What Hess’s law lets you do
Hess’s law states that the total enthalpy change for a reaction is the same no matter how many steps the reaction is carried out in. This makes it possible to find the enthalpy of a reaction that is hard to measure directly by adding up the enthalpies of known steps that combine to give the same overall reaction.
How this calculator works
Enter the ΔH of up to three known step reactions, along with a multiplier for each: use a positive multiplier if the step is used as written and scaled by that factor, or a negative multiplier if the step must be reversed (which flips the sign of its ΔH) before it can be added. The calculator sums ΔH1·mult1 + ΔH2·mult2 + ΔH3·mult3 to get the overall reaction enthalpy. Leave a step’s ΔH at 0 if you only need two steps.
Note: You must first work out the correct multiplier and direction for each step yourself so that the steps add up to your target overall equation. This tool is for general chemistry education.