Enthalpy Calculator

Summary

The Enthalpy Calculator determines the standard enthalpy change (ΔH°rxn) of a chemical reaction via two methods: the Formation Method applies ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants), while Hess's Law adds a series of intermediate reaction enthalpies to arrive at the net ΔH. All computation is performed client-side with no data sent to any server.

How it works

1. Choose a calculation method: Standard Enthalpies of Formation or Hess's Law.
2. For the Formation Method: enter each product with its stoichiometric coefficient and ΔH°f value, then each reactant similarly.
3. The calculator applies ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants).
4. For Hess's Law: enter each intermediate reaction's ΔH value; reverse a reaction by checking the "Reverse" box, which negates its ΔH.
5. Click Calculate to see the net ΔH, a sign interpretation (exothermic / endothermic), and the full equation breakdown.
6. Use the Copy button to copy the result to your clipboard.

Use cases

• Determine whether a combustion reaction is exothermic and by how much.
• Solve Hess's law problems from general chemistry courses.
• Verify hand-calculated ΔH values against known formation enthalpies.
• Predict energy release or absorption for industrial chemical processes.
• Practice thermochemistry calculations for exams (AP Chemistry, university general chemistry).
• Calculate ΔH for reactions where direct measurement is impractical by combining known steps.
• Explore how reversing or scaling a reaction step changes overall ΔH.
• Cross-check textbook example answers instantly.

Frequently Asked Questions