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Do now!

Do now!. Can you stick the slides in your book please (cut round the edges, but don’t cut out each individual slide – it takes too long!). Bond energies. Let’s make some molecules!. One molecule of methane and two molecules of oxygen. Combustion of methane?. Combustion of methane.

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Do now!

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  1. Do now! Can you stick the slides in your book please (cut round the edges, but don’t cut out each individual slide – it takes too long!).

  2. Bond energies

  3. Let’s make some molecules! One molecule of methane and two molecules of oxygen

  4. Combustion of methane?

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

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

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

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

  9. Bond energies The energy released when a bond is formed or absorbed when it is broken is called the bond energy. e.g. the C-H bond in methane has a bond energy of 413 KJ/mol

  10. Examples of bond energies

  11. Energy level diagrams

  12. 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”

  13. 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”

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

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

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

  17. 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

  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 Bonds made = 4 x (O-H) + 2 x (C=O) = 4 x -464 + 2 x -805 = -1856 + -1610 = -3466 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 Overall Energy change = 2658 + -3466 = -808 KJ/mol (Exothermic)

  20. Let’s try some questions! Chemistry for you (OLD) P198 Qs 7,8,9.

  21. How well have you understood?

  22. Bond energies?

  23. Energy level diagrams?

  24. Using bond energies in calculations?

  25. Homework • Read pages 160 to 163 • Answer ALL questions on those pages • Due Monday 11th Jan

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