Free Energy

Bioenergetics describes the transfer and utilization of energy in biologic systems. It concerns the initial and final energy states of the reaction components, not the reaction mechanism or how much time it takes for the chemical change to occur. Bioenergetics makes use of a few basic ideas from the field of thermodynamics, particularly the concept of free energy. Because changes in free energy provide a measure of the energetic feasibility of a chemical reaction, they allow prediction of whether a reaction or process can take place. In short, bioenergetics predicts if a process is possible, whereas kinetics measures the reaction rate.
FREE ENERGY
The direction and extent to which a chemical reaction proceeds are determined by the degree to which two factors change during the reaction. These are enthalpy (ΔH, a measure of the change [Δ] in heat content of the reactants and products) and entropy (ΔS, a measure of the change in randomness or disorder of the reactants and products), as shown in Figure 1. Neither of these thermodynamic quantities by itself is sufficient to determine whether a chemical reaction will proceed spontaneously in the direction it is written. However, when combined mathematically (see Fig. 1), enthalpy and entropy can be used to define a third quantity, free energy (G), which predicts the direction in which a reaction will spontaneously proceed.

Figure 1:  Relationship between changes in free energy (G), enthalpy (H), and entropy (S). T is the absolute temperature in Kelvin (K), where K = °C + 273.