chemical structure: chemical bonding. polar bonds
DESCRIPTION
Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.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
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
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
Homonuclear bonds Hetronuclear bonds
Ethane (C2H6)Hydrazine (N2H4)
Hydrogen peroxide (H2O2)
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
Determining Bond Energies
• Consider the 2 homonuclear diatomics H2 and F2
• 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
Bond EnergiesBond Dissociation Energy (kJmol-1)
X Y X–X Y–Y ½ (X–X + Y–Y)
ExptlX–Y
H F 436 159 298 570*
H Cl 436 242 339 432*
H Br 436 193 315 366*
H I 436 151 294 298
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
Anomalous Bond EnergiesMolecule Expected
Bond Energy
(kJmol-1)
Measured Bond
Energy (kJmol-1)
E
H–F 298 570 272
H–Cl 339 432 93
H–Br 315 366 51
H–I 294 298 4
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
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 BONDSIn 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
Energ
y
2s 2s
σ*(2s)
σ*(2s)
LiLi
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
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:
H–F–+
The electrons are sitting closer to the F atom
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
Why does this happen?
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
ElectronegativityPauling 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
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
Electronegativity
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
…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
So, 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
Examples of Polar Bonds
F H
Cl H
–
+
+
+-
+-
The slight charges on each end of the molecule lead to electrostatic attraction between adjacent molecules – HYDROGEN BONDING
OH
H
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
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 ()
H–F
H–F
H–F
H–F
H–F
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
Hydrogen bonding affects the physical properties of
molecules with polar bonds
150170190210230250270
NH3 PH3 AsH3 SbH3Molecule
Boi
ling
poi
nt (K)
NH3, H2O and HF all have anomalously HIGH boiling points, since extra energy is needed to break the hydrogen bonds
15
20
25
NH3 PH3 AsH3 SbH3Molecule
Δvap
H/k
J mol
-1
150
200
250
300
350
400
H2O H2S H2Se H2TeMolecule
Boili
ng p
oint
(K)
150
200
250
300
350
HF HCl HBr HIMolecule
Boi
ling
poi
nt (K)
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
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…ATOMIC Orbitals
MOLECULAR Orbitals
H + H H2
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
F2
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
F F
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
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?
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
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
Energ
y
2p 2p
σ*(2pZ)
σ (2pZ)
π*(2py)π*(2px)
π(2py)π(2px)
2s 2s
σ*(2s)
σ (2s)F F
Consider the MO diagram of F2
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
Energ
y
2s
2s
σ*(2s)
σ*(2s)X Y
Heteronuclear Diatomic molecule MO
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
Energ
y
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
LiH molecule
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
The 2pz(F) can overlap with the 1s(H). The orbitals that do not overlap form NON-BONDING MOs
Energ
y 1s
2p
σ*
σ
2s
H F
Non-bonding
Non-bonding
HF
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 2pz on the F to form a -bond. No overlap can occur between the 1s and the 2px or 2py, as
these are pointing in the wrong direction
1s 2pz
1s 2px
H F
H F
Z Z
x
Bonding
Anti- Bonding
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
HF
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
Energ
y 1s
2p
σ*
σ
2s
H F
Non-bonding
Non-bonding
HF
H–F+ -
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
Energ
y 2s
2p
σ*
σ
2s
Li F
Non-bonding
Non-bonding
LiF
Li–F+ -
LiF
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
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
Acknowledgements
• JISC• HEA• Centre for Educational Research and
Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org