Everything Is Measured From The Elements

You cannot measure a substance’s enthalpy. You can only measure a change. So chemistry picks a zero and measures everything from it — and the zero it picks is the elements themselves. Once every compound has a number relative to that baseline, any reaction’s ΔH° is one subtraction: products minus reactants, in that order, always.

8 STEPS · 6 QUICK CHECKS · ΔH° = ΣnΔH_f°(products) − ΣnΔH_f°(reactants) · STANDARD STATE: 1 bar, 298 K · v1