Gibbs Free Energy and Spontaneity

Gibbs Free Energy (ΔG) is a thermodynamic property that determines whether a chemical reaction can occur spontaneously or not. The sign of ΔG determines the direction of the reaction, and its magnitude determines the extent of the reaction. ΔG can be determined using the following equation:

ΔG = ΔH – TΔS

where ΔH is the enthalpy change, T is the temperature in Kelvin, and ΔS is the entropy change. ΔG is negative when the reaction is spontaneous and positive when it is non-spontaneous.

Conditions for Negative ΔG

a) Process is spontaneous: When a process is spontaneous, it means that the reactants have a higher Gibbs free energy than the products. In other words, the products are more stable than the reactants, and the reaction can occur without any external intervention. In such cases, ΔG is negative, indicating that the reaction is thermodynamically favorable.

b) Process may or may not be spontaneous: When a process may or may not be spontaneous, it means that the reaction is at equilibrium. At equilibrium, ΔG is zero, indicating that the forward and reverse reactions are occurring at the same rate. However, if the conditions are changed, such as by altering the temperature or pressure, the reaction can become spontaneous or non-spontaneous.

c) Reaction is at equilibrium: As mentioned above, at equilibrium, ΔG is zero, indicating that the reaction is neither spontaneous nor non-spontaneous.

d) Reaction is exothermic: An exothermic reaction releases heat and has a negative ΔH. However, the spontaneity of the reaction depends on the entropy change (ΔS). If ΔS is negative, the reaction may be non-spontaneous, even though ΔH is negative. Therefore, the sign of ΔG depends on the relative magnitudes of ΔH and TΔS.

Conclusion

In conclusion, the correct answer is option 'A' - the Gibbs free energy change shall be negative when the process is spontaneous. This is because a spontaneous process is one where ΔG is negative, indicating that the reaction is thermodynamically favorable. The other conditions do not necessarily result in a negative ΔG, as the spontaneity of the reaction depends on various factors, including enthalpy and entropy changes.