1. and Chemistry? Joules or calories

2. How much energy is stored in glucose? C 6 H 12 O 6

3. Chemical Energy When chemical reactions occur, bonds are first broken and then formed. When chemical reactions occur, bonds are first broken and then formed. The amount of energy released / absorbed during a chemical reaction can be measured and calculated from the bond energies The amount of energy released / absorbed during a chemical reaction can be measured and calculated from the bond energies

4. Bond Energy Energy is absorbed by atoms when their bonds break. (+H) - bond dissociation Energy is released when bonds form between atoms. (- H) - bond formation

5. Bond Energy - Analogy Imagine stretching a rubber band until it breaks. You must do work to stretch the band because the tension in the band opposes your efforts. You lose energy; the band gains it. Something similar happens when bonds break in a chemical reaction. The energy required to break the bonds is absorbed from the surroundings.

6. Bond Energy Bond Energy (kJ/mol) H - H436 C - H413 N - H393 436 kJ is required to disrupt H-H 436 kJ is released when H-H is formed The higher the bond energy, the more work is required to break the bond, the more stable it is

7. Bond Energy Bond Energy (kJ/mol) Bond Energy (kJ/mol) H - H436N - N160 C - H413N = O631 N - H393N triple N941 P - H297N - O201 C - C347N - P297 C - O358O - H464 C - N305O - S265 C - Cl397O - Cl269 C = C607O - O204 C = O805C - F552 O = O498C - S259

8. Bond Energy Energy is absorbed when bonds break. Energy is released when bonds form. Energy is absorbed or released when the heat capacities of the products and reactants differ.

9. Calculating Energy of a Reaction H 2 + Cl 2 2HCl Breaking H bond = 436 kJ/mol Breaking Cl bond = 242 kJ/mol H-Cl bond forming = -431 kJ/mol

10. Reactions and Energy If heat is generated during a reaction exothermic If heat is absorbed during a reaction endothermic

11. Calculating Energy of a Reaction H 2 + Cl 2 2HCl Moles of Bonds Broken Energy absorbed (kJ) Moles of Bonds Formed Energy released (kJ) 1 H-H @ 431.2 kJ 436 2 H-Cl @427 kJ 854 1 Cl-Cl @ 243 679854 Heat of Reaction is 679 – 854 = -175kJ; energy is released during the reaction Endothermic or exothermic?

12. Chemical Reactions and Energy H 2 + Cl 2 Bonds are broken HCl Energy released

13. Calculating chemical energy N 2 + O 2 2NO Breaking one N bond = 946 kJ/mol Breaking one O bond = 498 kJ/mol Forming 2 N-O bonds = 2 x 631 kJ/mol = 1262 kJ/mol Net energy released = (946 + 498) + 2(-631) = +182 kJ/mol Endothermic or exothermic?

14. Chemical Reactions and Energy N 2 + O 2 Bonds are broken NO Energy absorbed

15. Calculating chemical energy 2H 2 + O 2 2H 2 O Endothermic or exothermic?

16. Calculating chemical energy CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (l) Endothermic or exothermic?