Chemistry / Unit 4 Chem 0706 worked example, 60 to 90 seconds
Hess's Law
The idea
Hess's law states that if a process is written as the sum of stepwise processes, the overall enthalpy change equals the sum of the enthalpy changes of those steps, because enthalpy is a state function that depends only on starting and ending conditions. This licenses combining known reactions algebraically — reversing a reaction flips the sign of its ΔH, and scaling a reaction by a factor scales its ΔH by that same factor — to build up a target reaction whose enthalpy change was never measured directly.
In practice
Carbon burning directly to carbon dioxide releases the identical total heat as carbon first forming carbon monoxide and that carbon monoxide then burning further to carbon dioxide, because both paths start and end at the same states. The two-step route's enthalpy changes sum to the one-step route's single value.
Where people slip
Adding two known reactions' enthalpy changes without first canceling any species that appears as a product in one step and a reactant in another produces a total that does not match the actual overall reaction.
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