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Can you guess the topic for today? PowerPoint Presentation
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Can you guess the topic for today?

Can you guess the topic for today?

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Can you guess the topic for today?

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  1. Can you guess the topic for today?

  2. Bond energies

  3. Combustion of methane One molecule of methane and two molecules of oxygen

  4. Combustion of methane CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

  5. Combustion of methane CH4(g) + 2O2(g) 2H2O(l) + CO2(g) All reactions involve bond breaking and bond making as the atoms “swap partners”

  6. Bond breaking - endothermic • Energy is always required to be inputted to break a bond. Bond breaking is always endothermic.

  7. Bond making - exothermic • Energy is always released when a bond is formed. Bond making is always exothermic.

  8. 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

  9. 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

  10. 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

  11. Bond Energy

  12. Energy level diagrams

  13. Exothermic reaction The energy need to break the bonds is less than the energy released when new bonds are made C + 4H + 4O energy Energy needed to break bonds Energy released by forming bonds CH4(g) + 2O2(g) Energy released CO2(g) + 2H2O(l) “reaction path”

  14. Endothermic reaction The energy need to break the bonds is more than the energy released when new bonds are made energy Energy released by forming bonds Energy needed to break bonds NH4NO3(l) Energy absorbed NH4NO3(s) + H2O (l) “reaction path”

  15. ΔH – Energy change in a complete reaction If heat is given out, the reaction has lost energy so ΔH is negative

  16. ΔH – Energy change in a complete reaction If heat is absorbed (reaction gets colder), the reaction has gained energy so ΔH is positive

  17. Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

  18. Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x 413 + 2 x 498 = 1662 + 996 = 2658 KJ/mol

  19. Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x 413 + 2 x 498 = 1662 + 996 = 2658 KJ/mol Bonds made = 4 x (O-H) + 2 x (C=O) = 4 x -464 + 2 x -805 = -1856 + -1610 = -3466 KJ/mol

  20. Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x 413 + 2 x 498 = 1662 + 996 = 2658 KJ/mol Bonds made = 4 x (O-H) + 2 x (C=O) = 4 x -464 + 2 x -805 = -1856 + -1610 = -3466 KJ/mol Overall Energy change = 2658 + -3466 = -808 KJ/mol (Exothermic)

  21. 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

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

  23. Calculating Energy of a Reaction H2 + Cl2  2HCl Endothermic or exothermic?

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

  25. 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?

  26. Chemical Reactions and Energy Energy absorbed N2 + O2 NO Bonds are broken

  27. Calculating chemical energy 2H2 + O2  2H2O Breaking H bond = 436 kJ/mol Breaking O bond = 498 kJ/mol O-H bond forming = -464 kJ/mol Net energy released = ((2*436) + 498) + 4(-464) = -442 kJ/mol When the net energy value is positive, the reaction is endothermic.