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Understanding Bond Energy and Chemical Reactions

Learn how energy is stored in glucose, and the concepts of bond energy in chemical reactions. Explore how bonds break and form, and the energy absorbed or released. Understand the analogy of bond energy to a stretched rubber band. Calculate energy in reactions and determine if they are endothermic or exothermic.

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Understanding Bond Energy and Chemical Reactions

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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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