Question

If an endothermic reaction is not spontaneous at the freezing point of water but becomes visible at its boiling point, then:

a. ΔH is negative
b. ΔH is positive
c. ΔS is positive
d. ΔS is negative

Answer 1

An endothermic reaction becomes visible at the boiling point of water, signifying that the change in entropy (δS) is positive.

Step-by-step explanation:

When the reaction is exothermic (negative ΔH) but undergoes a decrease in entropy (negative ΔS), it is the enthalpy term that favors the reaction. In this case, a spontaneous reaction is dependent upon the ΔS 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. Above 273 K, the larger ΔS value causes the sign of ΔG to be positive, and freezing does not occur.