AP Chemistry / Unit 6: Thermochemistry
6.9 Hess's Law
Notes
$\Delta H$ for an overall reaction is the sum of $\Delta H$ for steps that add up to it.
- Reverse a step → flip the sign. Multiply a step → multiply $\Delta H$.
Practice
Step 1: $\Delta H=-200$. Step 2 (reversed from $+50$). Total:
$-200+(-50)$.
If you multiply an equation by 3, its $\Delta H$:
Enthalpy is extensive.
Hess's law works because enthalpy is a:
Only start and end matter.
Ready for more? Take the Unit 6 test.