The question asks about the implications of a negative free energy change ($\Delta G$) when elements react to form compounds.

Understanding Free Energy Change ($\Delta G$)

Free energy change, often represented as $\Delta G$ (Gibbs free energy change), is a thermodynamic potential that measures the maximum or minimum electrical work that may be extracted from a thermodynamic system at constant temperature and pressure. It is a key indicator of whether a reaction will occur spontaneously.

Analyzing the Statements

• Statement (A): spontaneous reaction A negative free energy change ($\Delta G < 0$) is the fundamental criterion for a process to be spontaneous under constant temperature and pressure conditions. This means the reaction can proceed without continuous external energy input. Therefore, statement (A) is correct.
• Statement (B): free energy of the products are higher than that of reactants The change in free energy ($\Delta G$) is calculated as $\Delta G = G_{products} - G_{reactants}$. If $\Delta G$ is negative, it implies that $G_{products} < G_{reactants}$. This means the free energy of the products is lower than that of the reactants. Therefore, statement (B) is incorrect.
• Statement (C): very high activation barrier Free energy change ($\Delta G$) determines the overall spontaneity (thermodynamics) of a reaction, while the activation energy ($E_a$) determines the reaction rate (kinetics). A reaction with a negative $\Delta G$ can still have a high activation barrier, making it slow, or a low activation barrier, making it fast. A negative $\Delta G$ does not necessarily imply a high activation barrier. Therefore, statement (C) is incorrect.
• Statement (D): exergonic reactions Reactions that release free energy are called exergonic reactions. This corresponds exactly to a negative free energy change ($\Delta G < 0$). Therefore, statement (D) is correct.

Conclusion on Correct Statements

Based on the analysis, statements (A) and (D) are correct, while statements (B) and (C) are incorrect.

Identifying the Correct Option

The option that includes only the correct statements (A) and (D) is option 4.