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Chemical Structure: Chemical Bonding. Polar Bonds
 

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.

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 Chemical Structure: Chemical Bonding. Polar Bonds Presentation Transcript

    • 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
    • Definitions…
      • A HOMONUCLEAR BOND is a bond between two identical atoms
      • A HETERONUCLEAR BOND is a bond between different atoms
      This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
    • 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 )
    • Determining Bond Energies
      • Consider the 2 homonuclear diatomics H 2 and F 2
      • The bond energy of H – F would be expected to be the mean of the bond energies of H–H and F–F
      • Is this right?
      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
    • 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
    • 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
    • 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
    • SYMMETRICAL BONDS
      • In a HOMONUCLEAR diatomic molecule, the electrons within the bond are shared equally between the two atoms – a symmetrical bond:
      • The electrons sit in molecular orbitals which lie EQUI-DISTANT from each atom
      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
    • ASYMMETRICAL BONDS
      • In a HETERONUCLEAR diatomic molecule, the electrons within the bond are NOT always shared equally between the two atoms – an asymmetrical bond.
      • In an assymetrical bond, the electrons sit closer to one atom than the other, leading to a POLAR BOND :
      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
    • Why does this happen? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
    • Electronegativity
      • Pauling defined ELECTRONEGATIVITY as:
      • “ the power of an atom in a molecule to attract electrons to itself”
      • This is an atomic property, but only applies when the atoms are in a bond
      This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
    • 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
    • … 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?
    • 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
    • Definition…
      • 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
      • The strongest hydrogen bonds involve the first row elements F, O or N
      This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
    • 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
    • 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
    • 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
    • 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
    • F 2
      • Electronic configuration of 9F is:
      • 1s2 2s2 2p5 (9 electrons)
      • 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)
      • Hence, we know we have a single bond in F2: F–F
      This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License F F
      • BUT we know that the F–F molecule has 18 electrons (2 x 9)
      • How can we arrange 18 electrons in molecular orbitals and end up with only ONE bond ?
      This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
      • SOLUTION:
      • For every bonding orbital there must be an ‘anti-bonding orbital’
      • An electron in a bonding orbital is cancelled out by an electron in an anti-bonding orbital
    • This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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  +  -
    • 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  +  -
    • 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
    • Summary This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
    • What you should know…
      • Difference between homonuclear and heteronuclear bonds
      • Explain why some heteronuclear bonds are harder than expected to break
      • How the presence of hydrogen bonding in molecules affects some of their physical properties, like boiling points
      • How to draw the MO diagram of a heteronuclear diatomic molecule, and understand how bonding, anti-bonding and non-bonding orbitals are formed
      • Use the MO diagram to determine whether the bonding is likely to be polar
      This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
    • Definitions…
      • Homonuclear bond
      • Heteronuclear bond
      • Polar bond
      • Hydrogen bond
      • Electronegativity
      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 Acknowledgements
      • JISC
      • HEA
      • Centre for Educational Research and Development
      • School of natural and applied sciences
      • School of Journalism
      • SirenFM
      • http:// tango.freedesktop.org