1. Hess’s Law

2. Enthalpy is a State Variable ‘State variable’ just means something that doesn’t change depending on the path you take to get from A to B. If you start at the bottom of K2, your overall change in altitude is the same no matter which route you take up.

3. For Chemistry Purposes This means that if you don't know the  H for a reaction, you can figure it out by using the ones for any other reactions that start and end at the same point: A A B C If you put 100 kJ in to go from A to C and get 40 kJ out from C to B then it must be 60 kJ in from A to B

4. Example Reaction Let's burn some random compound: 2C 7 H 10 O 2 + 17O 2 → 14CO 2 + 10H 2 O  H = ???? Since we picked this compound arbitrarily, we probably have no idea how much energy we'll get from burning it. BUT! If we can find a set of reactions that, when combined, start with 2C 7 H 10 O 2 and 17 oxygens, and end with 14 carbon dioxides and 10 waters, their individual  H values will add up to our desired value

5. This Is Not Easy Finding the right reactions can be...problematic. There are some shortcuts, though: If you're interested, or planning on taking AP chem, look up “heat of formation” or “enthalpy of formation”

6. Summary Enthalpy is a state function (path doesn't matter) You can use a series of reactions to get  H The details can be messy, but there are shortcuts