Hydrogen Worksheet
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1
2
3C
F
hem
actsheet
Number 77
The Importance of Hydrogen Bonding
To succeed in this topic you need to:
The Water Molecule
•
Be able to recall the features of hydrogen bonding as represented in this
The distribution of charges on a water molecule is important for showing
Factsheet.
how hydrogen bonds are formed. The hydrogen atoms have their electrons
•
Have a thorough knowledge of the importance of hydrogen bonding.
involved in covalent bonds with oxygen. This leaves the hydrogen nuclei
•
Be able to decide between which molecules hydrogen bonding will
exposed and gives a small positive area of charge to this area of the molecule
occur, and draw them in diagrams.
(δ+). The oxygen atom of the water molecule has all of the electrons in the
•
Have read factsheets and revised electronegativity and hydrogen bonding
molecule gathered around it, giving this part of the molecule a small negative
charge (δ-).
After working through this Factsheet, you will:
•
Have revised the chemistry relating to hydrogen bonds that are required
δ+
δ+
for AS and A2 chemistry modules including option modules.
H
H
•
Have a reference paper with you as you start to work through questions
about hydrogen bonding.
O
Electronegativity
δ-
Electronegativity is a measure of how good atoms are at attracting electrons.
The value given to any element is just an index, a way of comparing it with
In diagrams hydrogen bonds are usually drawn as dashed lines - - - - - . In
other elements. Electronegativity values have no units. You are not required
exam questions candidates are often asked to draw in the hydrogen bonds
to remember electronegativity values but it is a good idea to learn that the
between molecules
bottom left hand side of the periodic table has the lowest electronegativity
and the top right hand side (excluding group VIII) has the highest. If a
The structure of ice
covalent bond is formed between two atoms of differing electronegativity,
As water gets colder the molecules lose kinetic energy and the random
the electrons are unevenly distributed and the bond is described as polar. A
movement of particles gives way to the more regular structure of ice. In
bond between two atoms of similar electronegativity is not going to have
this structure oxygen atoms are held in position by two hydrogen bonds
areas of charge and the bond is non-polar. In the diagram below all the
and two covalent bonds. The hydrogen bonds keep the molecules further
bonds are polar except the H−P bond.
apart than in the liquid state. This accounts for the fact that ice is less dense
δ δ δ δ δ + H- - - - - - - -x- -N δ δ δ δ δ -
than water. Snow flakes have a structure based on a six pointed star which
δ+ = small amount of positive charge
relates to the position of the hydrogen bonds.
δ δ δ δ δ + H- - - - - - - - x -O δ δ δ δ δ -
δ- =small amount of negative charge
δ δ δ δ δ + H - - - - - - - - -x-F δ δ δ δ δ -
H
H
x = theoretical position of pair of
H- - - - - -x - - - - - -P
electrons
H
O
O
H
Hydrogen bonds are intermolecular forces that occur when hydrogen
H
H
is covalently bonded to another atom with a high electronegativity
H
value. In practise, H-bonds occur when hydrogen is bonded to oxygen,
O
O
fluorine or nitrogen atoms. Oxygen in a molecule where it is only joined to
H
H
H
carbon ( C=O ) form hydrogen bonds with hydrogen that is joined to
oxygen in another molecule (O-H).
O
The strength of Hydrogen Bonds
Importance of hydrogen bonding in water
Hydogen bonding is an example of an intermolecular force which can be
Physical properties
defined as weak attraction between molecules caused by weak electrostatic
charge. You do not have to give a value for the strength of hydrogen bonds in
The fact that hydrogen bonds form between water molecules accounts for
exam answers but remember that hydrogen bonds are stronger than Van der
a large number of the unusual physical properties of water;
Waals forces but nowhere near the strenghth of a typical covalent bond
(between 5 and 10% the strength)
•
Water is a liquid at room temperature. If we look at hydrogen sulphide
Hydrogen bonds are formed when propanone (which has no hydrogen
H
S, we see that it is closely related to water with the oxygen being
2
bonds) is added to water in a test tube. This is an exothermic reaction and
replaced by the next element down the group - sulphur. H
S is a gas at
2
you can feel the heat given off as the two liquids mix together and the
room temperature and we would expect water to be a gas as well. H-
hydrogen bonds are formed.
bonds are present in H
O but not in H
S.
2
2
H
•
Liquids generally contract in volume as the temperature decreases. In
H
H
C
o
the case of water however, it starts to expand at 4
C. This is because
Propanone
H
the H-bonds prevent the water molecules from coming close together.
H
C
C
As water molecules lose kinetic energy, the H-bonds form and a lattice
O
H
starts to emerge. This accounts for the fact that ice is less dense than
H
H
water and therefore floats on the water surface.
O
Water
1
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