Explanation:
1. Spontaneous as written at all temperatures
C. When ΔH is negative and ΔS is positive, the sign of ΔG will always be negative, and the reaction will be spontaneous at all temperatures.
2. Spontaneous in reverse at all temperatures
A. When ΔH is positive and ΔS is negative, the sign of ΔG will always be positive, and the reaction can never be spontaneous.
3. Spontaneous as written above a certain temperature
B. ΔH is positive and ΔS is positive - an endothermic reaction (positive ΔH) that also displays an increase in entropy (positive ΔS). It is the entropy term that favors the reaction. Therefore, as the temperature increases, the TΔS term in the Gibbs free energy equation will begin to predominate and ΔG will become negative.
4. Spontaneous as written below a certain temperature
D. ΔH negative and ΔS is negative - When the reaction is exothermic (negative ΔH) but undergoes a decrease in entropy (negative ΔS), it is the enthalpy term which favors the reaction. In this case, a spontaneous reaction is dependent upon the TΔS term being small relative to the ΔH term, so that ΔG is negative. The freezing of water is an example of this type of process. It is spontaneous only at a relatively low temperature.
