A spontaneous reaction is characterized by which of the following?

• A. An increase in activation energy
• B. A decrease in Gibbs free energy
• C. An increase in temperature
• D. An external input of energy

Answer: (B)

A spontaneous reaction is characterized by a decrease in Gibbs free energy, which indicates that the reaction can occur without needing an external energy input. In thermodynamics, Gibbs free energy (( \Delta G )) is a criterion for spontaneity: if ( \Delta G ) is negative, the process is spontaneous under constant temperature and pressure. This means that the products of the reaction have lower free energy than the reactants, favoring the reaction's progress.

In contrast, an increase in activation energy would make the reaction less likely to occur spontaneously, as higher energy barriers must be overcome for the reaction to proceed. An increase in temperature does not inherently indicate spontaneity; while certain reactions may become spontaneous at higher temperatures, this is not a defining characteristic of spontaneous reactions overall. Lastly, an external input of energy is contradictory to the definition of spontaneity; spontaneous reactions occur without such input. Thus, the correct identification of a decrease in Gibbs free energy aligns directly with the fundamental principles of thermodynamic spontaneity.