Hess Law Calculator

Solve enthalpy changes using combined reaction steps. Reverse equations, apply multipliers, and inspect heat totals. Build faster thermochemistry workflows with clear exports and visuals.

Hess Law Calculator Form

Enter only the steps you need. Reverse any step when required.

Each row supports one reference reaction. Enter the raw enthalpy value, apply a multiplier, and reverse the step if needed.

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Formula Used

Overall enthalpy:

ΔHtarget = Σ (si × ni × ΔHi)

Here, si equals +1 for the same direction and -1 for a reversed reaction.

ni is the multiplier applied to the reaction. The calculator adds every adjusted step to get the net enthalpy change.

How to Use This Calculator

  1. Type the target reaction you want to reconstruct.
  2. Enter each known thermochemical step in a separate card.
  3. Add the published raw ΔH for every step.
  4. Set the multiplier needed to match stoichiometric coefficients.
  5. Check the reverse box when a reference equation must flip.
  6. Choose your preferred output unit label and decimal precision.
  7. Press the calculate button to view the total enthalpy.
  8. Review the table, cumulative chart, and export files.

Example Data Table

Example target reaction: C(graphite) + O2(g) → CO2(g)

Step Reaction Raw ΔH Multiplier Direction Adjusted ΔH
Step 1 C(graphite) + 1/2 O2(g) → CO(g) -110.50 kJ/mol 1 Same direction -110.50 kJ/mol
Step 2 CO(g) + 1/2 O2(g) → CO2(g) -283.00 kJ/mol 1 Same direction -283.00 kJ/mol
Total ΔH -393.50 kJ/mol

Frequently Asked Questions

1. What does Hess’s law state?

Hess’s law states that total enthalpy change depends only on initial and final states. Any valid pathway of intermediate reactions gives the same net heat change.

2. Why does reversing a reaction change the sign?

Reversing a reaction swaps reactants and products. The heat flow also reverses, so a positive enthalpy becomes negative, and a negative enthalpy becomes positive.

3. Why does multiplying a reaction also multiply ΔH?

Enthalpy is an extensive property. When you multiply every stoichiometric coefficient by a factor, the heat change scales by the same factor.

4. Do all steps need the same unit?

Yes. Use one consistent unit through the full pathway. Mixing kJ, J, or per-mole values without conversion can produce a wrong overall result.

5. Can I combine combustion, formation, and measured reactions?

Yes, if every step is thermochemically valid and scaled consistently. Different reference reactions can be combined inside one Hess cycle.

6. What does a positive final value mean?

A positive total ΔH means the target reaction absorbs heat. A negative value means it releases heat. A zero value indicates a thermoneutral process.

7. How many steps should I include?

Use only the steps required to reconstruct the target equation. Extra steps can still work, but they increase the chance of sign or scaling errors.

8. Why might my answer differ from a textbook table?

Small differences may come from rounding, phase labels, reference states, or temperature assumptions. Matching data sources and states improves agreement.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.