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Gibbs Free Energy Calculator

Solve delta G = delta H minus T delta S for any variable, get a spontaneity verdict, the crossover temperature and the equilibrium constant both ways.

-32.9576 kJ/mol
ΔG
Spontaneous (exergonic)
Verdict at this temperature
464 K
Crossover temperature

ΔG = ΔH − TΔS

  • ΔG = -92.2 − (298.15 × -198.7 ÷ 1000) = -32.9576 kJ/mol
  • TΔS = -59.2424 kJ/mol — the entropy term is worth 59.2424 kJ/mol against the products.
  • Temperature used: 298.15 K = 25 °C. ΔS is divided by 1000 so both energy terms are in kJ/mol.
  • Spontaneous below this temperature, non-spontaneous above it.
  • Sign flips at T = ΔH ÷ ΔS = 464.0161 K (190.8661 °C).

Equilibrium constant bridge — ΔG° = −RT ln K

From ΔG° = -32.9576 kJ/mol at 298.15 K: K = 5.9418e+5 — products are favoured.

K = 1 gives ΔG° = −RT ln K = -0 kJ/mol. R = 8.314463 J/(mol·K).

ΔG here is the standard free energy change ΔG°, which assumes every species is at its standard state — 1 bar for gases, 1 mol/L for solutes, the pure substance for solids and liquids. It also assumes ΔH and ΔS themselves do not change with temperature, which is a good approximation over a few tens of kelvin but drifts over hundreds. A negative ΔG says a reaction is thermodynamically allowed, not that it will happen quickly: kinetics and activation energy decide the rate. Preset values are standard 298 K data from commonly cited thermodynamic tables.

What is the Gibbs Free Energy Calculator?

Gibbs free energy change is delta G = delta H - T x delta S. A negative value means the reaction is spontaneous at that temperature. The Haber process, with delta H of -92.2 kJ/mol and delta S of -198.7 J/mol/K, gives about -33 kJ/mol at 298 K.

  • Solves for delta G, delta H, delta S or temperature
  • Handles the kJ and J unit mismatch and shows it in the working
  • Spontaneity verdict plus the crossover temperature where the sign flips
  • Two-way bridge to the equilibrium constant through delta G = -RT ln K
  • Seven preloaded reactions including the Haber process and combustion of methane
  • Kelvin or celsius input, converted before any arithmetic

How to use the Gibbs Free Energy Calculator

  1. 1

    Choose which quantity to solve for: delta G, delta H, delta S or temperature.

  2. 2

    Pick a reaction preset, or leave it on Custom values and type your own numbers.

  3. 3

    Enter enthalpy in kJ/mol, entropy in J/mol/K and the temperature in kelvin or celsius.

  4. 4

    Read the answer, the spontaneity verdict and the crossover temperature.

  5. 5

    Use the equilibrium constant panel to convert between delta G and K in either direction.

About the Gibbs Free Energy Calculator

The ByteTools Gibbs Free Energy Calculator solves delta G = delta H minus T delta S for whichever of the four quantities you do not have. Enthalpy goes in as kJ/mol, entropy as J/mol/K, and the temperature in kelvin or celsius; the division by 1000 that trips up so many students is handled for you and shown in the working.

Beyond the number itself, the page gives a spontaneous, non-spontaneous or equilibrium verdict, the crossover temperature where the sign of delta G flips, and the bridge to the equilibrium constant through delta G equals minus RT ln K — usable in both directions, so you can go from a known K back to a free energy. Seven common reactions are preloaded, from the Haber process to melting ice.

All the maths runs in JavaScript in your browser and no values are sent anywhere. The page states plainly that these are standard-state quantities and that a negative delta G says a reaction is allowed, not that it will be fast.

Frequently asked questions

What does a negative Gibbs free energy mean?

It means the reaction is thermodynamically favourable, or spontaneous, in the direction written at that temperature. It says nothing about speed — a reaction with a very negative delta G can still sit unchanged for years if its activation energy is high.

How do you find the temperature where a reaction becomes spontaneous?

Set delta G to zero and rearrange to T = delta H divided by delta S. Above or below that crossover temperature the sign of delta G flips, depending on whether the two terms are positive or negative. This tool prints it and says which direction the change goes.

Why do I have to divide entropy by 1000?

Enthalpy is normally tabulated in kilojoules per mole while entropy is in joules per mole per kelvin. The two terms must be in the same energy unit before you subtract them, so entropy is divided by 1000 to bring it into kJ. Forgetting this makes the answer wrong by a factor of a thousand.

How is Gibbs free energy related to the equilibrium constant?

Through delta G standard equals minus RT ln K. A negative standard free energy gives a K above 1, meaning products are favoured at equilibrium; a positive one gives a K below 1. This page converts in both directions using the gas constant 8.314 J/mol/K.

Do delta H and delta S change with temperature?

Slightly, yes. Treating them as constant is a good approximation across a few tens of kelvin but drifts over hundreds, so a crossover temperature far from where the data was measured should be treated as an estimate rather than an exact figure.

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