (∆S) of the system and its surroundings must be positive (∆S > 0). This means that the products of the reaction have a higher degree of disorder than the reactants.

(ii) Gibbs energy: The criterion for spontaneity in terms of Gibbs energy is based on the concept of free energy change (∆G). The Gibbs energy change (∆G) of a reaction is related to both the enthalpy change (∆H) and the entropy change (∆S) of the system through the equation ∆G = ∆H - T∆S, where T is the temperature in Kelvin. For a reaction to be spontaneous, the overall change in Gibbs energy (∆G) must be negative (∆G < 0) at a given temperature. This means that the reaction is energetically favorable and can proceed spontaneously.

In summary, for a reaction to be spontaneous, the criteria are: (i) In terms of entropy, the overall change in entropy (∆S) of the system and its surroundings must be positive (∆S > 0). (ii) In terms of Gibbs energy, the overall change in Gibbs energy (∆G) of the system must be negative (∆G < 0) at a given temperature.