Starter
• Define the groups of compounds shown by the three
lines
• What could you label the x-axis as?
• Describe the pattern shown in the three lines and what
types of bonding are present?
Starter answers
•
•
•
•
Red = Group 5 hydrides
Green = Group 6 hydrides
Blue = Group 7 hydrides or Hydrogen halides
X-axis could be labelled as “Period” (2,3,4 and 5)
• NH3, H2O, and HF all polar.
• The rest are non-polar but b.p. increases due to
increased spontaneous dipole-induced dipole
(Van der Waals) bonding
Dipole-Dipole Interactions &
Hydrogen Bonding
Objectives
Be able to...
• Apply knowledge of electronegativity to
determine polarity of covalent bond
• Describe permanent dipole-induced dipoles
• Define hydrogen bonding as a special type of
permanent dipole-induced dipole
• Be able to recognise biochemical and material
examples of hydrogen bonding
Specification...
Where this comes from...
Intermolecular Forces
• Van der Waals forces (instantaneous dipoleinduced dipole interactions) exist between all
molecules whether polar or non polar
• They are:
▫ Weak intermolecular forces
▫ Caused by very small or instantaneous
dipoles in molecules , caused by the
random movement of electrons within
electron clouds
Induced Dipoles
• Movement of electrons produces an oscillating
dipole (temporary) – this oscillating dipole
induces a dipole in a neighbouring molecule
which is induced onto further molecules –
induced dipoles attract one another.
• As electrons are constantly moving, dipoles are
always being created and destroyed,
however the overall effect is constant and the
atoms are attracted to each other.
Factors which affect the strength:
• The greater the number of electrons in each
molecule
• The larger the oscillating and induced
dipoles
• The greater the attractive forces between
molecules
• How well the molecules fit together in 3D
space... Tetris theory!
Questions
• Grades C-D
• Grades A-B
Permanent Dipole-Dipoles
• What is a polar molecule?
▫ (a molecule which atoms have significant
differences in their electronegativity and a dipole
is created)
• + and - charges on polar molecules cause
weak electrostatic forces of attraction
between molecules E.g. in HCl(g)
H-Cl….. H-Cl….. H-Cl
• Although they are weak forces they are still
much greater then VdWaals
Hydrogen Bonding
• Special type of permanent dipole – dipole
interaction
• Hydrogen bonding can only occur when Hydrogen
is bonded covalently to Fluorine, Nitrogen, or
Oxygen.
• Hydrogen has a high charge density as it is so
small and F, N & O are highly electronegative.
• The bond is highly polarised
• Molecules which have hydrogen bonding are
usually organic
Examples of H-Bonding
Effects of H-Bonding
• Hydrogen bonding has a huge effect on the
properties of substances. They are soluble in
water and have higher boiling and freezing
points than non-polar of similar size.
• Why does ice float?
▫ (less dense than liquid water due to rigid Hydrogen
bonds which are relatively long, molecules held in an
open lattice structure)
H-bonding and Ice
• Ice has relatively high melting point, and water a
relatively high boiling point
• Relatively strong H-bonds between H2O
molecules
• Extra forces on top of VdWaals
• The extra intermolecular forces from H-bonds
also explains high surface tension and viscosity
in water
Exam Question
• State and explain the overall trend in boiling
points for Group 6 Hydrides. Why is water’s
boiling
point
higher
than
expected
comparison to the other group 6 hydrides?
[Total 5 marks]
in
Exam Answers
1. Except for water there is an increase in boiling
point going down the group
2. The increase in the size/Mr and number of
electrons…
3. Leads to an increase in Van der Waals
4. More energy…
5. Is needed to break the hydrogen bonds between
water molecules
Three types of intermolecular bonding...
• Instantaneous dipole-induced dipole (otherwise
known as Van der Waals)
• Permanent dipole-induced dipole
• Permanent dipole-Permanent dipole (of which
Hydrogen bonding is an extreme example)
Three types of intermolecular bonding...
Home Learning
• Complete worksheet on Intermolecular Bonding
for next lesson.
Objectives
Be able to...
• Apply knowledge of electronegativity to
determine polarity of covalent bond
• Describe permanent dipole-induced dipoles
• Define hydrogen bonding as a special type of
permanent dipole-induced dipole
• Be able to recognise biochemical and material
examples of hydrogen bonding