What does Hess's law say?
The total heat of a reaction is the same no matter how many steps it takes - because energy depends only on the start and end states (enthalpy is a 'state function').
// chemistry › Reaction Enthalpy
Hess's Law
Calculate reaction enthalpy ΔH = ΣΔH(products) − ΣΔH(reactants), with symbol legend and real-world examples.
Δ H = Σ Δ Hproduᴄts - Σ Δ Hreₐᴄtₐnts
A mind behind this: Germain Hess 1802–1850
Frequently asked questions
Why is it useful?
It lets you find the heat of a reaction that is hard to measure directly, by adding up easier reactions that sum to it.
What does 'state function' mean?
A quantity that depends only on the current state, not the path taken - like altitude between two towns being fixed whatever road you drive. Enthalpy is one.
Exothermic vs endothermic?
Negative ΔH = releases heat (exothermic, like burning). Positive ΔH = absorbs heat (endothermic, like a cold pack). The sign tells you which.
How does the example give −150?
ΔH = −400 − (−250) = −150 kJ/mol. Products sit lower in energy than reactants, so it releases 150 kJ - exothermic.