Which statement is correct for a reaction with ΔH > 0 and ΔS > 0 (for example, the thermal decomposition of a carbonate)? Assume ΔH and ΔS do not depend on temperature.

Answer: C. It becomes spontaneous above a certain temperature

With ΔG = ΔH − TΔS and both ΔH and ΔS positive, the ΔH term wins at low temperature and ΔG > 0, but as the temperature rises the −TΔS term grows, and ΔG < 0 for T > ΔH/ΔS. Decomposition reactions that release a gas and raise the entropy, and the melting of solids and evaporation of liquids, are of this type; the melting and boiling points are exactly the temperatures at which ΔG = 0. Proceeding more readily at low temperature and not at high temperature is the case ΔH < 0 and ΔS < 0, which includes the oxidation of most metals. Not proceeding at any temperature is the case ΔH > 0 and ΔS < 0.