1. Bill Vining 232 PSCI 436-2698 viningwj@oneonta.edu
Chemistry 112 Sections 1-7
Bill Vining
232 PSCI

2. Hess’s Law
If you can add the reactions, you can add the H’s.

3. Hess’s Law
Enthalpy is a
State Function.

4. Rules for manipulating Reaction Equations
Reverse reactions: Change sign of H
Multiple by a constant: Multiply sign of H by that constant

5. A. Mg + 2 HCl  MgCl2 + H2 B. MgO + 2 HCl  MgCl2 + H2O
Hess’s Law: Your Lab
A. Mg + 2 HCl  MgCl2 + H2
0.52 g Mg mL 1 M HCl leads to 21 oC temperature increase
B. MgO + 2 HCl  MgCl2 + H2O
0.71 g MgO mL 1 M HCl leads to a 4 oC temperature increase

6. Hess’s Law: Your Lab
Mg(s) + ½ O2(g)  MgO(s) H = ???

7. Standard Enthalpy of Formation
Formation Reaction: Reaction that forms one mole of a compound from its elements in their natural states.
K2CO3(s)??
Standard enthalpy of formation: Enthalpy change on forming one mole of a compound from its elements under standard conditions.

9. Elements!

10. Using Standard Enthalpy of Formation: Diagramatically

11. Using Standard Enthalpy of Formation: Mathematically
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)

12. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l)
+2220 kJ
-2220 kJ
+574 kJ
-574 kJ