1. Thermochemistry Exothermic reactions release heat to the surroundings. Fe 2 O 3 + 2 Al  2 Fe + Al 2 O 3 + 851.5 kJ Potassium Permanganate Reaction Demo Or ΔH = -851.5 kJ Thermite Reaction Demo

2. Thermochemistry Endothermic reactions absorb heat from the surroundings. Ba(OH) 2 + 2 NH 4 NO 3 + 102.2 kJ  Ba(NO 3 ) 2 + 2 NH 3 + 10 H 2 O Barium Hydroxide Reaction Demo Or ΔH = +102.2 kJ

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4. Enthalpy Energy that is gained or lost by substances during a reaction. Symbolized by the letter H. [http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG07_14.JPG]

5. Enthalpy Example: Given equal amounts (mass) of both, which produces more energy: Methane (Natural Gas!) or Octane (Gasoline!)

6. Enthalpy Combustion of Methane (Natural Gas!) CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) ΔH = -890.4 kJ (Exothermic!) 22

7. Enthalpy Taken from reaction ratio! Your furnace turns on and burns 32.0 g of methane, how much heat is produced? 32.0 g CH 4 16.0 g CH 4 1 mol CH 4 -890.4 kJ = -1780 kJ CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g) ΔH = -890.4 kJ

8. Enthalpy Combustion of Octane (Gasoline!) C 8 H 18 (l) + O 2 (g)  CO 2 (g) + H 2 O(g) ΔH = -5430 kJ (Exothermic!) 2 25 16 18

9. Enthalpy Taken from reaction ratio! Your car turns on and burns 32.0 g of octane, how much heat is produced? 32.0 g C 8 H 18 114.0 g C 8 H 18 1 mol C 8 H 18 2 mol C 8 H 18 -5430 kJ = -762 kJ 2 C 8 H 18 (l) + 25 O 2 (g)  16 CO 2 (g) + 18 H 2 O(g) ΔH = -5430 kJ

10. Enthalpy Methane produces more energy when burned…as long as mass is constant.

11. Enthalpy of Formation This is the amount of energy (enthalpy) that is involved in “creating” a compound. [http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/TB07_02.JPG]

12. Enthalpy of Formation The “ΔH” given in a reaction is calculated by using these values in this equation:

13. Enthalpy of Formation Example: CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O (l) What is ∆H for this reaction? 1 st Step - Balance the Equation 22

14. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (l) Enthalpy of Formation 2 nd Step – Find Enthalpy Values Using Chart -75 kJ/mol-393.5 kJ/mol -286 kJ/mol Any element that is “pure”, has an enthalpy of 0 kJ/mol

15. Enthalpy of Formation 3 rd – Plug into ΔH equation and solve: ΔH = [ (-286) + (-393.5)] – [(-74.8) + 0] Coefficient from balanced equation! ΔH = (H products ) – (H reactants ) 2

16. Enthalpy of Formation Final Answer: -890.7 kJ/mol No significant digits needed since all the numbers are found on tables!

17. Hess’s Law A series of reactions can be added together to find their overall enthalpy. N 2 + O 2  2 NO 2 NO + O 2  2 NO 2 + N 2 + 2 O 2  2 NO 2 ΔH = +181 kJ ΔH = -131 kJ ΔH = +68 kJ

18. Hess’s Law Example: C 2 H 2 + H 2  C 2 H 4 ΔH = -174.4 kJ C 2 H 6  C 2 H 4 + H 2 ΔH = 137.0 kJ What is the enthalpy for the net reaction: C 2 H 2 + 2 H 2  C 2 H 6

19. C 2 H 2 + H 2  C 2 H 4 C 2 H 6  C 2 H 4 + H 2 + C 2 H 2 + 2 H 2  C 2 H 6 ΔH = -174.4 kJ ΔH = 137.0 kJ ΔH = - 311.4kJ If you cannot simply add to find the net reaction, “flip” a reaction to make it work! C 2 H 4 + H 2  C 2 H 6 ΔH = -137.0 kJ