What it does
Whether a reaction gives out heat, whether it goes by itself and how far it goes all follow from three numbers per substance: the standard enthalpy of formation ΔfH°, the standard entropy S° and the standard Gibbs energy of formation ΔfG°. This tool adds them up for you using Hess’s law.
- ΔrH° = Σ ν ΔfH°(products) − Σ ν ΔfH°(reactants). Negative means exothermic (heat given out).
- ΔrS° = Σ ν S°(products) − Σ ν S°(reactants), in J/(mol·K). Positive usually means more gas on the product side.
- ΔrG° at 298.15 K from the ΔfG° values when every species has one, otherwise from ΔH° − TΔS°. Negative means the reaction goes forward as written under standard conditions.
- K = exp(−ΔG°/RT), the equilibrium constant, and whether products or reactants are favoured.
- ΔG and K at your temperature, with ΔH° and ΔS° kept constant.
- The turnover temperature T = ΔH°/ΔS°, where ΔG changes sign, and which side of it is spontaneous.
- A plot of ΔG° against temperature, with zero, the turnover point and your temperature marked.
How to use it
- Type the balanced reaction with state symbols, such as
CaCO3(s) → CaO(s) + CO2(g). Use→,->or=for the arrow. Coefficients can be whole numbers, decimals or fractions such as1/2O2(g). - The table below fills in values for any substance in the built-in table. Water appears twice, as liquid and vapour, so the state symbol matters. For anything not in the table, type ΔfH° (kJ/mol), S° (J/(mol·K)) and, if you have it, ΔfG° (kJ/mol). You can also overwrite any value.
- Set the temperature in kelvin if you want ΔG and K somewhere other than 25 °C.
- Read the headline and facts, check the plot, and download a PNG, PDF or Word report if you need one.
If the elements on the two sides do not match, a warning names them, so you can fix the coefficients (the equation balancer can do this for you).
Worked example: burning methane
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).
- ΔrH° = −393.51 + 2(−285.83) − (−74.6) − 0 = −890.6 kJ/mol, strongly exothermic: this is the heat of combustion of natural gas.
- ΔrS° = 213.79 + 2(69.95) − 186.3 − 2(205.15) = −242.9 J/(mol·K): three moles of gas become one, and the water is liquid.
- ΔrG° = −394.37 + 2(−237.14) − (−50.5) = −818.2 kJ/mol, so K is about 10¹⁴³: the reaction goes to completion.
- Since ΔH and ΔS are both negative, ΔG turns positive above ΔH/ΔS ≈ 3670 K, far above any real flame, and only in the constant-ΔH approximation.
For the decomposition of limestone, CaCO₃(s) → CaO(s) + CO₂(g), ΔH° = +179.2 kJ/mol and ΔS° = +160.2 J/(mol·K). It is not spontaneous at room temperature, but turns over at about 1119 K (845 °C), which is why lime kilns run at around 900 °C.
Questions
Why are the elements zero? ΔfH° and ΔfG° of an element in its standard state (O₂ gas, graphite, iron metal) are zero by definition. Their entropy is not zero, because S° is an absolute value.
Why does my answer differ slightly from my textbook? Tables differ in the last digit or two, and older ones use 1 atm instead of 1 bar. The built-in values come from the CRC Handbook and NIST at 298.15 K and 1 bar. Overwrite them with your textbook’s numbers if you need to match exactly.
How accurate is ΔG at other temperatures? The tool assumes ΔH° and ΔS° do not change with temperature. That is good to a few kJ/mol over a few hundred kelvin, but not across a phase change: if water boils on the way, use the vapour values.
Does spontaneous mean fast? No. A negative ΔG says the reaction can go, not how quickly. Methane and air sit together for years until a spark starts the reaction.
Is anything sent anywhere? No. Everything runs in your browser and works offline once the page has loaded.
Where the numbers come from
- Atomic weights: IUPAC/CIAAW standard atomic weights (abridged values).
- Standard formation values (ΔfH°, S°, ΔfG°) for 38 substances: NIST Chemistry WebBook and CRC Handbook values at 298.15 K, which you can type over.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-08.