Gibbs free energy facts for kids
Gibbs free energy is the available energy in a chemical reaction. 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 686kcal. 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:
 U is the internal energy
 p is pressure
 V is Volume
 T is the temperature
 S is the entropy
 H is the enthalpy
Images for kids

The reaction C(s)diamond → C(s)graphite has a negative change in Gibbs free energy and is therefore thermodynamically favorable at 25 °C and 1 atm. However, the reaction is too slow to be observed, because of its very high activation energy. Whether a reaction is thermodynamically favorable does not determine its rate.

American scientist Willard Gibbs' 1873 figures two and three (above left and middle) used by Scottish physicist James Clerk Maxwell in 1874 to create a threedimensional entropy, volume, energy thermodynamic surface diagram for a fictitious waterlike substance, transposed the two figures of Gibbs (above right) onto the volumeentropy coordinates (transposed to bottom of cube) and energyentropy coordinates (flipped upside down and transposed to back of cube), respectively, of a threedimensional Cartesian coordinates; the region AB being the firstever threedimensional representation of Gibbs free energy, or what Gibbs called "available energy"; the region AC being its capacity for entropy, what Gibbs defined as "the amount by which the entropy of the body can be increased without changing the energy of the body or increasing its volume.