Hess’s Law Calculator

This Hess’s law calculator adds up the enthalpy changes of each step in a reaction pathway to work out the overall enthalpy change for the reaction as a whole, which matters because enthalpy is a state function: the total change depends only on the starting materials and final products, never on the route taken. You enter the enthalpy change of every step in kJ/mol, separated by commas, using the correct sign for each: reverse a step and flip its sign, scale a step and multiply its enthalpy by the same factor. The calculator adds every value you enter and returns the overall enthalpy change in kJ/mol, the number of steps entered, and whether the reaction is exothermic, meaning energy is released and the value is negative, or endothermic, meaning energy is absorbed and the value is positive. This makes it useful for checking homework answers, verifying a Hess’s law cycle before a test, or double-checking a lab calculation where a reaction has been broken into steps with known enthalpies. Because the result depends entirely on the figures you supply, accuracy relies on entering correct, properly signed values, ideally taken from a reliable table of standard enthalpies of formation or combustion. The formula, a worked example combining two steps into one overall value, and the assumptions behind the calculation are below, with answers to common questions about reversing and scaling steps.

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