Energetics & electrochemistry · SL/HL
Hess Law Formation Enthalpy Calculator
Enter each side as “coefficient ΔHf°” in kJ mol⁻¹. Elements in their standard states have ΔHf° = 0.
Your values
Start with the example or enter your own values. Use a dot for decimals and e for scientific notation.
Calculated locally. No account or input upload.
Result & working
ExampleThe calculator is loading. The worked example is also shown below.
Step-by-step working
The formula
ΔH°reaction = ΣνΔHf°(products) − ΣνΔHf°(reactants)
| Symbol | Meaning |
|---|---|
| ν | Stoichiometric coefficient. |
| ΔHf° | Standard enthalpy of formation. |
When to use this calculator
This is one application of Hess’s law. Check that physical states and thermochemical data correspond to the reaction being analysed.
Worked example
The page opens with these example values. The result below is pre-generated for reference.
- Product: 1 × (-890.3) = -890.3
- Reactant: 1 × (-393.5) = -393.5
- Reactant: 2 × (-285.8) = -571.6
- ΔH° = Σproducts − Σreactants = -890.3 − (-965.1) = 74.8 kJ mol⁻¹
74.8 kJ mol⁻¹
Common mistakes
- Include stoichiometric coefficients.
- Do not reverse the products-minus-reactants sign.