Gibbs free energy is the available energy in a chemical reaction.[1] It was calculated by Josiah Willard Gibbs.
Purpose
Systems have a tendency to achieve as low of a free energy state as possible. The Gibbs free energy number is how far away the current system is from this state. It is analogous to a number line, with zero being the final state. Scientists use Gibbs free energy to determine the energy in a system that can be used. A negative Gibbs number indicates that you can get energy out of the reaction. It is a favorable reaction, which means that the chemical reaction "wants" to move towards its lower state. A positive Gibbs number shows that you must put energy in to get energy out.
An example can be seen in respiration. The Gibbs value of one mole of glucose is -686kcal.[2] This means that we can get 686 kcal of energy from one mole of glucose. This means if we put in energy in respiration, we can theoretically get 686 kcal out of the mole of glucose.
Calculation
The equation for Gibbs free energy is G(p,T) = U + pV − TS.
This is often simplified as G(p,T) = H − TS.
(For the second equation, U + pV is replaced by H because H = U + pV).
where:
Notes