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Calculation Machine

Hess's Law Calculator

Add reaction steps to get the overall enthalpy change.

Reviewed by James Whitfield · How we check our maths

Your values

reverse a step by flipping its sign

ⓘ Live results

Results
Overall ΔH
211 kJ/mol
Steps used
3
Reaction type
Endothermic — absorbs heat

Working

  1. 1Step 1: -393.5 kJ/mol
  2. 2Step 2: -285.8 kJ/mol
  3. 3Step 3: 890.3 kJ/mol

Quick answer

Hess's law says enthalpy is a state function, so the overall ΔH equals the sum of the ΔH of every step, whatever route is taken.

The formula

ΔH(total) = ΣΔH(steps)

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Step enthalpies (comma separated)-393.5, -285.8, 890.3 kJ/mol
  • Multiply total by1 ×

Result

  • Overall ΔH211 kJ/mol
  • Steps used3
  • Reaction typeEndothermic — absorbs heat

How to use the Hess's Law Calculator

  1. 1List each step's ΔH separated by commas.
  2. 2Flip the sign of any step you reversed.
  3. 3Scale the total if you doubled the target equation.
Step enthalpies (comma separated)(kJ/mol)
reverse a step by flipping its sign

Frequently asked questions

How do you reverse a step in Hess's law?

Change the sign of its ΔH: reversing a −200 kJ/mol step makes it +200 kJ/mol.

What if a step is doubled?

Double its ΔH before entering it, because enthalpy scales with the amount of substance.

About this calculator