Calculate the Gibbs Free Energy (ΔG) of a chemical reaction and determine its spontaneity. Enter enthalpy change (ΔH), entropy change (ΔS), and temperature (T) to get ΔG in kJ/mol along with a spontaneity verdict — or switch to advanced mode to compute ΔG° from an equilibrium constant (K) or find ΔG from the reaction quotient (Q).
Gibbs Free Energy (ΔG)
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Spontaneity
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Temperature Used (K)
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ΔH (kJ/mol)
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T·ΔS (kJ/mol)
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Gibbs free energy is calculated using the equation ΔG = ΔH − T·ΔS, where ΔH is the enthalpy change (in kJ/mol), T is the absolute temperature in Kelvin, and ΔS is the entropy change (in kJ/mol·K). Make sure units are consistent before applying the formula — convert J to kJ as needed.
A negative ΔG (ΔG < 0) means the reaction is spontaneous — it can proceed on its own without requiring external energy input. The more negative the value, the greater the thermodynamic driving force for the reaction.
A positive ΔG (ΔG > 0) means the reaction is non-spontaneous under the given conditions. External energy must be supplied to drive the reaction forward. The reverse reaction would be spontaneous instead.
At equilibrium, there is no net change in the concentrations of reactants or products, meaning no useful work can be extracted from the system. At this point ΔG = 0, and the forward and reverse reaction rates are equal.
Gibbs free energy predicts the spontaneity and direction of a chemical reaction at constant temperature and pressure. It also quantifies the maximum useful work that can be obtained from the process, and through ΔG° = −RT ln K, it is directly related to the equilibrium constant.
Temperature affects spontaneity through the T·ΔS term. If ΔH and ΔS are both positive (endothermic, entropy-increasing), the reaction becomes spontaneous only above a certain temperature. If both are negative, it is spontaneous only below that temperature. When ΔH and ΔS have opposite signs, temperature determines which factor dominates.
ΔG° is the standard Gibbs free energy change measured under standard conditions (1 bar pressure, 1 M concentration, 298.15 K). ΔG is the actual free energy change under non-standard conditions, calculated as ΔG = ΔG° + RT ln Q, where Q is the reaction quotient at the actual conditions.
Gibbs free energy is typically expressed in kJ/mol (kilojoules per mole) or J/mol (joules per mole). When applying ΔG = ΔH − T·ΔS, ensure ΔH and T·ΔS are in the same units. Since ΔS is usually given in J/(mol·K), divide by 1000 to convert T·ΔS to kJ/mol before subtracting from ΔH in kJ/mol.