Question

Which of the following must be true about a reaction if it is only spontaneous at high temperatures?

It is exothermic, with both negative enthalpy and entropy changes.

It is endothermic, with both positive enthalpy and entropy changes.

It is exothermic, with a negative enthalpy and positive entropy change.

It is endothermic, with a positive enthalpy and negative entropy change.

Answer 1

please help :(

Answer 2

A reaction is spontaneous if the tendency towards minimum enthalpy (heat content in a system) and maximum entropy (degree of randomness) both favour the forward direction.

For minimum enthalpy to be favoured in the forward direction the reaction would have to be exothermic (an endothermic reaction would favour the lower temperature on the reactant side and the reaction would not go forward completely). Because it's exothermic, energy is released, so the enthalpy value would decrease (and become negative). Entropy would have to increase for the reaction to be spontaneous so it should be a positive value. The correct answer should be the third choice you listed.

Try sketching out Potential Energy vs Progress of Reaction diagrams for questions like these, they really help with visualizing the entropy and enthalpy trends.