Boiling Points and Intermolecular Forces
Intermolecular forces
https://www.youtube.com/watch?v=S8QsLUO_tgQ
Explaining trends and anomalies in boiling points
Boiling points of halogens
•
Fluorine (F
2
)
Bp = -188
o
C
•
Chlorine (Cl
2
)
Bp = - 34
o
C
•
Bromine (Br
2
)
Bp = 59
o
C
•
Iodine (I
2
)
Bp = 184
o
C
Question: What is the reason for
the trend?
18e-
34e-
59e-
106e-
•
The reason for the increase in Bp is that,
as the size of the entire electron cloud
increases so does the size of the
instantaneous diplole moment.
•
Stronger forces of attraction between
molecules.
•
So a greater amount of energy is needed
to overcome these forces to vaporise.
Boiling points of isomers with same formula
Task: Use molymods to make models of pentane and 2,2 dimethyl propane.
•
Pentane
Bp = 36
o
C
•
2,2 dimethyl propane
Bp = 10
o
C
Question: Why can two non-polar molecules with the same molecular formula have
different Bp’s?
•
Straight chain isomers are attracted more strongly to their neighbours because
more atoms/electron clouds are in close enough proximity for instantaneous
dipole forces to act.
•
Therefore more energy is needed to overcome these attractive forces.
Boiling points of molecules with similar Mr
Task: Draw Lewis diagrams then 3D representations of BCl
3
and NCl
3
•
BCl
3
Bp = 12.6
o
C
M= 117 g/mol
•
NCl
3
Bp = 71
o
C
M = 120 g/mol
Question: Why does NCl
3
have a much higher Bp than BCl
3
?
•
BCl
3
is non-polar because, despite having polar bonds, it has a trigonal
symmetrical trigonal planar shape where the bond dipoles cancel.
•
NCl
3
is polar because it has a non-symmetrical triangular pyramidal
shape where bond dipoles do not cancel.
•
Since polar molecules have stronger dipole-dipole attractive forces,
NCl
3
has higher Bp.
Boiling points of polar molecules
Task: Complete the Practical 7.1 on p95-96 of Continuing Chemistry
Why do HF, H
2
0 and NH
3
have anomalously high Bp’s?
•
All three of these molecules have strong hydrogen bonding between
molecules.
•
A large amount of energy is required to break these forces of
attraction so they have a higher Bp than they would if they only had
ordinary dipole-dipole interactions.
Exploring the relationship between intermolecular forces and boiling points, this content discusses trends and anomalies in boiling points of halogens, isomers with the same molecular formula, molecules with similar Mr, and polar molecules. It explains how molecular size, structure, and interactions affect boiling points, highlighting concepts such as dipole moments, hydrogen bonding, and asymmetrical shapes.
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Presentation Transcript
Intermolecular forces https://www.youtube.com/watch?v=S8QsLUO_tgQ Explaining trends and anomalies in boiling points
Boiling points of halogens Fluorine (F2) Chlorine (Cl2) Bromine (Br2) Iodine (I2) Bp = -188oC Bp = - 34oC Bp = 59oC Bp = 184oC 18e- 34e- 59e- 106e- The reason for the increase in Bp is that, as the size of the entire electron cloud increases so does the size of the instantaneous diplole moment. Stronger forces of attraction between molecules. So a greater amount of energy is needed to overcome these forces to vaporise. Question: What is the reason for the trend?
Boiling points of isomers with same formula Task: Use molymods to make models of pentane and 2,2 dimethyl propane. Pentane Bp = 36oC 2,2 dimethyl propane Bp = 10oC Question: Why can two non-polar molecules with the same molecular formula have different Bp s? Straight chain isomers are attracted more strongly to their neighbours because more atoms/electron clouds are in close enough proximity for instantaneous dipole forces to act. Therefore more energy is needed to overcome these attractive forces.
Boiling points of molecules with similar Mr Task: Draw Lewis diagrams then 3D representations of BCl3 and NCl3 BCl3 Bp = 12.6oC M= 117 g/mol NCl3 Bp = 71oC M = 120 g/mol Question: Why does NCl3 have a much higher Bp than BCl3? BCl3 is non-polar because, despite having polar bonds, it has a trigonal symmetrical trigonal planar shape where the bond dipoles cancel. NCl3 is polar because it has a non-symmetrical triangular pyramidal shape where bond dipoles do not cancel. Since polar molecules have stronger dipole-dipole attractive forces, NCl3 has higher Bp.
Boiling points of polar molecules Task: Complete the Practical 7.1 on p95-96 of Continuing Chemistry Why do HF, H20 and NH3 have anomalously high Bp s? All three of these molecules have strong hydrogen bonding between molecules. A large amount of energy is required to break these forces of attraction so they have a higher Bp than they would if they only had ordinary dipole-dipole interactions.
