Determine reaction spontaneity using the fundamental thermodynamic equation ΔG = ΔH − TΔS. Leave one field blank to calculate it instantly.
ΔG = ΔH − TΔS
Given ΔH = −92.2 kJ/mol and ΔS = −198 J·mol−1·K−1 (e.g., water formation).
Given ΔH = +40.0 kJ/mol, ΔS = +125 J·mol−1·K−1. Find T where ΔG = 0.
Given ΔG = −12.0 kJ/mol, ΔH = −25.0 kJ/mol at T = 350 K.
Gibbs free energy is a thermodynamic quantity that measures the maximum useful work that can be performed by a system at constant temperature and pressure. It's crucial for predicting whether a reaction will occur spontaneously. A negative ΔG indicates a spontaneous reaction, while a positive ΔG indicates a non-spontaneous reaction.
The equation ΔG = ΔH − TΔS combines two factors: enthalpy (ΔH), which represents heat energy change, and entropy (TΔS), which represents disorder change multiplied by temperature. Calculate ΔH and ΔS from standard tables or experiments, multiply entropy by absolute temperature in Kelvin, then subtract from ΔH to find ΔG. This calculator performs all conversions automatically.
A negative ΔG means the reaction is thermodynamically spontaneous under the given conditions. The system will release free energy and proceed in the forward direction as written. However, negative ΔG doesn't indicate reaction speed—use kinetics and reaction rates to predict how fast the reaction occurs.
When ΔG = 0, the system is at equilibrium—there is no net tendency for the reaction to proceed forward or backward. At this point, the forward and reverse reactions occur at equal rates. This condition is temperature-dependent; changing temperature often shifts a system from equilibrium to spontaneous or non-spontaneous.
Temperature directly affects ΔG through the TΔS term. For reactions with positive entropy change (ΔS > 0), higher temperatures favor spontaneity (make ΔG more negative). Conversely, for reactions with negative entropy change (ΔS < 0), higher temperatures disfavor spontaneity (make ΔG more positive). This explains why some reactions are spontaneous at high temperatures but non-spontaneous at low temperatures.
Common units for ΔG are kJ/mol (kilojoules per mole), J/mol (joules per mole), and kcal/mol (kilocalories per mole). Temperature must be in Kelvin (K). Entropy units are J/mol·K. This calculator automatically converts between unit systems, so you can enter values in any supported unit and receive results in your preferred unit.
