Reverse or multiply given reaction steps so they add up to a target equation.
Hess's law states that the total enthalpy change of a reaction is independent of the pathway taken to get from reactants to products — a direct consequence of enthalpy being a state function, and the basis for calculating otherwise hard-to-measure ΔH values throughout AP Chemistry Unit 6.
Because ΔH is path-independent, any target reaction can be constructed by algebraically combining a series of other reactions with known ΔH values, as long as those reactions sum (after any needed manipulation) to exactly the target equation. Two manipulation rules govern how ΔH transforms alongside the equation: reversing a reaction flips the sign of its ΔH (since going backward releases what going forward absorbed, or vice versa), and multiplying a reaction by any factor scales its ΔH by that same factor.
Standard enthalpies of formation are the most common building blocks for Hess's law calculations, since ΔHf° values for a huge range of compounds are tabulated and can be combined to calculate ΔH for reactions that were never directly measured. Combustion enthalpies serve the same building-block role for organic compounds specifically, since combustion reactions are experimentally convenient to measure calorimetrically.
The formation of acetylene (C2H2) from its elements is a classic Hess's law puzzle precisely because it cannot be measured directly — carbon and hydrogen don't cleanly combine to form pure acetylene under laboratory conditions — but by combining the known combustion enthalpies of carbon, hydrogen, and acetylene itself (reversing the acetylene combustion equation, since we want acetylene as a product, not a reactant), the target formation enthalpy, ΔHf ≈ +226.7 kJ/mol, can be calculated exactly, revealing acetylene as an unusually high-energy, endothermic-to-form molecule.
This Hess's law builder presents a target equation and a set of given reaction steps, and lets you reverse or scale each step until they sum correctly to the target — with ΔH tracked automatically through every manipulation, reinforcing exactly how the sign and magnitude rules work rather than just stating them.