Final answer:
If a system has less potential energy after a reaction than before, the reaction is thermodynamically favorable because it is exothermic and releases energy.
Step-by-step explanation:
If the system after the reaction has less potential energy than the system before the reaction, this indicates that the reaction has released energy to its surroundings, which is characteristic of an exothermic reaction. In thermodynamic terms, when the enthalpy change (ΔH) of a reaction is negative, it means that the system has lost energy, and hence, the products of the reaction have lower energy compared to the reactants. Such a scenario is depicted as energetically downhill on an energy scale.
Therefore, the statement can be completed as "If the system after the reaction has less potential energy than the system before the reaction, the reaction is thermodynamically favorable." This is because exothermic reactions, where energy is released, tend to be spontaneous and thermodynamically favorable, leading to products that are more stable.