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and Chemistry?

Energy. and Chemistry?. Joules or calories. Joules or calories. Joules or calories. Joules or calories. How much energy is stored in glucose?. C 6 H 12 O 6. Chemical Energy. When chemical reactions occur, bonds are first broken and then formed.

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and Chemistry?

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  1. Energy and Chemistry? Joules or calories Joules or calories Joules or calories Joules or calories

  2. How much energy is stored in glucose? C6H12O6

  3. Chemical Energy • 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

  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 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

  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 H2 + Cl2  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 H2 + Cl2  2HCl Endothermic or exothermic?

  12. Chemical Reactions and Energy Energy released H2 + Cl2 HCl Bonds are broken

  13. Calculating chemical energy N2 + O2  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 Energy absorbed N2 + O2 NO Bonds are broken

  15. Calculating chemical energy 2H2 + O2  2H2O Endothermic or exothermic?

  16. Calculating chemical energy CH4(g) + 2O2(g)  CO2(g) + 2H2O (l) Endothermic or exothermic?

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