1. Not all chemical energy changes can be studied conveniently using simple calorimetry.
Methods used to study these reactions are based on the principle that net changes in all properties of a system are independent of the way the system changes from the initial state to the final state.

2. Predicting ΔrH: Hess’ Law
Hess’ law states that the addition of chemical equations yields a net chemical equation whose enthalpy change is the sum of the individual enthalpy changes.
ΔrH° = Δ1H° + Δ2H° + Δ3H° + … = ΣΔrH°
Two things to remember:
If a chemical equation is reversed, then the sign of ΔrH changes.
If the coefficients of a chemical equation are altered by multiplying or dividing by a constant factor, then the ΔrH is altered by the same factor.

3. Use the following steps:
Write the net reaction equation, if it is not given.
Manipulate the given equations so they will add to yield the net equation.
Multiply, divide, &/or reverse the sign of the enthalpy of reaction.
Cancel & add the remaining reactants & products to yield the net equation.
Add the component enthalpy changes to obtain the net enthalpy change.
Determine the molar enthalpy for a reactant or product, if required.

4. Example Problem Calculate the enthalpy change of the reaction
2 Fe(s) /2O2(g) → Fe2O3(s)
using the following equations:
Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3CO2(g) ∆rH˚ = kJ
CO(g) + ½ O2(g) → CO2(g) ∆rH˚ = kJ

5. Step 1.
Net 2 Fe(s) + 3/2O2(g) → Fe2O3(s) ∆rH˚ = ?
Step 2 and 3.
2 Fe(s) + 3CO2(g) → Fe2O3(s) + 3 CO(g) ∆rH˚ = kJ
3 CO(g) + 3/2 O2(g) → 3 CO2(g) ∆rH˚ = kJ
Remember, every term is multiplied by 3, including the ∆rH.
This equation also allows the unwanted substances from Equations 1 & 2 to cancel out.

6. Step 4 and 5.
(1) 2 Fe(s) + 3CO2(g) → Fe2O3(s) + 3 CO(g) ∆rH˚ = kJ
(2) 3 CO(g) + 3/2 O2(g) → 3 CO2(g) ∆rH˚ = kJ
2 Fe(s) + 3/2 O2(g) → Fe2O3(s) ∆rH˚ = kJ

7. Use the following given equations and their standard enthalpy changes.

10. Homework: Read pgs. 502 – 504 Practice #’s 1 – 4 (p. 504, 505)
Section 11.4 Questions #’s 2,3,9,10 (p )