The mathematical definition of this property is enthalpy minus the product of temperature and entropy. This relationship is written as G = H – TS. For changes at constant temperature and pressure, the equation becomes ΔG = ΔH – TΔS.
A negative ΔG value means a reaction is spontaneous, or thermodynamically favored. A positive value means the reaction is nonspontaneous and needs energy to proceed. When ΔG equals zero, the reaction has reached equilibrium.
A common student misconception is that spontaneous reactions must happen quickly. In chemistry, spontaneous only means a reaction is energetically favored to occur. It does not describe the speed of the reaction.
For example, the conversion of diamond to graphite is spontaneous, but it takes millions of years. Another error is confusing standard free energy change with non-standard free energy change. The standard value applies only to set conditions, while the non-standard value changes as the reaction goes on.
This thermodynamic tool is vital for engineers designing chemical reactors and batteries. The free energy value also links directly to the equilibrium constant of a reaction. This connection is expressed by the equation ΔG° = -RT ln K. Here, R is the gas constant, T is temperature, and K is the equilibrium constant.
A large negative standard free energy change means the reaction will produce mostly products. Knowing this relationship helps chemists predict how far a reaction will proceed.