Final answer:
A nonspontaneous reaction must have positive ΔH and negative ΔS, which results in a positive ΔG, indicating that the reaction cannot occur spontaneously under any temperature conditions.
Step-by-step explanation:
A reaction that is unambiguously nonspontaneous under a given set of conditions would have positive ΔH (enthalpy change) and negative ΔS (entropy change). Per the equation ΔG = ΔH - TΔS, if ΔH is positive and ΔS is negative, the term -TΔS will also be positive at all positive Kelvin temperatures, thus ΔG will always be positive, indicating nonspontaneity according to the Gibbs free energy equation.