Energy Changes

1. Energy Changes • Phase Changes of a System • Melting • Solid  liquid • Endothermic (energy put in) • Boiling • Liquid  Gas • Endothermic • Condensing • Gas  Liquid • Exothermic • Freezing • Liquid  Solid • Exothermic • Sublimation • Solid  Gas • Endothermic

2. Energy Changes • Phase Changes Involve Enthalpy Changes • They are exothermic or endothermic • The change in enthalpy is specific for each substance

3. Energy Changes • Molar Enthalpy • The enthalpy change per mole of a substance undergoing a change • Example: • 6.03 kJ to change 1 mole of ice to water • 40.8 kJ to change 1 mole of water to steam • Molar Enthalpy - H

4. Energy Changes • Molar Mass • The mass of one mole of a substance • Use periodic table

5. Energy Changes • Molar Mass • Example: • Molar mass of CH4 • Molar Mass of C12H22O11

6. Energy Changes • Enthalpy Calculations • ΔHvap = nHvap • n = number of moles • H = enthalpy • Vap = vaporization

7. Energy Changes • Enthalpy Calculations • Example: • ΔHvap = nHvap • CCl2F2 =Freon • What is the enthalpy change for boiling 500g of Freon? • Molar Enthalpy = 34.99kJ/mol

8. Energy Changes • Enthalpy Calculations • Try: • Pg. 349 • 15, 16b, 17, 18, 19

9. Energy Changes • Total Energy Change • Example: • Heat 2.5 L water from 12 degrees to 100 degrees and THEN vaporize it • A change in temperature AND a phase change • Requires two sets of calculations

10. Energy Changes • Total Energy Change • First • q = vcΔt • Then • ΔHvap = nHvap

11. Energy Changes • Try • Pg. 351 • 21d, 22a, b • Pg. 365 • 15, 21, 22 • 18?

12. Energy Changes • Enthalpy Communication • Exothermic Reactions • Negative enthalpy • Use a minus sign • Because the system LOSES energy • Endothermic Reactions • Positive enthalpy • Use a plus sign • Because the system GAINS energy