Hess's law
ScienceDefinition
Hess's law states that the total enthalpy change for a chemical reaction is the same regardless of whether the reaction occurs in one step or multiple steps. This allows you to calculate ΔH for a reaction by adding the enthalpy changes of intermediate reactions.
How It Works
- Identify the target reaction whose ΔH you want to find.
- Find a set of reactions with known ΔH values whose steps add up to the target reaction.
- Reverse any reactions as needed (which changes the sign of ΔH) and multiply by coefficients to match the target.
- Add the adjusted ΔH values together to get the overall enthalpy change.
Examples
- Calculating the enthalpy of formation of CO from C and O₂ using combustion data for C and CO
- Determining the ΔH of a reaction that is too slow or dangerous to measure directly in a calorimeter
- Using standard enthalpies of formation to calculate ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants)
Key Fact
ΔH_total = ΔH₁ + ΔH₂ + ΔH₃ + … (enthalpy is a state function, path-independent)
Study This Concept
Practice Hess's law with free review games in these units: