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Intermolecular Forces: Types, Properties, and Examples, Exercises of Biochemistry

An in-depth exploration of intermolecular forces (ifs), their types, properties, and examples. Ifs are electrostatic interactions between opposite charges, weaker than chemical bonds due to smaller charges and longer distances. Various types of ifs, including van der waals forces, ion-dipole forces, dipole-dipole forces, hydrogen bonding, charge-induced dipole forces, and dispersion forces. It also discusses the properties of liquids, such as surface tension, capillary action, and viscosity, and how they relate to intermolecular forces.

What you will learn

  • How do intermolecular forces affect the properties of liquids?
  • Which intermolecular forces are most common in substances and why?
  • What are the main differences between various types of intermolecular forces?

Typology: Exercises

2021/2022

Uploaded on 09/12/2022

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12.3 Types of Intermolecular Forces (
I
F)
IFs are based on electrostatic interactions
between opposite charges, but they are much
weaker than chemical bonds for two reasons:
Smaller el. charges are involved
The distances between the el. charges are longer
Covalent radius ½ the distance between two
identical bonded atoms
van der Waals radius ½ the distance between two
adjacent, identical (nonbonded) atoms from different
molecules The van der Waals radius of an atom is
always longer than its covalent radius
IFs are also called van der Waals forces
Stronger IFs lead to higher Tband Tmvalues
Ion-Dipole forces – IFs between an ion and a
nearby polar molecule (ionic compounds like
NaCl dissolved in polar solvents like H2O)
Dipole-Dipole forces – IFs between two polar
molecules (δ+of one molecule attracts δ-of
the other)
Increase with increasing the dipole moment
Example:
CH3CH2CH3and CH3CN have almost the
same molecular weight and size. Since
CH3CN is much more polar, its dipole-dipole
forces are much stronger and its boiling and
melting points, Tband Tm, are much higher.
Hydrogen Bonding IF between molecules
containing H-atoms connected to highly
electronegative small atoms with lone pairs
like F, Oand N
Due to the very high polarity of the H-F, H-O and
H-N bonds (the δ+of His attracted to the δ-of
the lone pair of F, Oor N)
Example:
CH3OH has an O-H
bond H-bonding
Charge-Induced Dipole forces
Induced dipole a dipole induced in a particle
through a distortion of its electron cloud by an
electric field such as a nearby charge or another
dipole
Polarizability the ease with which the electron
cloud of a particle can be distorted (increases with
increasing the size of the electron cloud and
therefore with the molar mass of the particle)
¾Ion-Induced Dipole forces – IFs between an ion
and a dipole it induces in a nearby particle
¾Dipole-Induced Dipole forces – IFs between a
dipole and a dipole it induces in a nearby particle
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12.3 Types of Intermolecular Forces (

I

F

IF

s are based on electrostatic interactions

between opposite charges, but they are muchweaker than chemical bonds for two reasons:– Smaller el. charges are involved– The distances between the el. charges are longer

-^

Covalent radius

  • ½ the distance between two

identical bonded atoms

-^

van der Waals radius

  • ½ the distance between two

adjacent, identical (nonbonded) atoms from differentmolecules

The van der Waals radius of an atom is

always longer than its covalent radius

IF

s are also called

van der Waals forces

  • Stronger

IF

s lead to higher

T

b

and

T

m

values

Ion-Dipole

forces –

IF

s between an ion and a

nearby polar molecule (ionic compounds likeNaCl dissolved in polar solvents like H

2

O)

Dipole-Dipole

forces –

IF

s between two polar

molecules (

of one molecule attracts

of

the other)– Increase with increasing the dipole moment

Example:

CH

3

CH

2

CH

3

and

CH

3

CN

have almost the

same molecular weight and size. Since CH

3

CN

is much more polar, its dipole-dipole

forces are much stronger and its boiling andmelting points,

T

b

and

T

m

, are much higher.

Hydrogen Bonding

IF

between molecules

containing

H

-atoms connected to highly

electronegative small atoms with lone pairslike

F

,^

O

and

N

  • Due to the very high polarity of the

H-F

,^

H-O

and

H-N

bonds (the

δ

of

H

is attracted to the

δ

-^

of

the lone pair of

F

,^

O

or

N

Example: CH

3

OH has an O-H

bond

H-bonding

Charge-Induced Dipole

forces

Induced dipole

  • a dipole induced in a particle

through a distortion of its electron cloud by anelectric field such as a nearby charge or anotherdipole

Polarizability

  • the ease with which the electron

cloud of a particle can be distorted (increases withincreasing the size of the electron cloud andtherefore with the molar mass of the particle) ¾

Ion-Induced Dipole

forces –

IF

s between an ion

and a dipole it induces in a nearby particle ¾

Dipole-Induced Dipole

forces –

IF

s between a

dipole and a dipole it induces in a nearby particle

Dispersion

London

) forces

Instantaneous dipole

  • a dipole that results from

an instantaneous fluctuation of the electron cloudin a particle

  • The instantaneous dipole can induce a dipole in a

nearby particle and create an

induced dipole

in it

Dispersion

London

) forces –

IF

s between an

instantaneous dipole

and an

induced dipole

in a

nearby particle

  • Dispersion forces

exist between

any

particles

, but

they are

the

only

type of

IF

s for non-polar

molecules and atoms of noble gases Examples:

CH

4

, Cl

, CO 2

2

, Ar, Kr, Xe … are all

non-polar

only dispersion forces are present

  • The

strength

of the dispersion forces depends on:

  • The

polarizability

(size, molar mass) of the particles

size, molar mass

dispersion forces

  • The

shape

of the particles – dispersion forces

between rod-shaped molecules are stronger comparedto spherical molecules of the same size (rod-shapesprovide more points of contact)

Example:

The boiling points of the noble gases increase downthe group as the molar mass increases

Noble gases

He

Ne

Ar

Kr

Xe

T

b^

C)

**-

-**

Molar mass

Dispersion forces

T

b

Intermolecular

Intramolecular

Capillary action

  • the rising of liquids in

narrow tubes (capillaries) against the force ofgravity (results from the competition of twotypes of forces)^ ¾

Adhesive forces

  • between the molecules of the

liquid and the walls of the capillary ¾

Cohesive forces

  • between the molecules within

the liquid (

IF

s)

Examples:

The

H

O 2

level rises inside

a narrow glass tube

the adhesive

forces are stronger; The

Hg

level drops

inside a narrow glass tube

the

cohesive forces are stronger

Viscosity (

  • the resistance to flow

η↑

with increasing the strength of the

IF

s

η↓

with increasing the temperature (the kinetic

energy of the molecules increases relative to the IF

s of attraction between them)

Examples:CH

3

CH

OH 2

is more viscous than

CH

CH 3

CH 2

3

H-bonding

London forces

Oil and honey flow easier at higher

T