Hess\u2019s Law Calculator

This Hess\u2019s law calculator adds up the enthalpy changes of the steps in a reaction pathway to give the overall enthalpy change. Hess\u2019s law says that because enthalpy is a state function, the total enthalpy change of a reaction depends only on the start and end points, not on the route taken. So if you can break a reaction into steps with known enthalpies, you just add them. Enter the step values, separated by commas and with the correct signs (reverse a step by flipping the sign, scale it by multiplying), and the calculator returns the total. The law, a worked example and the assumptions are below.

Step enthalpy changes, comma separated (kJ/mol)

Number of steps

Steps

The formula

Hess\u2019s law: the overall enthalpy change equals the sum of the enthalpy changes of the individual steps, because enthalpy is a state function. Reverse a step by changing its sign; scale a step by multiplying its enthalpy by the same factor.

Worked example

Combining steps of minus 393.5 and minus 285.8 kJ/mol gives an overall enthalpy of minus 679.3 kJ/mol, an exothermic reaction. Enter -393.5, -285.8 to confirm.

Frequently asked questions

What is Hess\u2019s law?

The total enthalpy change of a reaction is the same regardless of the pathway, so step enthalpies simply add.

How do I handle a reversed step?

Flip the sign of its enthalpy change. Reversing a reaction reverses the sign of its enthalpy.

What if a step is scaled?

Multiply its enthalpy by the same factor you multiply the equation by, then include that scaled value.

Who this calculator is for

This calculator is for chemistry students, teachers and lab workers.

What this calculator assumes

You enter values in the units shown.
Standard conditions unless stated.
Results are rounded for display.

Formula and sources

Related calculators

Molarity Calculator: concentration in mol/L.
pH Calculator: acidity from H+ concentration.
Ideal Gas Law Calculator: PV = nRT.

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