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Bond…. Bond Energy

Bond…. Bond Energy. All physical stuff is made of … Matter! (Chemicals – atoms and molecules) In Chemical reactions, atoms rearrange to form new substances (new molecules). Conservation. When new substances are made, the atoms themselves do not change.

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Bond…. Bond Energy

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  1. Bond….Bond Energy

  2. All physical stuff is made of … • Matter! (Chemicals – atoms and molecules) • In Chemical reactions, atoms rearrange to form new substances • (new molecules)

  3. Conservation • When new substances are made, the atoms themselves do not change. • They break their old bonds and form new ones. • Mass is CONSERVED! • ENERGY is stored in a chemical bonds

  4. Bond energies

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

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

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

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

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

  10. 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

  11. Examples of bond energies

  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 2H2(g) + O2(g) 2H2O(l)

  18. Calculating ΔH 2H2(g) + O2(g) 2H2O(l) Bonds broken = 2 x (H-H) + 1 x (O=O) = 2 x 436 + 1 x 498 = 872 + 498 = 1370 KJ/mol

  19. Calculating ΔH 2H2(g) + O2(g) 2H2O(l) Bonds broken = 2 x (H-H) + 1 x (O=O) = 2 x 436 + 1 x 498 = 872 + 498 = 1370 KJ/mol Bonds made = 4 x (O-H) = 4 x -464 = -1856 KJ/mol

  20. Calculating ΔH 2H2(g) + O2(g) 2H2O(l) Bonds broken = 2 x (H-H) + 1 x (O=O) = 2 x 436 + 1 x 498 = 872 + 498 = 1370 KJ/mol Bonds made = 4 x (O-H) = 4 x -464 = -1856 KJ/mol Overall Energy change = 1370 + (-1856) = -486 KJ/mol (Exothermic)

  21. Energy in the Chemical Equations Because Energy is released (exothermic), include it as a product: 2H2(g) + O2(g) 2H2O(l) + Energy • If Energy is absorbed (endothermic), you would include it as a reactant!

  22. How well have you understood?

  23. Bond energies?

  24. Energy level diagrams?

  25. Using bond energies in calculations?

  26. Let’s try Another! CH4(g) + 4Cl2(g) CCl4(g) + 4HCl(g)

  27. Homework • Homework Book pages 60 – 61 • Due TOMORROW • Lab Tomorrow as well!

  28. Combustion of methane?

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