Not all spontaneous reactions occur instantly because of the availability or absence of free energy that the reacting species need to carry out the reaction. This energy is known as the Gibbs free energy.
Every chemical reaction involves a change in the Gibbs free energy, denoted as delta G. This energy change is calculated by subtracting the total energy of the reaction products from that of the reactants. The second law of thermodynamics states that the total energy state of a system tends towards an increase in entropy, which corresponds to a decrease in energy of the system constituents present after the reaction has been carried out.
When the change in Gibbs free energy of a reaction is negative, this indicates that the energy of the products is lower than the energy of the reactants, meaning that the reaction is spontaneous, because it obeys this second law of thermodynamics. However, it is the magnitude of this change in Gibbs free energy, and not just its sign, that indicates how fast the reaction is likely to proceed. Reactions with larger negative differences in Gibbs free energy happen faster than those with a smaller magnitude of free energy change. The energy difference in spontaneous reaction is dissipated in different forms, such as the heat from a fire, or the electrons from a battery.