Free Energy Change
Definition and meaning of Free Energy Change in chemistry.
Free energy change (ΔG) is the change in Gibbs free energy accompanying a chemical reaction or physical process at constant temperature and pressure, and its sign determines whether that process is thermodynamically spontaneous.
In more detail
ΔG combines the enthalpy change (ΔH, heat content) and entropy change (ΔS, disorder) of a system through the relation ΔG = ΔH − TΔS, where T is the absolute temperature. A negative ΔG means the process is spontaneous (exergonic) as written, a positive ΔG means it is non-spontaneous and requires an input of energy (endergonic), and ΔG = 0 means the system is at equilibrium. Because ΔG accounts for both energy release and entropy change, it predicts spontaneity more reliably than enthalpy alone, and its standard value relates directly to the equilibrium constant K through ΔG° = −RT ln K.
Key facts
| Symbol | ΔG |
|---|---|
| Defining Equation | ΔG = ΔH − TΔS |
| Typical Units | kJ/mol or J/mol |
| Field | Physical Chemistry |
For the combustion of methane, CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), ΔG° is about −818 kJ/mol, confirming the reaction is highly spontaneous and releases usable energy under standard conditions.
Frequently asked questions
What does a negative ΔG mean?
A negative ΔG means the process is thermodynamically spontaneous under the given conditions, releasing free energy that can in principle be harnessed to do work.
How does ΔG relate to the equilibrium constant?
The standard free energy change is related to the equilibrium constant by ΔG° = −RT ln K; a large positive K (favoring products) corresponds to a large negative ΔG°.