Chemical Structure: Chemical Bonding. Polar Bonds

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Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

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Chemical Structure: Chemical Bonding. Polar Bonds

  1. 1. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Chemical Bonding 3 POLAR BONDS University of Lincoln presentation
  2. 2. Definitions… <ul><li>A HOMONUCLEAR BOND is a bond between two identical atoms </li></ul><ul><li>A HETERONUCLEAR BOND is a bond between different atoms </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  3. 3. Homonuclear & Heteronuclear bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Homonuclear bonds Hetronuclear bonds Ethane (C 2 H 6 ) Hydrazine (N 2 H 4 ) Hydrogen peroxide (H 2 O 2 )
  4. 4. Determining Bond Energies <ul><li>Consider the 2 homonuclear diatomics H 2 and F 2 </li></ul><ul><li>The bond energy of H – F would be expected to be the mean of the bond energies of H–H and F–F </li></ul><ul><li>Is this right? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  5. 5. Bond Energies This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond Dissociation Energy (kJmol -1 ) 298 294 151 436 I H 366* 315 193 436 Br H 432* 339 242 436 Cl H 570* 298 159 436 F H Exptl X–Y ½ (X–X + Y–Y) Y–Y X–X Y X
  6. 6. Anomalous Bond Energies This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 4 298 294 H–I 51 366 315 H–Br 93 432 339 H–Cl 272 570 298 H–F  E Measured Bond Energy (kJmol -1 ) Expected Bond Energy (kJmol -1 ) Molecule
  7. 7. Why are some heteronuclear bonds much stronger than expected? ? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  8. 8. SYMMETRICAL BONDS <ul><li>In a HOMONUCLEAR diatomic molecule, the electrons within the bond are shared equally between the two atoms – a symmetrical bond: </li></ul><ul><li>The electrons sit in molecular orbitals which lie EQUI-DISTANT from each atom </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2s 2s σ * (2s) σ * (2s) Li Li
  9. 9. ASYMMETRICAL BONDS <ul><li>In a HETERONUCLEAR diatomic molecule, the electrons within the bond are NOT always shared equally between the two atoms – an asymmetrical bond. </li></ul><ul><li>In an assymetrical bond, the electrons sit closer to one atom than the other, leading to a POLAR BOND : </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License H–F  –  + The electrons are sitting closer to the F atom
  10. 10. Why does this happen? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  11. 11. Electronegativity <ul><li>Pauling defined ELECTRONEGATIVITY as: </li></ul><ul><li>“ the power of an atom in a molecule to attract electrons to itself” </li></ul><ul><li>This is an atomic property, but only applies when the atoms are in a bond </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  12. 12. Electronegativity This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The higher the electronegativity, the stronger the ‘pulling’ power of the atom within a bond O 3.4 F 4.0 N 3.0 C 2.6 Cl 3.2 H 2.2 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.8 Mg 1.3 Be 1.6 Ca 1.0 Sr 0.9 Ba 0.9 S 2.6 P 2.2 B 2.0 Si 1.9 Al(III) 1.6 Se 2.6 Br 3.0 As(III) 2.2 Ge(IV) 2.0 I 2.7 Te 2.1 Sb 2.1 Ga(III) 1.8 Sn(IV) 2.0 In(III) 1.8 At 2.2 Po 2.0 Bi 2.0 Pb(IV) 2.3 Tl(III) 2.0
  13. 13. … When electrons are held tightly by an atom in a bond, due to the high electronegativity of that atom, the bond is much harder to break This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License So, why are some heteronuclear bonds much stronger than expected?
  14. 14. Examples of Polar Bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License  –  +  +  +  -  +  - The slight charges on each end of the molecule lead to electrostatic attraction between adjacent molecules – HYDROGEN BONDING
  15. 15. Definition… <ul><li>A HYDROGEN BOND is an interaction between a hydrogen atom attached to an electronegative atom, and an electronegative atom which possesses a lone pair of electrons </li></ul><ul><li>The strongest hydrogen bonds involve the first row elements F, O or N </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  16. 16. HYDROGEN BONDING ( ) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License H – F H – F H – F H – F H – F
  17. 17. Hydrogen bonding affects the physical properties of molecules with polar bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License NH 3 , H 2 O and HF all have anomalously HIGH boiling points, since extra energy is needed to break the hydrogen bonds
  18. 18. Can Molecular Orbital Theory account for polar bonds? ? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  19. 19. A quick recap… This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License ATOMIC Orbitals MOLECULAR Orbitals H + H H 2
  20. 20. F 2 <ul><li>Electronic configuration of 9F is: </li></ul><ul><li>1s2 2s2 2p5 (9 electrons) </li></ul><ul><li>The F atom needs 1 more electron to give it a full valence shell (8 outer electrons)– it does this by forming a single covalent bond (in this case with another F atom) </li></ul><ul><li>Hence, we know we have a single bond in F2: F–F </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License F F
  21. 21. <ul><li>BUT we know that the F–F molecule has 18 electrons (2 x 9) </li></ul><ul><li>How can we arrange 18 electrons in molecular orbitals and end up with only ONE bond ? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License <ul><li>SOLUTION: </li></ul><ul><li>For every bonding orbital there must be an ‘anti-bonding orbital’ </li></ul><ul><li>An electron in a bonding orbital is cancelled out by an electron in an anti-bonding orbital </li></ul>
  22. 22. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  23. 23. Consider the MO diagram of F 2 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2p 2p σ * (2 p Z ) σ (2 p Z ) π * (2p y ) π * (2p x ) π (2p y ) π (2p x ) 2s 2s σ * (2 s ) σ (2 s ) F F
  24. 24. Heteronuclear Diatomic molecule MO This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Homonuclear MO diagrams are symmetrical . Heteronuclear MOs are asymmetrical – the energies of equivalent atomic orbitals are DIFFERENT Energy 2s 2s σ * (2s) σ * (2s) X Y
  25. 25. LiH molecule This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2s 2s σ * (2s) σ * (2s) Li H Only valence orbitals shown. The 1s (H) and 2s (Li) overlap to form the  and  * molecular orbitals
  26. 26. HF This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The 2p z (F) can overlap with the 1s(H). T he orbitals that do not overlap form NON-BONDING MOs Energy 1 s 2p σ * σ 2s H F Non-bonding Non-bonding HF
  27. 27. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The 1s orbital on the H overlaps with the 2p z on the F to form a  -bond. No overlap can occur between the 1s and the 2p x or 2p y , as these are pointing in the wrong direction 1 s 2 p z 1 s 2 p x H F H F Bonding Anti- Bonding
  28. 28. HF This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The electrons are sat closer to the F atomic orbitals than the H atomic orbitals. Therefore it is predicted that the H–F bond would be POLAR Energy 1 s 2p σ * σ 2s H F Non-bonding Non-bonding HF H–F  +  -
  29. 29. LiF This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2 s 2p σ * σ 2s Li F Non-bonding Non-bonding LiF Li–F  +  -
  30. 30. Hence, the MO theory can predict POLAR bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  31. 31. Summary This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  32. 32. What you should know… <ul><li>Difference between homonuclear and heteronuclear bonds </li></ul><ul><li>Explain why some heteronuclear bonds are harder than expected to break </li></ul><ul><li>How the presence of hydrogen bonding in molecules affects some of their physical properties, like boiling points </li></ul><ul><li>How to draw the MO diagram of a heteronuclear diatomic molecule, and understand how bonding, anti-bonding and non-bonding orbitals are formed </li></ul><ul><li>Use the MO diagram to determine whether the bonding is likely to be polar </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  33. 33. Definitions… <ul><li>Homonuclear bond </li></ul><ul><li>Heteronuclear bond </li></ul><ul><li>Polar bond </li></ul><ul><li>Hydrogen bond </li></ul><ul><li>Electronegativity </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  34. 34. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acknowledgements <ul><li>JISC </li></ul><ul><li>HEA </li></ul><ul><li>Centre for Educational Research and Development </li></ul><ul><li>School of natural and applied sciences </li></ul><ul><li>School of Journalism </li></ul><ul><li>SirenFM </li></ul><ul><li>http:// tango.freedesktop.org </li></ul>

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