2 types of Intermolecular forces

1. Van der Waal’s forces(attraction between partial + charge on one molecule & partial - charge on another molecule)

2. Hydrogen bonding

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hydrogen bond

Formation of hydrogen bonds between HF molecules.

Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of F atom of another HF.

Formation of hydrogen bonds between H2O molecules.

hydrogen bond

Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of O atom of another H2O.

Formation of hydrogen bonds between NH3 molecules.

hydrogen bond

Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of N atom of another NH3.

Class practice 27.3

Identify the hydrogen atoms of the following species that are capable of forming hydrogen bonding with water molecules.

(a) CH3OH

(b) (c)

Hydrogen bond -- between H atom (bonded to F,O,N) and lone pair of electron (on F,O,N)

(c)

(a)

(b)

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(a) CH3OH

(b)

A27.3

(c)

(b)

Boili

ng p

oint

(°C)

Period

H2O

H2SH2Se

H2TeHF

HClHBr

HINH3

PH3

AsH3 SbH3

CH4

SiH4

GeH4

SnH4

• Molecular size of hydride molecules increases down a group

• the van der Waal’s forces between molecules increases down a group

Boili

ng p

oint

(°C)

Period

H2O

H2SH2Se

H2TeHF

HClHBr

HINH3

PH3

AsH3 SbH3

CH4

SiH4

GeH4

SnH4

• High electronegativities of F, O and N.• Besides van der Waal’s forces, there are hydrogen

bonds between molecules of NH3 , H2O and HF.• However, there is weak van der Waal’s forces

between other molecules only.• Hydrogen bond is stronger than van der Waal’s

forces• A lot more energy is needed to break hydrogen

bonds between molecules • The melting and boiling pt of NH3 , H2O and HF

are much higher than expected.

Molecular size of hydride molecules increases down a group the VDW forces between molecules increases down a group

V

• Besides van der Waal’s forces, there are hydrogen bonds between H2O molecules.

• However, In H2S, H2Se, H2Te , there are weak van der Waal’s forces between molecules only.

• Hydrogen bond is stronger than van der Waal’s forces

• A lot more energy is needed to break hydrogen bonds between molecules

• The melting and boiling pt of water are much higher than expected.

Fig. 27.11 Droplets of water are caused by high surface tension that pulls water molecules into a sphere.

2. Surface tension

2. Surface tensionThere are extensive hydrogen bonds between water molecules.

The surface tension of water is much higher than that of most other common liquids.

hydrogen bond

Liquid Relative surface tension

C6H12 18.4 ( no of H bonding per molecule= 0)

CH3OH(methanol) 22.6 ( no of H bonding per molecule= 1)

CH3CH2OH 22.8 ( no of H bonding per molecule= 1)

H2O 72.3 ( no of H bonding per molecule= 2)

hydrogen bond

3. Viscosity

The resistance of aliquid to flow.

The higher the viscosity of a liquid, the more slowly it flows.

Viscosity

Strong intermolecular forces hold molecules together and do not allow them to move past one another easily.

Liquid water

molecules

Benzene molecules

Hydrogen bonds

Weak intermolecular forces

held by

held by

Liquid Relative viscosity

Benzene 1

Water 15

Table 27.4 Relative viscosities of some liquids at 25°C.

Water has high melting and boiling points, high surface tension and is more viscous than benzene.

Experiment 27.1Experiment 27.1 Experiment 27.1Experiment 27.1

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The oxygen atom of each water molecule forms hydrogen bonds with two hydrogen atoms of nearby water molecules.

Structure and bonding of ice

a water molecule

hydrogenbond

hydrogenbond

hydrogen atom

oxygen atom

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The two hydrogen atoms of each water molecule also form hydrogen bonds with oxygen atoms of nearby water molecules.hydrogen

bond

hydrogenbond

hydrogen atom

oxygen atom

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2

1

3

4

The central oxygen atom of each water molecule has a tetrahedral arrangement of two lone pairs (forming hydrogen bonds) and two bond pairs.

Fig. 28.3A water molecule canform hydrogen bonds with four other water molecules.

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In solid ice, the tetrahedral arrangement repeats over and over again, resulting in a regular open network structure of water molecules.

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hydrogen bond

empty space

a water molecule

Fig. 28.4 The structure of ice.

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Fig. 28.5 The oxygen atoms in the structure of ice are arranged in a hexagonal shape.

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Fig. 28.6The hexagonal symmetry of a snowflake reflects the structure of ice.

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ExplanationIn ice, water molecules are arranged in an orderly manner in an open network structure because of extensive hydrogen bonding.

Open network structure!

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liquid water

In this open structure, water molecules are further apart than they are in liquid water.

melts

open structure collapses

water molecules tend to pack more closely together

Think aboutThink about

ice

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Ice

presence of extensive hydrogen bonding between water molecules

regular open network structure

low densityrelatively highmelting point

High viscosity

Ethanol CH3CH2OH

hydrogen bond

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule.

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water molecule.

Ethanol CH3CH2OH

hydrogen bond

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule.

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water molecule.

High Solubility in water

Ethanol CH3CH2OH

hydrogen bond

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule.

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water molecule.

High boiling point

Ethanol CH3CH2OH

hydrogen bond

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule.

The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water molecule.

High viscosity

Ethanol is completely miscible with water, and has high boiling point. It is as viscous as water.

Ethanoic acid CH3COOH

Fig. 27.16 There are hydrogen bonds between the base pairs on the nucleic acid chains.

hydrogen bonds

Effect of hydrogen bonding on DNA

Hydrogen bonds between specific base pairs hold two nucleic acid chains of a DNA molecule together.

The presence of hydrogen bonds helps maintain the double helical shape of the molecules.

b.p / density/ viscosity of molecules

Affected by

Strength of van der Waal’s forces Presence and no. of hydrogen bonds

Molecular size

Shape Polarity of molecules

Affected by(1. Presence of lone pair electrons on F,O,N on one molecule2. Presence of H attached to F,O,N on another molecule)

No. of hydrogen bonds per molecule = minimum no. of lone pair electrons on F,O,N / no, of H attached to F,O,N

Molecular crystalsCrystals having an ordered arrangement of molecules are called molecular crystals.

table sugarice

Examples: ice, table sugar and iodine

Fig. 28.1 The crystal structure of iodine.

Iodine molecules are arranged orderly in iodine crystal.

These molecules are closely packed together, but they are still separate molecules.These molecules are held together by relatively weak intermolecular forces.

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The oxygen atom of each water molecule forms hydrogen bonds with two hydrogen atoms of nearby water molecules.

Structure and bonding of ice

a water molecule

hydrogenbond

hydrogenbond

hydrogen atom

oxygen atom

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2

1

3

4

The central oxygen atom of each water molecule has a tetrahedral arrangement of two lone pairs (forming hydrogen bonds) and two bond pairs.

Fig. 28.3A water molecule canform hydrogen bonds with four other water molecules.

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In solid ice, the tetrahedral arrangement repeats over and over again, resulting in a regular open network structure of water molecules.

Learning tipLearning tip

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hydrogen bond

empty space

a water molecule

Fig. 28.4 The structure of ice.

ice (ball)

ice (ball)

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ExplanationIn ice, water molecules are arranged in an orderly manner in an open network structure because of extensive hydrogen bonding.

Open network structure!

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liquid water

In this open structure, water molecules are further apart than they are in liquid water.

melts

open structure collapses

water molecules tend to pack more closely together

Think aboutThink about

ice

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2. Melting pointWater has a high melting temperature compared with substances of similar molecular masses.

Substance Relative molecular mass Melting point (°C)

Nitrogen 18 −210

Ammonia 17 −78

Water 18 0

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Ice

presence of extensive hydrogen bonding between water molecules

regular open network structure

low densityrelatively highmelting point

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Structure and bonding of fullerenes

Fullerenes are molecules composed entirely of carbon atoms, in the form of hollow spheres or hollow tubes.

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Buckminsterfullerene (or buckyball)The first fullerene discovered was buckminsterfullerene.

Fig. 28.10 (a) The structure of buckminsterfullerene. (b) A soccer ball.

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Each carbon atom is connected to three other carbon atoms by one double covalent bond and two single covalent bonds.

The atoms are arranged in a pattern of 20 hexagons and 12 pentagons on the surface of the sphere.

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Do you know?Do you know?

Other related molecules composed of only carbon atoms were also discovered.

Class practice 28.2Class practice 28.2

C28 C32 C50 C70

Fig. 28.11 Some of the more stable members of the fullerene family. (a) C28 (b) C32 (c) C50 (d) C70

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GraphiteGraphite

DiamondDiamond

insolublein all liquid

solvents

insolublein all liquid

solvents

C60C60

dissolves in organic

solvent

dissolves in organic

solvent

1. Solubility

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buckminsterfullerene (C60) is an electrical insulator.

2. Electrical conductivity

Substance Electrical conductivity

Graphite √ (with delocalized e-)

Diamond X

C60

X (simple molecular structure,No ions, no delocalized electrons

http://en.wikipedia.org/wiki/Diamond_cubic

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Substance Melting point (°C)

Graphite 3730 (Giant covalent structure)

Diamond 3550(Giant covalent structure)

C60

1070(Simple molecular structure)

C60 molecules are held together by weak van der Waals’ forces.

1. Melting point

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buckminsterfullerenemolecule (C60)

Buckminsterfullerenes are relatively strong and hard compared with most other molecular solids.

Fig. 28.13 The C60 molecules are packed closely together in solid state.

3. Strength and hardness