Form 6 Physical Chemistry

1
Intermolecular Forces
11.1 Polarity of Molecules 11.2 Van der Waals
Forces 11.3 Van der Waals Radii 11.4 Molecular
Crystals 11.5 Hydrogen Bonding
2
Polarity of Molecules
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Polarity of molecules
11.1 Polarity of molecules (SB p.275)
(very weak when compared with covalent bond
between atoms in molecule)
Intermolecular forces
Van der Waals forces
hydrogen bonding
4
11.1 Polarity of molecules (SB p.275)
Polarity of molecules
3 types of dipoles
Permanent dipole
Instantaneous dipole
Induced dipole
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Permanent dipole
11.1 Polarity of molecules (SB p.275)
A permanent dipole exists in all polar molecules
as a result of the difference in the
electronegativity of bonded atoms.
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Instantaneous dipole
11.1 Polarity of molecules (SB p.276)
An instantaneous dipole is a temporary dipole
that exists as a result of fluctuation in the
electron cloud.
7
Induced dipole
11.1 Polarity of molecules (SB p.276)
An induced dipole is a temporary dipole that is
created due to the influence of neighbouring
dipole (which may be a permanent or an
instantaneous dipole).
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Van der Waals Forces
9
Van der Waals Forces
11.2 Van der Waals forces (SB p.276)
Van der Waals forces
Dipole-Dipole Interaction
Dipole-Induced Dipole Interaction
Instantaneous Dipole-Induced DipoleInteraction
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Dipole-dipole interactions
11.2 Van der Waals forces (SB p.277)

• Polar molecules have permanent dipole moments.
• They tend to orient themselves in such a way that
  the attractive forces between molecules are
  maximized while repulsive forces are minimized

11
Dipole-induced dipole interactions
11.2 Van der Waals forces (SB p.277)

• When a non-polar molecule approaches a polar
  molecule (with a permanent dipole), a dipole will
  be induced in the non-polar molecule
• The dipole induced will be in opposite
  orientation to that of the polar molecule.

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Instantaneous dipole-induced dipole interactions
11.2 Van der Waals forces (SB p.277)

• The instantaneous dipole will induce a dipole
  moment in the neighbouring atom by attracting
  opposite charges
• If the ve end of the dipole is pointing towards
  a neighbouring atom, the induced dipole will then
  have its -ve end pointing towards the ve pole of
  that dipole

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Instantaneous dipole-induced dipole interactions
11.2 Van der Waals forces (SB p.278)
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11.2 Van der Waals forces (SB p.279)
Strength of van der Waals forces
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11.2 Van der Waals forces (SB p.279)
Strength of van der Waals forces

• Two factors affecting the strength of van der
  Waals forces
• Sizes of electron clouds of molecules
• Surface area of molecules

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11.2 Van der Waals forces (SB p.279)
1. Size of electron cloud
The greater the no. of e-s in a molecule
The more weakly they are held by the nucleus
The easier the instantaneous dipole can be set up
(greater van der Waals forces)
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11.2 Van der Waals forces (SB p.280)
2. Surface area of molecule
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11.2 Van der Waals forces (SB p.280)
2. Surface area of molecule
The van der Waals forces also increase with the
surface area of the molecule.
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11.2 Van der Waals forces (SB p.280)
Change of states and intermolecular forces

• 3 different states solid, liquid and gas
• Molecules in different orders in the three
  states
• ? highest order in the solid state
• ? lowest order in the gas state
• Change of state is related to the strength of
  intermolecular forces of the molecular
  substances

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11.2 Van der Waals forces (SB p.281)
Pressure-temperature diagram of carbon dioxide
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11.2 Van der Waals forces (SB p.280)
Distinguishing features of the pressure-temperatur
e diagram of carbon dioxide

• Melting point curve has a ve slope
• ? melting of CO2 becomes more difficult with
  increase in temp.
• Triple point is at 5.1 atm and 57 oC
• ? at 1 atm, CO2 sublimes
• ? No liquid state of CO2 exists under normal
  atmospheric condition
• ? Dry ice

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Van der Waals Radii
23
Van der Waals Radii
11.3 Van der Waals radii (SB p.282)

• Van der Waals forces determine the closest
  distance between argon atoms

24
11.3 Van der Waals radii (SB p.283)
Radii of iodine

• The covalent radius is one half of the distance
  between two atoms in the same molecule.
• The van der Waals radius is one half of the
  distance between two atoms in adjacent molecule.

25
11.3 Van der Waals radii (SB p.283)
Covalent and van der Waals radii of some elements
26
11.3 Van der Waals radii (SB p.284)
Structure of graphite
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Molecular Crystals
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Molecular crystals
11.4 Molecular crystals (SB p.284)
A molecular crystal is a structure which consists
of individual molecules packed together in a
regular arrangement by weak intermolecular forces.
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Iodine
11.4 Molecular crystals (SB p.285)
A unit cell of iodine crystal showing the
orientation of I2 molecules
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Dry ice
11.4 Molecular crystals (SB p.285)
A unit cell of dry ice (CO2)
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Buckminsterfullerene
11.4 Molecular crystals (SB p.286)

• New form carbon, C60

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Hydrogen Bonding
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11.5 Hydrogen bonding (SB p.286)
HF molecule
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11.5 Hydrogen bonding (SB p.286)
Relative strength of van der Waals forces,
hydrogen bond and covalent bond
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11.5 Hydrogen bonding (SB p.287)
Formation of hydrogen bonds in hydrogen fluoride
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11.5 Hydrogen bonding (SB p.287)
Formation of hydrogen bonds in water
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11.5 Hydrogen bonding (SB p.287)
Formation of hydrogen bonds in ammonia
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11.5 Hydrogen bonding (SB p.287)
Formation of hydrogen bonds in methanol
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11.5 Hydrogen bonding (SB p.287)
Essential requirements for the formation of
hydrogen bond

• H atom must be directly bonded to a highly
  electronegative atom (e.g. F, O and N)
• An unshared pair of electrons (lone pair
  electrons) is present on the electronegative
  atom

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11.5 Hydrogen bonding (SB p.288)
Pressure-temperature diagram of water

• Quite similar to that of CO2
• One exception slope of melting point curve is
  negative

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11.5 Hydrogen bonding (SB p.288)
Extraordinary features in relation to hydrogen
bond formation

• High m.p. and b.p.
• Ice melts to give liquid water with a contraction
  in volume

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11.5 Hydrogen bonding (SB p.288)
Importance of hydrogen bonding in physical
phenomena
1. Anomalous properties of the second period
hydrides
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11.5 Hydrogen bonding (SB p.288)

• Explanation
• High electronegativities of F(4.0), N(3.0) and
  O(3.5)
• Formation of intermolecular hydrogen bonds
• Hydrogen bonds are much stronger than van der
  Waals forces
• ? more energy is needed to break the hydrogen
  bonds
• ? abnormally high b.p.

44
11.5 Hydrogen bonding (SB p.289)
Enthalpy of vaporization

• Energy required to vaporize 1 mole of liquid
• Related to the strength of intermolecular forces
  that exist in the liquid

45
11.5 Hydrogen bonding (SB p.289)
Enthalpy of vaporization of Group VI hydrides

• Abnormally high enthalpy of vaporization
• ? formation of intermolecular hydrogen bonds

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11.5 Hydrogen bonding (SB p.290)
Boiling points and solubilities of alcohols

• B.p. of thiols are much lower than those of
  alcohols
• ? formation of intermolecular hydrogen bonds
  between alcohol molecules

47
11.5 Hydrogen bonding (SB p.290)
Dimerization of carboxylic acids

• When ethanoic acid is dissolved in non-polar
  solvents, the molecular mass of found to be 120
  (not 60)
• Formation of dimer

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11.5 Hydrogen bonding (SB p.291)
Hydrogen bonding in water and ice

• In water, the molecules are in constant motion.
  H bonds are formed and broken continually. The
  arrangement of molecules are thus in random.

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11.5 Hydrogen bonding (SB p.292)

• In ice, the molecular motion is of a minimum and
  the molecules are oriented in such a way that the
  max. no. of H bonds are formed. This creates an
  open structure. (density of ice lt density of
  water)

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11.5 Hydrogen bonding (SB p.293)
Hydrogen bonding in proteins

• Primary structure of protein polymer of amino
  acids

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11.5 Hydrogen bonding (SB p.293)
Hydrogen bonding in proteins

• H bonds formed between NH and CO groups of
  protein chains
• creates the secondary coiled (helix) structure of
  the protein chain

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11.5 Hydrogen bonding (SB p.293)
Hydrogen bonding in DNA

• DNA (deoxyribonnuclei acid) is present in the
  nuclei of living cells
• carries genetic information
• consists of two macromolecular strands spiraling
  round each other in the form of a double helix

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11.5 Hydrogen bonding (SB p.294)
Hydrogen bonding and the double helix of DNA
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The END
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11.2 Van der Waals forces (SB p.280)
Back
Let's Think 1
How is the enthalpy of vaporization related to
intermolecular forces of a simple molecular
substance like neon?
Answer
The enthalpy of vaporization of a substance is
the energy needed to vaporize one mole of the
substance at its boiling point. Consider a
substance like neon, which consists of single
atoms, Neon liquefies when the temperature is
lowered to 246 oC at 1 atm. The enthalpy of
vaporization of the liquid at this temperature is
1.77 kJ mol-1. Some of this energy is needed to
push back the atmosphere when the vapour forms.
The remaining energy must be supplied to overcome
the intermolecular attractions. Because each
molecule in a liquid is surrounded by several
neighbouring molecules, this remaining energy is
some multiple of a single molecule-molecule
interaction. Typically, this multiple is about 5.
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11.2 Van der Waals forces (SB p.280)
Check Point 11-2

• Comment on the relative strength of van der
  Waals forces in solid, liquid and gaseous
  bromine.

Answer

• The relative strength of van der Waals forces
  decreases in the order
• Solid bromine gt liquid bromine gt gaseous bromine
• The van der Waals forces are highly dependent
  on the distance between adjacent molecules. It
  decreases exponentially with the separation
  between the molecules. Going from solid to liquid
  and then to gaseous state, the separation between
  molecules increases, so the van der Waals forces
  become weaker and weaker.

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11.2 Van der Waals forces (SB p.280)
Check Point 11-2

• (b) Plastics are substances which have very
  strong van der Waals forces. Explain why the van
  der Waals forces are so strong in plastics.

Answer
(b) A large size of a molecule of plastics
indicates that it has a large electron cloud
which is more easily polarized. Therefore, the
molecule of plastics is more likely induced to
form an instantaneous dipole. Moreover, the
molecule of plastics has an extensive surface
area. These make plastics have very strong van
der Waals forces between the molecules.
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11.2 Van der Waals forces (SB p.280)
Back
Check Point 11-2

• Arrange the following substances in an increasing
  order of boiling point
• (i) N2, O2, Cl2, Ne
• (ii) H2, Br2, He

Answer
(c) (i) Ne lt N2 lt O2 lt Cl2 (ii) He lt H2 lt Br2
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11.3 Van der Waals radii (SB p.284)
Back
Check Point 11-3

• What is the consequence of two molecules
  approaching each other at a distance less than
  the sum of their van der Waals radii?

Answer
The electron clouds of the two molecules will
repel each other, and the distance between the
two molecules will increase until the repulsion
is just balanced by the attraction.
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11.5 Hydrogen bonding (SB p.291)
Example 11-5A
The relative molecular masses and boiling points
of five compounds are given below
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11.5 Hydrogen bonding (SB p.291)
Example 11-5A

• Ammonia, hydrogen fluoride and water have similar
  relative molecular masses, yet their boiling
  points are different. Explain why.

Answer
(a) H2O can form 2 hydrogen bonds per molecule
while NH3 and HF can only form 1 hydrogen bond
per molecule. Thus, the boiling point of water is
higher than those of NH3 and HF. Besides, as F is
more electronegative than N, the intermolecular
hydrogen bond formed between HF molecules is
stronger than that between NH3 molecules.
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11.5 Hydrogen bonding (SB p.291)
Back
Example 11-5A

• Ethanol and methanol have similar structures, yet
  their boiling points are different. Explain why.

Answer
(b) For molecules with similar structures, their
boiling points depend on their relative molecular
masses. As the relative molecular mass of ethanol
is greater than that of methanol, the boiling
point of ethanol is higher.
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11.5 Hydrogen bonding (SB p.293)
Let's Think 2
Why it takes much longer time to boil an egg on a
mountain peak?
Answer
The boiling point of water decreases with
decreasing pressure. Although water boils easily
at mountain peak, the cooking of an egg takes
longer time. It is because the amount of heat
delivered to the egg is proportional to the
temperature of water.
Back
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11.5 Hydrogen bonding (SB p.296)
Example 11-5B

• The formation of a hydrogen bond between two
  molecules RAH and RB may be represented as
• R ? A ? H B ? R
• (i) Suggest possible elements for A and B. What
  are their common features?
• (ii) In which of the following ranges would you
  expect the strength of hydrogen bonds to lie?
• 0.1 10 kJ mol-1
• 10 50 kJ mol-1
• 100 400 kJ mol-1

Answer
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11.5 Hydrogen bonding (SB p.296)
Example 11-5B

• (i) A and B can be nitrogen, oxygen or fluorine.
  All of them are highly electronegative atoms,
  thus they form highly polar molecules, resulting
  in the formation of hydrogen bonds.
• (ii) 10 50 kJ mol-1

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11.5 Hydrogen bonding (SB p.296)
Example 11-5B
(b) Benzoic acid has an apparent relative
molecular mass of 244 in hexane, but only 122 in
aqueous solution. With the aid of diagrams,
explain this phenomenon.
Answer
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11.5 Hydrogen bonding (SB p.296)
Example 11-5B
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11.5 Hydrogen bonding (SB p.296)
Example 11-5B

• Cyclohexane (C6H12) is insoluble in water whereas
  glucose (C6H12O6) is miscible with water in all
  proportions.

Answer
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11.5 Hydrogen bonding (SB p.296)
Back
Example 11-5B
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11.5 Hydrogen bonding (SB p.297)
Check Point 11-5

• Name the types of bonding or intermolecular
  forces that are broken and formed in the
  following processes.
• H2O(s) ?? H2O(g)
• 2Mg(s) O2(g) ?? 2MgO(s)
• H2(g) F2(g) ?? 2HF(g)
• 2Na(s) 2H2O(l) ?? 2NaOH(aq) H2(g)
• CH3CH2OH(l) 3O2(g) ?? 2CO2(g) 3H2O(l)

Answer
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11.5 Hydrogen bonding (SB p.297)
Back
Check Point 11-5

• Bond broken hydrogen bond
• Bonds broken metallic bond and covalent bond
• Bond formed ionic bond
• Bond broken covalent bond
• Bonds formed covalent bond and hydrogen bond
• Bonds broken covalent bond, metallic bond and
  hydrogen bond
• Bonds formed ionic bond and covalent bond
• Bonds broken covalent bond and hydrogen bond
• Bonds formed covalent bond and hydrogen bond