Chapter 12
Pages 64 - 65
 Why do snow flakes form 6
sided shapes?
Hydrogen bonding
Learning outcomes
• Describe hydrogen bonding between molecules
containing –OH and –NH groups.
• Describe and explain the anomalous properties of
water resulting from hydrogen bonding.
 https://www.youtube.com/watch?v=bOchZanTWHE&fea
ture=plcp
A hydrogen bond
 A special case of dipole-dipole interaction
 Hydrogen bonding affects molecules containing O-H and
N-H bonds (and F-H but you don’t need to know this for
your exam)
 These bonds are polar with permanent dipoles.
 It is a particularly strong attractions (the strongest of all
of the intermolecular forces you need to know for your
exam)
Why?
 Hydrogen atoms are very small and often carry a small
positive charge
 It can approach very closely to a small negative atoms
(often a lone pair or a very electronegative atom on a
different molecule)
Key features for
hydrogen bonding
Hydrogen bonds
 So, as in all other dipole-
dipole interactions, there is
attraction between the
lone pair on the negative
atom and the small
positive hydrogen atom
Why do snowflakes form
six sided shapes
 Because ice is an open
network of water molecules
 Each molecule has four
bonds – two covalent and
two hydrogen
 Hydrogen bonds are slightly
longer
 The open structure is made
up of rings of 6 oxygen
atoms – hence snowflakes
shape!
The weirdness of water
 Catalyst article
 Read the article on water
 What are some of its unique properties?
Exceptional properties of
water
 The hydrogen bond in water is 5% of the strength of the
O-H covalent bonds but this is still enough to have an
effect on the physical properties
 Those properties affected are;
Ice is less dense than water
2. Water has high mp and bp
1.
WEIRD!
WEIRD!
Can you explain points 1 and 2 in your own words?
Ice is less dense than
water
 Ice has an open lattice with the hydrogen bonds holding
the water molecules apart
 Ice melts the hydrogen bonds collapse allowing the water
molecules closer to each other
High m.p. and b.p.
 Compare water with a similar molecule hydrogen
sulfide.
 Water has a boiling point of 373K whereas hydrogen
sulfide has a boiling point of 212K. Much lower even
though its relative molecular mass is much greater!
 These hydrogen bonds are an extra force over and
above van der Waals and these forces have to be
overcome in order to melt or boil water.
Other properties
 Insects walking on water are actually using a raft of
hydrogen bonds!
 Clever little bugs!
I love
bonding
me
Hydrogen bonding in
biological molecules
 Really important in organic compounds containing O-H
and N-H bonds (we’ll meet lots of these later in the year,
oh yeah!)
 Hydrogen bonds play essential roles in many molecules
e.g.
 Shape determination (e.g. Protein molecules)
 Holding together the double helix shape of DNA
 In DNA, three hydrogen
bonds link C and G
 In DNA, two hydrogen
bonds link A and T
 Which is all pretty
important for all of us...