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Brown's Chemistry Theoretical
Chemistry - Equilibria - Chemical Equilibrium 8.6
8.6
The evidence and theory of hydrogen bonding in simple covalent hydrides
Revision notes for GCE Advanced Subsidiary Level AS
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Notes reversible reactions-equilibrium *
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Part 8 sub-index:
8.1 Vapour pressure origin and examples * 8.2.1
Introduction to Intermolecular Forces * 8.2.2
Detailed comparative discussion of boiling points of 8 organic molecules * 8.3
Boiling point plots for six
organic
homologous series * 8.4 Other case studies of
boiling points related to intermolecular forces * 8.5
Steam
distillation - theory and practice * 8.6 Evidence and theory
for hydrogen bonding in simple covalent hydrides *
8.7 Solubility of covalent compounds, miscible and
immiscible liquids
Advanced Equilibrium Chemistry Notes Part 1. Equilibrium,
Le Chatelier's Principle-rules * Part 2. Kc and Kp equilibrium expressions and
calculations * Part 3.
Equilibrium and industrial processes * Part 4.
Partition,
solubility product and ion-exchange * Part 5.
pH, weak-strong acid-base theory and
calculations * Part 6. Salt hydrolysis,
Acid-base titrations-indicators, pH curves and buffers *
Part 7. Redox equilibria, half-cell electrode potentials,
electrolysis and electrochemical series * Part 8.
Vapour pressure, enthalpy of vaporisation, boiling point, inter-molecular
forces, steam distillation, covalent compound solubility, miscible/immiscible
liquids
8.6
The evidence and theory of hydrogen bonding in simple covalent hydrides
The anomalous boiling points of NH3,
H2O and HF from the period 2 (group head) elements N, O and F
| |
Group 4 (14) hydride |
Group 5 (15) hydride |
Group 6 (16) hydride |
Group 7 (17) hydride |
Group 0 (18) Noble Gas |
| Period |
XH4 |
Bpt/K |
XH3 |
Bpt/K |
H2X |
Bpt/K |
HX |
Bpt/K |
atom |
Bpt/K |
| 2 |
CH4 |
112 |
NH3 |
240 |
H2O |
373 |
HF |
293 |
Ne |
27 |
| 3 |
SiH4 |
161 |
PH3 |
185 |
H2S |
212 |
HCl |
188 |
Ar |
87 |
| 4 |
GeH4 |
184 |
AsH3 |
218 |
H2Se |
232 |
HBr |
206 |
Kr |
121 |
| 5 |
SnH4 |
221 |
SbH3 |
256 |
H2Te |
271 |
HI |
238 |
Xe |
166 |
|
6 |
PbH4 |
260 |
BiH3 |
295 |
H2Po |
310 |
HAt |
277 |
Rn |
211 |

-
The graphs of boiling
point versus period for the Group 4 hydrides and the noble gases all
show the expected gradual rise in boiling point due to the greater
number of electrons in the bigger molecule, facilitating a greater
number of transient dipole - induced dipole interactions, therefore
increasing the intermolecular forces.
-
However, NH3,
H2O and HF
show considerably higher boiling points than 'expected'.
-
This is accounted by the
phenomena of hydrogen bonding.
-
Hydrogen bonding is the
strongest of the permanent dipole - permanent dipole intermolecular
force, though it is not a true ionic or covalent bond.
-
Generally speaking, it
only occurs where hydrogen is bonded to the three most
electronegative elements, namely nitrogen, oxygen and fluorine (*
there are some exceptions)
-
These elements can pull
electrons the most and will be more partially negative than other
elements.
-
Hydrogen
only has one electron, so if that electron is pulled away, there is
a just a minute proton left behind that will be particularly partially
positive.
-
Molecules with this type of
highly polar bond with have
much strong permanent dipole - permanent dipole forces, and these
are significant enough to have their
own category which we call 'hydrogen bonding' BUT this is still a
type of intermolecular force of attraction between molecules which results from a type of
bond within the molecule, namely N-H, O-H and H-F.
-
In the hydrogen bond
A-Hδ+
llll :Bδ-
, A and B are both very electronegative giving the partial charge
distribution as shown, and strong electrostatic
attraction between H and B.
-
Extra note:
-
There is
electrostatic repulsion between the two highly electronegative
elements A and B which tends to keep
the hydrogen bond linear - the delta minus is effectively the lone
pair on the highly electronegative atom.
-
Because A is very electronegative, the electron
density on the H atom is relatively small, and the repulsion between
B and H is not very strong so the A B distance can be
relatively short in the strongest hydrogen bonds and causes
repulsion between the atoms A and B, which tends to keep the
bond linear.
-
This directional nature
of the hydrogen bond helps explain why ice is quite a hard material
and proteins, enzymes and DNA etc. can have quite stable 3D
structures.
-
Check out the zig-zag
shape of (HF)n (n = 2 to 7?) in the vapour just above the
boiling point of hydrogen fluoride and the hydrogen bonds between
the bases in RNA and DNA.
-
Hydrogen bonding is
widespread
in many homologous series of organic compounds (as discussed in
section 8.2.2) such as alcohols,
carboxylic acids, amino acid derivatives like proteins, RNA and DNA
and acid amides etc.
-
Even though hydrogen
chloride is a polar molecule, the permanent dipole from is not sufficient to
give hydrogen bonding.
-
(*) Some of the
exceptions to the N-H, O-H and H-F hydrogen bonding situations.
-
Trichloromethane will
hydrogen bond with propanone in a mixture of the liquids because the
combined effect of the three quite electronegative chlorine atoms
on one carbon atom makes the CCl3 grouping behave like a very
electronegative atom.
-
Cl3δ-C-Hδ+
llll δ-O=Cδ+(CH3)2
permanent dipole - permanent dipole interactions of a hydrogen bond
nature (llll).
-
-
-
8.2.1 A summary of Van der
Waals forces and an introduction to intermolecular forces

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