Which combination represents a thermally favorable (exothermic) but entropically unfavorable process?

Which combination represents a thermally favorable (exothermic) but entropically unfavorable process?

• A. Thermally favorable (exothermic) and entropically unfavorable
• B. Thermally favorable and entropically favorable
• C. Thermally unfavorable and entropically favorable
• D. Thermally unfavorable and entropically unfavorable

Answer: (A)

A process that releases heat has a negative enthalpy change, and if it also becomes more ordered, its entropy change is negative. The spontaneity of a process is governed by G = H − TΔS. With H < 0 and ΔS < 0, the first term favors the process (negative H), while the second term becomes positive because −TΔS is positive, opposing the process as temperature rises. This means the reaction can be favorable at low temperatures (where the negative enthalpy dominates) but may lose favor at higher temperatures due to the entropy term growing in importance. So the combination described—exothermic (negative ΔH) and entropically unfavorable (negative ΔS)—is exactly the situation characterized by negative ΔH and negative ΔS.

A process that releases heat has a negative enthalpy change, and if it also becomes more ordered, its entropy change is negative. The spontaneity of a process is governed by G = H − TΔS. With H < 0 and ΔS < 0, the first term favors the process (negative H), while the second term becomes positive because −TΔS is positive, opposing the process as temperature rises. This means the reaction can be favorable at low temperatures (where the negative enthalpy dominates) but may lose favor at higher temperatures due to the entropy term growing in importance. So the combination described—exothermic (negative ΔH) and entropically unfavorable (negative ΔS)—is exactly the situation characterized by negative ΔH and negative ΔS.