Advanced Placement (AP) Chemistry Practice Exam

Which is a characteristic of nonspontaneous processes?

1. ΔG is negative

2. ΔG is zero

3. ΔG is positive

4. ΔG is always constant

The correct answer is: ΔG is positive

A characteristic of nonspontaneous processes is that the change in Gibbs free energy (ΔG) is positive. This means that for a process to be considered nonspontaneous, it requires an input of energy to occur because the products have a higher free energy than the reactants. In thermodynamic terms, a positive ΔG indicates that the system is not stable in its current state, and therefore the reaction does not proceed in the forward direction under standard conditions without the continuous addition of energy. In contrast, spontaneous processes are characterized by a negative ΔG, indicating that they occur naturally without an external energy source. A ΔG of zero corresponds to a system at equilibrium, where there is no net change in the concentrations of reactants and products, meaning that both the forward and reverse reactions occur at the same rate. The notion that ΔG is always constant does not specifically pertain to nonspontaneity; rather, ΔG can change depending on the conditions of a system. Therefore, the correct interpretation of characteristics leading to nonspontaneous processes hinges on the positive value of ΔG.