Chapter 11

New Way Chemistry for Hong Kong A-Level Book 11

Chapter 11Chapter 11Intermolecular ForcesIntermolecular Forces

11.111.1 Polarity of MoleculesPolarity of Molecules

11.211.2 Van der Waals’ ForcesVan der Waals’ Forces

11.311.3 Van der Waals’ RadiiVan der Waals’ Radii

11.411.4 Molecular Crystals Molecular Crystals

11.511.5 Hydrogen Bonding Hydrogen Bonding


Polarity of Molecules

Intermoleculear forcesIntermoleculear forces

Van der Waal’s forces

Van der Waal’s forces

hydrogen bonding

hydrogen bonding

(very weak when compare with covalent

bond between atoms in molecule)

electrostatic attraction between dipoles, i.e. the attraction between the +ve end of one molecule and the -ve end of another molecule

11.1 Polarity of Molecules (SB p.257)


3 types of dipoles3 types of dipoles

Permanent dipole

Permanent dipole

Instantaneous dipole

Instantaneous dipole

Induced dipole

Induced dipole


Polarity of Molecules


Permanent Dipole

A permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms.

A permanent dipole exists in all polar molecules as a result of the difference in the electronegativity of bonded atoms.



Instantaneous Dipole

An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.

An instantaneous dipole is a temporary dipole that exists as a result of fluctuation in the electron cloud.



Induced Dipole

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).

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).



Van der Waals’ Forces

Van der Waals’ forces

Van der Waals’ forces

Dipole-Dipole

Interaction

Dipole-Dipole

Interaction

Dipole-Induced Dipole

Interaction

Dipole-Induced Dipole

Interaction

Instantaneous Dipole-

Induced DipoleInteraction

Instantaneous Dipole-

Induced DipoleInteraction

11.2 Van der Waal’s Forces (SB p.258)


Dipole-Dipole Interactions

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.

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.



Dipole-Induced Dipole Interactions

When a non-poar 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 orintation to that of the polar molecule.

When a non-poar 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 orintation to that of the polar molecule.



Instantaneous Dipole-Induced Dipole Interactions

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.

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.




Strength of Van der Waals’ Forces

Type of interaction Magnitude (kJ mol-

1)

Dipole-dipole 5-25

Dipole-induced dipole 2-10

Instantaneous dipole-induced dipole

0.05-50


The greater the no. of e-s in a molecule

The greater the no. of e-s in a molecule

The more weakly they are held by the nucleus

The more weakly they are held by the nucleus

The easier the instantaneous dipole can be set up (greater van der Waals’ forces)

The easier the instantaneous dipole can be set up (greater van der Waals’ forces)


Molecule Boiling point (oC)

Helium

Neon

Argon

-269

-246

-186

Fluorine

Chlorine

Bromine

-188

-34.7

58.8

Methane

Ethane

Propane

-162

-88.6

-42.2


The van der Waals’ forces also increase with the surface area of the molecule.

The van der Waals’ forces also increase with the surface area of the molecule.


Surface Area of Molecule


Van der Waals’ Radii11.3 Van der Waal’s Radii (SB p.262)


Radii of iodine

The covalent radius is one half of the distance between two atoms in the same molecule.

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.

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

11.3 Van der Waal’s Radii (SB p.262)



Radii of some elements


sum of covalent radii

sum of covalent radii

sum of van der Waals’ radii

sum of van der Waals’ radii


Structure of graphite


Molecular Crystals

A molecular crystal is a structure which consists of individual molecules packed together in a regular arrangement by weak intermolecular forces.

A molecular crystal is a structure which consists of individual molecules packed together in a regular arrangement by weak intermolecular forces.

11.4 Molecular Crystals (SB p.264)


very +vevery +veF being very

electronegativeF being very

electronegative

11.5 Hydrogen Bonding (SB p.264)

HF molecule

F atom being small enough to approach very close to the H atom in the neighbouring molecule

F atom being small enough to approach very close to the H atom in the neighbouring molecule



The relative strength of van der Waals’ forces, hydreogen bond and covalent bond

Phenomenon Energy involved

(kJ mol-1)

Forces overcome

Sublimation of solid helium

0.11 Van der Waals’ forces

Sublimation of

ice

46.90 Hydrogen bonds

Dissociation of hydrogen molecules

436.00 Cocalent bonds



Formation of hydrogen bonds in hydrogen fluoride



Formation of hydrogen bonds in water


Formation of hydrogen bonds in ammonia11.5 Hydrogen Bonding (SB p.265)



Formation of hydrogen bonds in methanol


Experimental Determination of the Strength of Hydrogen Bond

trichloromethane

Cl

Cl

Cl

Cl

very +vevery +ve

?Any H-bond formed?



Experimental Determination of the Strength of Hydrogen Bond

ethyl ethanoate


?Any H-bond formed?


Experimental Determination of the Strength of Hydrogen Bonds

?Any H-bond formed?

(YES!)

H bond formed between trichloromethane & ethyl ethanoate

How strong is it?

Hint: When you mix the 2 liquids together, what will happen?Hint: When you mix the 2 liquids together, what will happen?



Intramolecular Hydrogen bonding

Butenedioic acid

C

C

O

OH

H

C

H

C

O

OH

CC

H

C

O

OH

C

O

OH

H

cis- butenedioic acid trans- butenedioic acidm.p. = 1300C m.p. = 2900C

cis-trans isomers(geometric isomers)

cis-trans isomers(geometric isomers)

C C

O

OH

H

C

H

C

O

OH




m.p. = 1300C

cis- butenedioic acid trans- butenedioic acid

m.p. = 2900C

Owing to the formation of intramolecular H bonds, cis-butenedioic acid forms less extensive intermolecular H bonds with neighbouring molecules.

Owing to the formation of intramolecular H bonds, cis-butenedioic acid forms less extensive intermolecular H bonds with neighbouring molecules.




NO2

OH

2-nitrophenol

NO2

OH

4-nitrophenol

Can you match the two compounds with the following m.p.’s?

m.p. = 2160Cm.p. = 2160C m.p. = 2590Cm.p. = 2590C




m.p. = 2160Cm.p. = 2160C m.p. = 2590Cm.p. = 2590C



Anomalous Properties of the Second Period Hydrides

Molecular mass Molecular mass

Van der Waals’ forces (b.p. )

Van der Waals’ forces (b.p. )

There must be some type of intermolecular force (which is much stronger than van der Wa

als’ forces) in NH3, H2O & HF

There must be some type of intermolecular force (which is much stronger than van der Wa

als’ forces) in NH3, H2O & HF

(Hydrogen bonding)



Essential requirements for the formation of a hydrogen bond

1. A hydrogen atom must be directly bonded to a highly electronegative atom (F, O, N).1. A hydrogen atom must be directly bonded to a highly electronegative atom (F, O, N).

2. An unshared pair of electrons (lone pair electrons) on the electronegative atom.2. An unshared pair of electrons (lone pair electrons) on the electronegative atom.



Boiling Points and Solubilities of Alcohol11.5 Hydrogen Bonding (SB p.268)


Organic compounds are usually insoluble in water,e.g. ethane (C2H6) or chloroethane (C2H5Cl)Organic compounds are usually insoluble in water,e.g. ethane (C2H6) or chloroethane (C2H5Cl)

But alochols of low molecular mass are soluble in water, e.g methanol (CH3OH) and ethanol (C2H5OH).

But alochols of low molecular mass are soluble in water, e.g methanol (CH3OH) and ethanol (C2H5OH).


Boiling Points and Solubilities of Alcohol


Dimerization of Carboxylic Acids

In vapour phase or in organic solvents, carboxylic acids (alkanoic acids) exist as dimers.In vapour phase or in organic solvents, carboxylic acids (alkanoic acids) exist as dimers.

A dimer of ethanoic acid



Hydrogen Bonding in Water and Ice

H bonding in water

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

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



H-bonding in ice

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 < density of water)

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 < density of water)



Hydrogen Bonding in Proteins

The primary structure of a protein consists of a sequence of amino acids.

The primary structure of a protein consists of a sequence of amino acids.




Hydrogen Bonding in Proteins

H bonds formed bewteen NH and CO groups of protein chains. This creates the secondary coiled (helix) structure of the protein chain.

H bonds formed bewteen NH and CO groups of protein chains. This creates the secondary coiled (helix) structure of the protein chain.


Hydrogen Bonding in DNA

A model of the DNA helix

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



H bonding in the double helix of DNA



The END

Chapter 11

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