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Bond….. Bond Energy!

Bond….. Bond Energy!. Bond energies. Let ’ s make some molecules!. One molecule of methane and two molecules of oxygen. Combustion of methane. CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g). Combustion of methane. CH 4(g) + 2O 2(g) 2H 2 O (l) + CO 2(g)

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Bond….. Bond Energy!

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  1. Bond….. Bond Energy!

  2. Bond energies

  3. Let’s make some molecules! 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. 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

  9. Examples of bond energies

  10. Energy level diagrams

  11. Exothermic reaction The energy needed 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”

  12. Endothermic reaction The energy needed 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”

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

  14. ΔH – Energy change in a complete reaction If heat is absorbed , the reaction has gained energy so ΔH is positive

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

  16. 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

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

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