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Bond Energies. James Bond Energies

Bond Energies. James Bond Energies. What Is Bond Energy?. Think of a bond like a spring holding two atoms together: If you pull them apart a little bit, they will spring back together If you squish them together a little bit, they will push back apart

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Bond Energies. James Bond Energies

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  1. Bond Energies. James Bond Energies

  2. What Is Bond Energy? Think of a bond like a spring holding two atoms together: If you pull them apart a little bit, they will spring back together If you squish them together a little bit, they will push back apart There is a natural length to the spring (or bond) that it wants to be. BUT! If you pull them hard enough apart, you can break the spring. Bond energy is how much energy you have to use to pull the atoms apart and break the bond.

  3. Bond Energy In Graph Form This is called the Lennard-Jones potential (you don’t need to know the name): (The values on the y-axis are somewhat arbitrary—only the difference between them matters) The atoms want to be 74 pm apart—at the bottom of the hill. This is the bond length. If we push them closer together (point 4) or pull them farther apart (point 2), they will spring back to 74 pm apart (point 3). But if we put 432 kJ/mol in, we can pull them all the way to point 1 and they are no longer attached to each other. This value is the bond energy.

  4. Bond Energy In Graph Form Some bonds are shorter/longer, and some are weaker/stronger The X2 bond is very short but not very strong: it is two very small atoms that can get close together, but only make one bond (like H2) The Z2 bond is much longer, but not much stronger. This would be larger atoms making one bond (like Cl2), or maybe making an unusually weak double bond (like O2) Y2 is shorter than Z2 and very strong, so this is either a very strong double bond, or a triple bond (like N2).

  5. Bond Energy In Graph Form Of course, if we can put 432 kJ/mol in to pull these atoms apart and break the bond, then we can take two atoms apart, let them bond, and get 432 kJ/mol back out. Think of it as “rolling down the hill”.

  6. KEY STATEMENT! Breaking bonds is endothermic (energy in) Making bonds is exothermic (energy out)

  7. Using Bond Energies What’s the point? If we have a reaction that uses and makes only covalent compounds, we can find DH using bond energies. Conveniently, people have put together huge tables showing the bond energies for all kinds of different bonds (these are always given as positive—breaking the bond)

  8. Using Bond Energies What’s the point? If we have a reaction that uses and makes only covalent compounds, we can find DH using bond energies. Conveniently, people have put together huge tables showing the bond energies for all kinds of different bonds (these are always given as positive—breaking the bond) Less conveniently, these values are only general estimations—not every single Carbon—Hydrogen bond in the world is the same as all the others. Our DH will be an approximation.

  9. Example Bond Energy Table Pay close attention to units here. Some tables will be in kcal/mol instead of kJ/mol. People like calories here because the most common bonds are all around 100 kcal/mol.

  10. How to do the Math We’ll start with a simple reaction—combustion of methane: CH4 + 2O2 CO2 + 2H2O Hess’s Law says any path from methane and oxygen to carbon dioxide and water has the same energy.

  11. CH4 + 2O2 CO2 + 2H2O Breaking all the reactants apart: We broke (values from earlier table): 4 C-H bonds: 413 kJ/mol each * 4 = 1652 kJ/mol 2 O=O bonds: 495 kJ/mol each * 2 = 990 kJ/mol(making sure to use the double bond value) So in total we had to put in 2642 kJ/mol to break the atoms apart.

  12. CH4 + 2O2 CO2 + 2H2O Putting the products together We made (values again from earlier table. Negative because we’re making bonds): 4 O-H bonds: -463 kJ/mol each * 4 = -1852 kJ/mol 2 C=O bonds: -799 kJ/mol each * 2 = -1598 kJ/mol(making sure to use the double bond value) So in total we had got out -3450 kJ/mol by putting atoms together.

  13. CH4 + 2O2 CO2 + 2H2O All together now: 2642 kJ/mol put in to break the bonds -3450 kJ/mol out by making the new bonds So overall our process has a DH = -808 kJ/mol and is exothermic.

  14. CH4 + 2O2 CO2 + 2H2O All together now: 2642 kJ/mol put in to break the bonds -3450 kJ/mol out by making the new bonds So overall our process has a DH = -808 kJ/mol and is exothermic. Actual DH = -882 kJ/mol, so we have about 10% error. This is again because the bond energies are averages for all bonds of that type, not necessarily these specific ones which may be a little stronger or weaker. Not bad for a quick method, though.

  15. Another Example CH3Br + HI CH3I + HBr Broken: 3 C-H = 1239 kJ/mol 1 C-Br = 276 kJ/mol 1 H-I = 299 kJ/mol Made: 3 C-H = -1239 kJ/mol 1 C-I = -240 kJ/mol 1 H-Br = -366 kJ/mol DH = -31 kJ/mol

  16. Another Example CH3Br + HI CH3I + HBr Broken: 1 C-Br = 276 kJ/mol 1 H-I = 299 kJ/mol Made: 1 C-I = -240 kJ/mol 1 H-Br = -366 kJ/mol It’s totally ok to recognize that nothing changes with the C-H bonds and just leave them off the lists. However, I find that people make more mistakes when they do this for complicated structures/reactions.

  17. Summary What do you need to know? • Breaking bonds is endothermic. Making bonds is exothermic. • How to recognize bond length and bond energy from the graphs. • How to compare two bonding graphs. • Be able to use bond energies to approximate DH.

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