Hess's Law Strategy
- Write the target reaction and the given reactions.
- Inventory the substances you need from the given reaction. (just checking that the base reactions exist even if the moles aren’t correct yet or they are on the wrong side)
- we do this if we need to eliminate any reactions that don’t contain any substances we need.
- Find substances that appear only once out of all of the reactions and fix those equations first.
- Manipulate your equations
- Reverse equations if substances are on the wrong side.
- Scale equations if coefficients are wrong. (multiply by scale factor)
- If everything you need exists, is on the correct side, and in the correct moles → Add all equations together. (left side substances go together and right side substances go together)
- Cancel identical substances on opposite sides.
- Verify the remaining reaction matches the target exactly.
- Add the adjusted ΔH values.
**remember that to be identical substances they must also be in the same state (gas, liquid, or solid)