Calculate ΔG from enthalpy, entropy, and temperature, then review the TΔS contribution, spontaneity, and temperature regime. Use it to test different inputs quickly, compare outcomes, and understand the main factors behind the result before moving on to related tools or deeper guidance.
Last updated
Estimate ΔG and check the temperature regime for spontaneity Enter enthalpy change, temperature, and entropy change to calculate Gibbs free energy, the TΔS contribution, and whether the process is favored at the current conditions.
Enter ΔH, temperature, and ΔS Add all three thermodynamic inputs to calculate Gibbs free energy and the implied spontaneity at the selected temperature.
Gibbs free energy calculator guide: ΔG, spontaneity, and temperature thresholds
A Gibbs free energy calculator is most useful when it does more than output one number. This page combines ΔH, temperature, and ΔS to calculate ΔG, show the TΔS contribution, and explain whether the process is favored at the current temperature or only within a particular temperature range.
What Gibbs free energy is showing
Gibbs free energy combines enthalpy and entropy into one temperature-dependent criterion for thermodynamic favorability at constant pressure and temperature. A negative ΔG means the process is thermodynamically favored under the entered conditions, a positive ΔG means it is not favored, and ΔG = 0 marks equilibrium.
That is why a useful worksheet should show more than the final ΔG value. Seeing the entropy contribution and the implied temperature regime makes it easier to understand why the sign changes and how the same reaction can behave differently at different temperatures.
The formula used here
This page uses the standard relation ΔG = ΔH − TΔS. The calculator expects ΔH in kJ/mol, temperature in kelvin, and ΔS in J/(mol·K), so the entropy term is converted into kJ/mol before subtraction.
That unit conversion matters. If ΔS is entered in joules per mole-kelvin, multiplying by temperature gives joules per mole. Dividing by 1,000 puts the entropy term onto the same kJ/mol basis as ΔH so the subtraction is dimensionally consistent.
ΔG = ΔH − TΔS
Standard Gibbs free energy relation for constant-pressure, constant-temperature analysis.
TΔS (kJ/mol) = T × ΔS / 1000
Converts the entropy contribution from joules into kilojoules so it matches the enthalpy units used in the worksheet.
Worked example
Suppose ΔH = -200 kJ/mol, T = 298 K, and ΔS = -100 J/(mol·K). First convert the entropy term: TΔS = 298 × (-100) / 1000 = -29.8 kJ/mol.
Then apply the Gibbs relation: ΔG = -200 - (-29.8) = -170.2 kJ/mol. The result is negative, so the process is thermodynamically favored at 298 K. Because both ΔH and ΔS are negative, this is a lower-temperature-favored case with a positive equilibrium threshold temperature.
How temperature changes the answer
The sign combination of ΔH and ΔS tells you whether spontaneity depends on temperature. If ΔH is negative and ΔS is positive, the process is favored at all temperatures. If ΔH is positive and ΔS is negative, it is unfavored at all temperatures.
When ΔH and ΔS have the same sign, temperature matters. Negative ΔH with negative ΔS favors lower temperatures, while positive ΔH with positive ΔS favors higher temperatures. In those cases the worksheet can also show the equilibrium threshold where ΔG would equal zero.
Further reading
OpenStax - Free Energy — Chemistry reference explaining the Gibbs free energy relationship and the effect of temperature on spontaneity.
No. A negative ΔG describes thermodynamic favorability, not reaction speed. A process can be thermodynamically favored and still proceed slowly if the activation barrier is high.
Why is ΔS entered in J/(mol·K) instead of kJ/(mol·K)?
Entropy data are commonly tabulated in joules per mole-kelvin. This page converts the resulting TΔS term into kJ/mol so it can be subtracted from ΔH on the same unit basis.
When does the equilibrium temperature matter?
It matters when ΔH and ΔS have the same sign and spontaneity changes with temperature. In those cases the threshold shows where ΔG would cross zero and the process would switch between favored and unfavored behavior.
Can I use this page for electrochemistry or biochemical free energy directly?
Only when the problem is appropriately expressed as ΔH, T, and ΔS for the process of interest. More specialized electrochemical or biochemical cases often need additional assumptions, concentration terms, or standard-state corrections.
