Hess's Law Calculator to Calculate Enthalpy of Reaction
Apply Hess's Law to calculate enthalpy changes for reactions that cannot be directly measured. Use standard enthalpies of formation to find ΔH for complex reactions.
What is Hess's Law?
Hess's Law states that the total enthalpy change of a reaction is independent of the pathway — only the initial and final states matter. This allows chemists to calculate ΔH for reactions that cannot be directly measured by combining known enthalpy values from other reactions.
🔥 Hess's Law Calculator
Use our free calculator for instant, accurate results.
📐 Formula
ΔH_rxn = Σ ΔH_f°(products) − Σ ΔH_f°(reactants)
ΔH_f° = standard enthalpy of formation (kJ/mol). Multiply by stoichiometric coefficients. Negative ΔH = exothermic reaction.
📝 Worked Example
Combustion of C to CO₂:
C + O₂ → CO₂, ΔH = −393.5 kJ/mol
Using formation enthalpies: ΔH_f°(CO₂) = −393.5, ΔH_f°(C) = 0, ΔH_f°(O₂) = 0
ΔH_rxn = −393.5 − (0+0) = −393.5 kJ/mol ✓
📝 How to Use the Calculator
❓ FAQ
What is standard enthalpy of formation?
The enthalpy change when 1 mol of a compound forms from its elements in their standard states at 298 K and 1 bar. By definition, ΔH_f° of elements in standard state = 0.
Why can't some enthalpies be measured directly?
Some reactions are too slow, produce multiple products simultaneously, or would require impractical conditions. Hess's Law lets us bypass direct measurement.
Veer Kumavat
Founder & AuthorVeer is a 14-year-old student from Nashik, Maharashtra, who built SciFi Calculators to help students worldwide master STEM subjects. He is passionate about making complex science and math problems accessible through intuitive digital tools.