Hess's law
Hess's law says the total enthalpy change of a reaction is the same whether it happens in one step or many. So you can add the ΔH values of known steps to find the ΔH of a reaction you can't measure.
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Analogy
Climbing a mountain: the height gained is the same whichever path you take.
Example
C + O₂ → CO₂ ΔH = −393.5 kJCO₂ → CO + ½O₂ ΔH = +283.0 kJAdd: C + ½O₂ → COΔH = −110.5 kJ
In practice
Chemists use it to find the heat of reactions that are hard to run cleanly, such as forming CO without making CO₂.
Practice
Flashcards
- State Hess's law.Total ΔH is the same whichever route, one step or several, goes from reactants to products.
Questions people ask
Why does Hess's law work?
Enthalpy is a state function, so ΔH depends only on the starting and ending states, not on the path. Any route from reactants to products must therefore give the same total. That lets you add and subtract known reactions and arrive at the same answer.
What do I do with ΔH when I reverse or multiply an equation?
Reversing an equation changes the sign of ΔH. Multiplying an equation by a number multiplies ΔH by the same number. Then add the adjusted equations so the unwanted species cancel, and add their ΔH values to get the ΔH for the target reaction.