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

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

Hess’s Law Enthalpy is a State Function.

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

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 + 100 mL 1 M HCl leads to 21 oC temperature increase B. MgO + 2 HCl  MgCl2 + H2O 0.71 g MgO + 100 mL 1 M HCl leads to a 4 oC temperature increase

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

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.

Elements!

Using Standard Enthalpy of Formation: Diagramatically

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

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