Enter your reactions into Reaction 1 (and optionally Reaction 2), input each ΔH value, then use the Flip toggle to reverse any reaction and the Multiplier to scale it — the Enthalpy Calculator applies Hess's Law to combine your adjusted enthalpies and return the Total Enthalpy Change (ΔH) for your target reaction.
Total Enthalpy Change (ΔH)
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Adjusted ΔH₁
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Adjusted ΔH₂
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Adjusted ΔH₃
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Total Enthalpy Change
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Hess's Law states that the enthalpy change of a chemical reaction is the same regardless of the pathway taken. This means the total enthalpy change depends only on the initial and final states, not the intermediate steps.
Check the 'Flip (reverse)' checkbox for any reaction step. This reverses the reaction direction and changes the sign of its ΔH value (positive becomes negative and vice versa).
The multiplier scales the entire reaction equation and its ΔH value. For example, multiplying by 2 doubles all stoichiometric coefficients and doubles the ΔH value.
The calculator accepts ΔH values in kJ/mol and automatically converts results to both kJ/mol and kcal/mol. Use consistent units for all your input values (1 kcal = 4.184 kJ).
The calculator is mathematically precise for the arithmetic operations. However, accuracy depends on the quality of your input ΔH values from experimental data or thermodynamic tables.
Hess's Law applies to enthalpy changes at constant pressure and temperature. It requires accurate ΔH values for known reactions, and the target reaction must be achievable through the combination of given steps.
While you can enter unbalanced equations for reference, the ΔH values you input must correspond to properly balanced chemical equations. The stoichiometry affects the enthalpy values.
This calculator handles up to three reaction steps. For more complex calculations involving additional steps, you can perform multiple calculations and combine the results manually.