Measuring how much heat a chemical reaction releases or absorbs usually means actually running that reaction under controlled conditions and measuring the temperature change directly. Some reactions make that approach genuinely difficult or impossible — they might proceed too slowly to measure practically, produce dangerous intermediate products, or simply be impractical to isolate cleanly in a lab. Hess's Law offers a way around the problem entirely: it lets chemists calculate a reaction's total heat mathematically, without ever running the reaction itself, by adding together the heats of other, more convenient reactions that get from the same starting materials to the same final products.
Heat depends only on start and end, not the path taken
Hess's Law rests on a specific property of enthalpy (the thermodynamic quantity describing a reaction's heat content under constant pressure): it's a state function, meaning its value depends only on a system's initial and final states, not on the particular sequence of steps taken to get from one to the other. This has a genuinely useful practical consequence: if a reaction can be broken down, on paper, into a series of intermediate steps whose individual heat changes are already known or independently measurable, then adding up those individual heat changes gives the exact same total as measuring the overall reaction directly — because the overall enthalpy change only cares about where you started and where you ended up, not which specific route got you there.
Combining known reactions like an accounting ledger
In practice, chemists apply Hess's Law by writing out a target reaction and finding a combination of other reactions, each with a known, separately measured enthalpy change, that can be added together (reversing some, scaling others by a multiplying factor) to produce exactly the same net set of reactants and products as the original target reaction. Adding up the enthalpy changes of those component reactions, applying the same reversals and scaling factors used to combine the reactions themselves, gives the enthalpy change of the target reaction — all without ever needing to physically carry it out. This technique is routinely used for reactions that would otherwise be genuinely impractical to measure directly, including certain combustion reactions, reactions involving unstable intermediates, and reactions that proceed far too slowly under normal conditions to yield a clean, direct measurement.
What we're still unsure about
Hess's Law itself, resting directly on enthalpy being a state function, is a completely settled, rigorously confirmed principle of thermodynamics, with no scientific dispute about its validity. What does require care in practice is correctly identifying and combining an appropriate set of known reactions to reconstruct a specific target reaction — choosing which component reactions to use, correctly balancing and scaling them, and ensuring the combination truly reproduces the exact target reaction without extraneous species left over is a skill that takes practice, and errors in this bookkeeping process, rather than any flaw in the underlying law, are the most common source of mistakes when Hess's Law is applied to complex, multi-step calculations.
This sits inside Thermochemistry & Hess's Law, one of seven topics in Physical Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.