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And Chemistry? Joules or calories. How much energy is stored in glucose? C 6 H 12 O 6.

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Presentation on theme: "And Chemistry? Joules or calories. How much energy is stored in glucose? C 6 H 12 O 6."— Presentation transcript:

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 - H 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) kJ kJ 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 = ( ) + 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?


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