Thermodynamics and Enthalpy Calculator
Calculate Gibbs free energy and enthalpy changes for chemical reactions. Use this when predicting reaction spontaneity or thermodynamic feasibility at a given temperature and pressure.
Last updated: May 2026
About this calculator
Thermodynamics governs whether a chemical reaction will occur spontaneously. The Gibbs free energy (ΔG) combines enthalpy (ΔH) and entropy (ΔS) to give a single criterion for spontaneity: when ΔG < 0, a reaction proceeds spontaneously. The core relationship is ΔG = ΔH − T·ΔS, where T is temperature in Kelvin. This calculator computes the result as: ΔG = (ΔH × 1000 − T × ΔS) / 1000. The ΔH input is in kJ/mol (converted to J/mol internally by multiplying by 1000), while ΔS is in J/(mol·K), and the result is converted back to kJ/mol. A positive ΔG means the reaction is non-spontaneous under those conditions; a negative ΔG means it is spontaneous. Temperature plays a critical role — for endothermic reactions (ΔH > 0), high temperatures can drive spontaneity by making T·ΔS large enough.
How to use
Suppose a reaction has ΔH = 50 kJ/mol, ΔS = 150 J/(mol·K), and T = 400 K. Step 1: Convert ΔH to J/mol: 50 × 1000 = 50,000 J/mol. Step 2: Calculate T·ΔS: 400 × 150 = 60,000 J/mol. Step 3: Subtract and convert back to kJ/mol: (50,000 − 60,000) / 1000 = −10 kJ/mol. ΔG = −10 kJ/mol. Because ΔG < 0, the reaction is spontaneous at this temperature.
Frequently asked questions
What does a negative Gibbs free energy value mean for a chemical reaction?
A negative ΔG indicates that a reaction is thermodynamically spontaneous under the given conditions — it can proceed without external energy input. This does not mean the reaction is instantaneous; kinetics still control the rate. For example, diamond converting to graphite has a negative ΔG at room temperature but occurs imperceptibly slowly. Spontaneity is purely a thermodynamic, not a kinetic, concept.
How does temperature affect the spontaneity of endothermic reactions?
For endothermic reactions, ΔH is positive, which opposes spontaneity. However, if ΔS is also positive (increased disorder), the term T·ΔS grows with temperature. At a high enough temperature, T·ΔS exceeds ΔH, making ΔG negative and the reaction spontaneous. This is why some reactions like melting ice or dissolving certain salts only become spontaneous above a threshold temperature. The crossover point is found by setting ΔG = 0, giving T = ΔH / ΔS.
Why is entropy change given in J/(mol·K) while enthalpy is in kJ/mol?
The two quantities use different magnitude scales by convention in thermodynamic tables. Entropy changes are typically small numbers (tens to hundreds of J/(mol·K)), while enthalpy changes are reported in the more convenient kJ/mol unit. When computing ΔG = ΔH − T·ΔS, it is essential to convert both to the same unit (usually J/mol) before subtracting. Forgetting this unit conversion is one of the most common errors in thermodynamics problems, leading to results that are off by a factor of 1000.