Hess's Law Calculator
Add reaction steps to get the overall enthalpy change.
Enter your values
reverse a step by flipping its sign
Enter the multiply total by in ×.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Overall ΔH
- 211 kJ/mol
- Steps used
- 3
- Reaction type
- Endothermic — absorbs heat
Working
- 1Step 1: -393.5 kJ/mol
- 2Step 2: -285.8 kJ/mol
- 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.
ΔH(total) = ΣΔH(steps)
At a glance
| What it does | Add reaction steps to get the overall enthalpy change. |
|---|---|
| Category | Chemistry |
| Inputs needed | Step enthalpies (comma separated), Multiply total by |
| Main output | Overall ΔH |
| Formula | ΔH(total) = ΣΔH(steps) |
| Cost | Free — no sign-up, no download |
How to use the Hess's Law Calculator
- 1List each step's ΔH separated by commas.
- 2Flip the sign of any step you reversed.
- 3Scale the total if you doubled the target equation.
Inputs explained
- Step enthalpies (comma separated)(kJ/mol)
- reverse a step by flipping its sign
- Multiply total by(×)
- Enter the multiply total by in ×.
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 ×
Results
- Overall ΔH211 kJ/mol
- Steps used3
- Reaction typeEndothermic — absorbs heat
- Step 1: -393.5 kJ/mol
- Step 2: -285.8 kJ/mol
- Step 3: 890.3 kJ/mol
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.
What do I need to enter into the Hess's Law Calculator?
Just 2 values: step enthalpies (comma separated), multiply total by. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Hess's Law Calculator work out the 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. It applies the formula ΔH(total) = ΣΔH(steps) and shows the working so you can check each step by hand.
Is the Hess's Law Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.