The Gibbs free energy graph shows whether or not a reaction is spontaneous-- whether it is exergonic or endergonic. ΔG is the change in free energy. Generally, all reactions want to go to a lower energy state, thus a negative change is favored. Negative ΔG indicates that the reaction is exergonic and spontaneous.
Note that ΔG reveals nothing about the speed of the reaction. For example, diamond is supposed to be in a liquid state at room temperature, yet the rate of this reaction is so slow that this change does not seem to occur. Neither do peptide bonds spontaneously go through hydrolysis even though the reaction of such is highly exergonic. A positive ΔG indicates that the reaction is endergonic, or that it requires energy to go from reactants to products. The free energy graph can be used to determine whether the reaction will be spontaneous or not by evaluating the ΔG. ΔG can be found by subtracting the free energy of the reactants from the free energy of the products.
ΔG = Gproducts - Greactants
Therefore, if the reaction goes from higher free energy to lower free energy, there will be a negative ΔG, and the reaction will be spontaneous. However, if the reactants have a lower ΔG than the products, there will be an increase in free energy, and the reaction is nonspontaneous. In this situation, some form of energy (in the form of heat, light, etc.) will be required for the reaction to take place. It should be noted that a spontaneous reaction will not necessarily occur on its own. This is because an initial activation energy is needed in order to start the reaction and thus even a spontaneous reaction may need some form of energy input. A good example of this is the very exergonic combustion of octane, which still needs a flame in order to initiate.
Enzymes will affect this free energy graph by lowering the activation barrier, or the amount of energy needed for the reaction to occur, They do this by stabilizing the transition state, or the state of highest energy between reactants and products. Enzymes DO NOT affect the equilibrium constant in any way, shape, or form. File:Free energy diagram.jpg