Transcript London Forces

Shape slap
o
3 6bond
pairs
and
1olone
2o
Bond
pairs
of
angle
PCl
AlCl
:PH
electrons
90
Bond
pyramidal
octahedral
tetrahedral
methane
water
angle
angles
CO
SO
SF
BH
107
120
5
3
3
Trigonal bipyramidal
6323
electron pair
To explain trends in the boiling points of noble gases and alkanes
Noble gas
Tb/K
2He
4
10Ne
27
18Ar
87
36Kr
120
54Xe
165
86Rn
211
Boiling point against atomic number
for the noble gases
250
200
Tb/K
150
100
50
0
0
20
40
Atomic number
60
80
Questions
• What is the trend in boiling points for the noble
gases?
• Explain the trend in terms of the relative strength of
the intermolecular (well in this case interatomic)
forces
• Given that gravity is a negligible intermolecular
force why is it surprising there are any forces
between these elements?
Modelling a helium atom
4 He
2
• What does this tell you about a helium atom?
• Draw a helium atom
• List three things that your drawing doesn’t
show very well
Explaining the attractions
• In the middle of a piece of paper draw the nucleus of a
helium atom
• Use two ‘bonds’ to represent the electrons
• Move the electrons around randomly until told to stop
• STOP!
• Put the ‘electrons’ on the sheet and mark (i) their positions
and (ii) the midway point between them – this is the atom’s
centre of negative charge at this moment.
• Where is the centre of positive charge?
• Repeat the movement…
• …STOP!
Explaining the attractions
• What do you notice about the centre of positive
and negative charges in your atom?
• Line up your atoms and repeat the movements…
• …STOP!
• Look at the surrounding atoms
– Where are the centres of positive and negative charge?
– What effect would they have on your electrons?
– What effect would they have on your atom?
• Repeat the movements once more…
• …STOP!
• Can you explain the trend in boiling points down
group 0?
London Forces
Some definitions
• Dipole – separation of charge
δ+
δ• Temporary dipole – a separation of charge caused by the
random movements of negative electrons around the
positive nucleus
• Induced dipole – a separation of charge caused by a nearby
dipole
• London forces (often called van der Waals’ forces): weak
attractions between all molecules caused by attractions
between temporary induced dipoles
A task
• Use these terms to explain (i) why there are forces between
helium atoms and (ii) why these forces increase with
atomic number
London Forces and alkanes
Alkane
CH4
C2H6
Tb/K
109
185
C3H8
C4H10
C5H12
231
273
309
C6H14
C7H16
C8H18
C9H20
C10H22
2-methylpentane
2,3-dimethylbutane
342
372
399
424
447
333
331
Boiling points of the alkanes
• Plot a graph of boiling point against number of
carbons for the first ten unbranched alkanes
• Add a trendline
• Describe the trend in boiling points
• Explain the trend in terms of London Forces
• Add points for the two named compounds
• Make models of the three isomers of C6H14 and use
them to suggest reasons for the difference in boiling
points.
• Use skeletal formulae in your written explanation
Spot the mistake!
• Chemistry - a volatile history(50 minutes in)