Final answer:
A spontaneous reaction occurs when the Gibbs free energy (ΔG) is negative, indicating that the reaction releases free energy and is exergonic. Option C is the correct answer as it reflects the condition for spontaneity in thermochemical terms.
Step-by-step explanation:
A spontaneous reaction is one that proceeds on its own without the need for additional energy input from the surroundings. The spontaneity of a reaction is determined by the Gibbs free energy (ΔG) of the system. For a reaction to be spontaneous, the ΔG must be negative. This is an indication that the products of a chemical reaction have lower free energy than the reactants and is a sign that the reaction releases free energy, which is termed exergonic.
It is important to note that spontaneous reactions can occur at different rates. Some may happen almost instantly, while others, like the rusting of iron, may take a significant amount of time. Therefore, spontaneous does not necessarily mean immediate. In the context of thermochemistry, the correct option for a spontaneous reaction is C) when the Gibbs energy of a reaction is negative.
When the enthalpy (ΔH) of a reaction is negative and the entropy (ΔS) is positive, the ΔG will always be negative. However, if ΔH is positive and ΔS is negative, then the ΔG will be positive, and such a reaction cannot be spontaneous without the addition of energy. As a result, the assertion that a spontaneous reaction occurs with a positive Gibbs energy is incorrect, and the correct answer is C) when the Gibbs energy of a reaction is negative.