1. - “Heats” of formation fH° - DrU° ?
CHEM 433 – 10/5/11
III. 1st Law: Thermochemistry ( )
- H for chemical changes
- Hess’ Law
- “Heats” of formation fH°
- DrU° ?
READ: FINISH CHAPTER 2
HW #3 Due F. HW#4 out F.

2. ENTHALPY IS AN EXTENSIVE PROPERTY
1) How much heat is produced when 0.50 mol of H2O is produced via:
2 H2 (g) + O2 (g) —> 2 H2O (l) H = -572 kJ
2) What is H for:
4 H2 (g) + 2 O2 (g) —> 4 H2O (l) H = ?

4. Some DvapH° values …
Which value are highest?
H2O vs NH3 ?
Methane vs. propane vs. butane?
Does PH3 have H-bonding?

5. So, how do you go about getting DH?
CH4 (g) + 2 O2 (g) —> CO H2O (l) H = ?
Experimentally:
Measure qp & n => DrH° = qp / n (note ~ in kJ / mol)
Theoretically: ?
(CHEM 433 does not have a lab… or what if the reaciton does not go... or ….?)

6. Hess’ Law:
In pictures…

7. Use Hess’ Law and the reaction below to calculate cH for C6H6(l) - which is rxnH for:
__C6H6 (l) + __O2 (g) —> __CO2 (g) + __H2O (l)
(balanced for one mol C6H6…)
Data:
C6H12 (l) + 9 O2 (g) —> 6 CO2 (g) + 6 H2O (l) H° = kJ
C6H6 (l) H2 (g) —> C6H12 (l) H° = -205 kJ
H2 (g) /2 O2 (g) —> H2O(l) H° = -286 kJ

8. The Born-Haber Cycle - Hess’ Law in Action!
Why are ionic compounds so stable ?

9. Standard Enthalpies of Formation: rH° to produce 1 mol of a given substance from elements in their reference states (most stable from, 1 bar, some specified T - often 298K )

10. Using “heats” of Formation : rH° for a given reaction can be obtained via:
{The sum of rH°’s for the products} –
{The sum of rH°’s for the reactants}