Calculator inputs
Example data table
| Scenario | Given data | Equation | Expected insight |
|---|---|---|---|
| Thermal Gibbs evaluation | ΔH = -92.4 kJ/mol, ΔS = -198 J/mol·K, T = 25°C | ΔG = ΔH - TΔS | Tests spontaneity at room temperature. |
| Formation free energies | CO2 and H2O products, CH4 and O2 reactants | ΔG° = ΣνGf°(products) - ΣνGf°(reactants) | Builds reaction free energy from tabulated data. |
| Nonstandard conditions | ΔG° = -32.9 kJ/mol, T = 25°C, Q = 10 | ΔG = ΔG° + RT ln Q | Shows how composition shifts reaction drive. |
| Electrochemical cell | n = 2, E = 1.10 V | ΔG = -nFE | Converts cell voltage into free energy change. |
Formula used
1) Gibbs equation with enthalpy and entropy
ΔG = ΔH - TΔS
Use temperature in kelvin. Keep units consistent by converting entropy into kJ/mol·K when enthalpy is entered in kJ/mol.
2) Standard reaction free energy from formation values
ΔG°rxn = ΣνGf°(products) - ΣνGf°(reactants)
Multiply each standard formation free energy by its stoichiometric coefficient, sum products, sum reactants, then subtract.
3) Nonstandard free energy
ΔG = ΔG° + RT ln Q
Here, R is the gas constant, T is kelvin temperature, and Q is the reaction quotient. Q changes the reaction driving force away from standard conditions.
4) Electrochemical form
ΔG = -nFE
n is transferred electrons, F is Faraday’s constant, and E is cell potential. A positive cell potential gives a negative free energy change.
How to use this calculator
- Select the equation mode that matches your chemistry problem.
- Enter values with consistent units. The page converts temperature and entropy where needed.
- For formation data, fill product and reactant rows with coefficients and standard formation free energies.
- Press Calculate Free Energy to show results above the form, including the table and Plotly graph.
- Use the CSV button for spreadsheet-friendly exports and the PDF button for a quick report copy.
- Review the interpretation line to understand whether the reaction is spontaneous, non-spontaneous, or at equilibrium.
Frequently asked questions
1) Reactions in which there is a negative change in free energy (−ΔG) are:
They are thermodynamically spontaneous in the forward direction under the stated conditions. A negative ΔG means the process can proceed without added non-expansion work, although the reaction may still be slow if activation energy is high.
2) Calculate the standard free energy change for the following reaction at 25 C.
Use ΔG° = ΣνGf°(products) - ΣνGf°(reactants) at 25°C, which is 298.15 K. Multiply each tabulated formation free energy by its coefficient, sum the products, sum the reactants, then subtract reactants from products.
3) If the free energy change ΔG for a reaction is -46.11 kJ/mol, the reaction is:
It is thermodynamically spontaneous in the forward direction under those conditions. The negative value means products are favored energetically, but the reaction rate still depends on kinetics, catalysts, and activation energy barriers.
4) The standard free energy change for a reaction can be calculated using the equation:
The common equation is ΔG° = ΣνGf°(products) - ΣνGf°(reactants). It can also be related to equilibrium by ΔG° = -RT ln K, which connects thermodynamics to the equilibrium constant.
5) What is the free energy change (ΔG) of the hydrolysis of ATP to ADP?
Under biochemical standard conditions, ATP hydrolysis to ADP and phosphate has ΔG°′ of about -30.5 kJ/mol. In cells, the actual ΔG is often more negative because ATP, ADP, and phosphate concentrations are not standard.
6) What is the difference between ΔG and ΔG°?
ΔG describes the reaction under actual conditions, including real concentrations or pressures. ΔG° applies to standard conditions. The relationship ΔG = ΔG° + RT ln Q shows how the actual mixture changes the free energy.
7) Which units should I use for the Gibbs equation?
Use temperature in kelvin. Keep ΔH and TΔS in matching energy units, usually kJ/mol. If entropy is in J/mol·K, divide by 1000 before multiplying by temperature when enthalpy is in kJ/mol.
8) Why can changing Q alter whether a reaction is spontaneous?
Q measures the current product-to-reactant balance. When Q changes, the RT ln Q term changes too. A reaction with negative ΔG° can become non-spontaneous if products build up enough, and the reverse can also happen.