What is defined as the standard free energy of formation (ΔG°)?

The standard free energy of formation, denoted as ΔG°, specifically refers to the change in free energy that occurs when one mole of a compound is formed from its elements in their standard states. This definition highlights that ΔG° measures the spontaneity of the formation process under standard conditions (typically 1 bar of pressure and a specified temperature, often 25°C).

In this context, this value is critical for predicting whether a reaction will proceed spontaneously; a negative ΔG° indicates a spontaneous formation process, while a positive ΔG° suggests non-spontaneity. This concept is fundamental in thermodynamics as it relates to the stability of the compound relative to its elemental components.

While the other options touch upon important thermodynamic concepts, they do not pertain to the definition of standard free energy of formation. For instance, mixing substances relates to enthalpic or entropic changes during solution formation, combustion focuses on energy changes during reactions with oxygen, and ionic bond formation pertains to energy differences between gaseous ions and their solid state. Thus, none of these relate directly to the definition or application of standard free energy of formation.