Intermolecular Forces in Hydrogen Bonding

1. BY: Pooja N
198223
INTERMOLECULAR FORCES IN
HYDROGEN BONDING

2. • Ionic bond
• Covalent bond
• Metallic bond
INTRAMOLECULAR
FORCES

3.  Intermolecular forces are forces of attraction and repulsion between molecules that
hold molecules, ions, and atoms together.
 Attractive intermolecular forces are known as VANDER WALLS FORCES.
(1) LONDEN FORCES
(2) DIPOLE –DIPOLE INTERACTIONS
(3) DIPOLE –INDUCED DIPOLE INERACTION
Strong type of Dipole-Dipole interaction is called as HYDROGEN BONDING
They determine physical properties of molecule
like boiling point, melting point, density,
enthalpies of fusion.
INTERMOLECULAR FORCES

4. LONDON FORCES:
 They are weakest intermolecular forces.
 Force of attraction between two temporary dipole is known as LONDON
FORCES
DIPOLE –DIPOLE INTERACTION:
 Present between the molecules possessing permanent dipole.
 Partial negative portion of one molecule is attracted to the partial positive
portion of second polar molecule.
DIPOLE INDUCED DIPOLE INTERACTION:
 Attractive forces found between the polar molecules having permanent dipole
and the molecules lacking permanent dipole.
Covalent bonds > Hydrogen bonds > Dipole-Dipole interactions > London dispersion force
(400kcal) (12-16 kcal) (0.5-2 kcal) (<1 kcal)
STRENGTH OF FORCES

5. HYDROGEN BONDING
• This is a special kind of dipole-dipole interaction that occurs between a
hydrogen atom bonded to either an oxygen , nitrogen or fluorine atom
(electronegative atom).
• Partial positive end of hydrogen is attracted to partial negative end of
oxygen, nitrogen or fluorine of another molecule.
• It is relatively stronger forces of attraction between molecules and
considerable energy is required to break hydrogen bonds.

6. CONDITIONS FOR HYDROGEN BONDING
 A hydrogen bond H –Z is only effective when Z is strongly
electronegative. Higher is the electronegativity greater is the
polarisation of the molecule.
 The size of electronegative atoms should be small. Smaller is the size
greater is the electrostatic attraction. Thus only F,O and N atoms satisfy
these conditions and form hydrogen bond of higher electronegativities.
 Chlorine having same electronegativity as that of nitrogen does not
form hydrogen bond due to its large size.

7. TYPES OF HYDROGEN BONDING
 Intermolecular hydrogen bonding
 Intramolecular hydrogen bonding
INTERMOLECULAR HYDROGEN BONDING
 When hydrogen bonds formed between the two or more molecules of
the same or different compounds they are called intermolecular
hydrogen bonds.
 Hydrogen fluoride, water, alcohols, and solutions of alcohol or
ammonia in water involve intermolecular hydrogen bonding.

8. EXAMPLES OF INTERMOLECULAR HYDROGEN BONDING:
(1) WATER
 Water molecules contains a highly electronegative oxygen atom linked
to hydrogen atoms.
 Due to large electronegativity difference the oxygen end of the
molecule becomes negative, where as the H-atom becomes positive.
 The negative end of one molecule attracts the positive end of the other
and hydrogen bonding takes place.

9. (2) HYDROGEN FLUORIDE
 Fluorine , having the highest value of electronegativity, forms the
strongest hydrogen bonds.
 Orientations of various atoms and bond lengths

10. (3) AMMONIA
 It contains highly electronegative atom N linked to H-atoms.
(4) ALCOHOLS AND CARBOXYLIC ACIDS

11. INTRAMOLECULAR HYDROGEN BONDING
 These involve electrostatic force of attraction between hydrogen and an
electronegative element both present in the same molecule.

12. EFFECT OF HYDROGEN BONDING ON PHYSICAL PROPERTIES OF
COMPOUNDS
(1) Ethyl alcohol (C2H5OH) is liquid at room temperature and its boiling point
is high (780C) while dimethyl ether (CH3OCH3) is a gas its boiling point is 23.60C.
 Ethyl alcohol molecules are associated through intermolecular hydrogen
bond. So molecules become closer to each other therefor ethyl alcohol is
liquid at room temperature & more energy is required to break these forces. So
its boiling point Is more. While in Diethyl ether hydrogen bond is not present

13. (2) Alcohols are more soluble in water.
 Alcohols and water molecules are associated through intermolecuar
hydrogen bonding.
(3) The boiling point of monocarboxylic acid is higher than expected.
 Monocarboxylic acid molecules are associated through intermolecular
hydrogen bonding. So more amount of energy is required to break
these bonds

14. (4) Glucose, urea, sugar, alcohol ,glycerol etc. are covalent compound but
they are soluble in water.
• They form intermolecular hydrogen bond with water molecules.
(5) Hydrogen chloride ( HCl ) is a gas at room temperature while
Hydrogen fluoride (HF ) is liquid.
• HF molecules are associated through inter molecular hydrogen bond
and it exists as (HF)n. So molecules of HF come closer to each other .
Hence HF is liquid.

15. (6) p-nitrophenol is more soluble than o-nitrophenol in water
 p-nitrophenol forms intermolecular hydrogen bond with water and
dissolve in it. While in the case of o-nitrophenol groups are involved in
intramolecular hydrogen bond and are not free to form hydrogen bond
with water.

16. (7) Melting point of p-nitrophenol is more (i.e 114 0C) than o-nitrophenol
(i.e 450C).
 p-nitrophenol molecules are associated through intermolecular
hydrogen bond so more amount of energy is required to break these
force hence melting point of p-nitrophenol is more.
 In o-nitro phenol, groups are involved in intramolecular hydrogen bond
. So less amount of energy is required to break these forces .Hence
melting point of o- nitrophenol is less.

17. (8) Ice floats on water.
 Ice is solid even then it is lighter than water because in ice, water
molecules are associated through hydrogen bond . In Ice, water
molecules are tetrahedrally arranged, Ice has an open cage like
structure . In ice open space is present. Due to open space volume of
ice increases and density decreases, so ice floats on water.
IMPORTANCE OF HYDROGEN BONDING:
 Without hydrogen bonding water would exist as gas like hydrogen
sulphide.
 Hydrogen bonding is present in all living organisms. It is present in
tissues, organs, blood, bones in animal life. It pleays important role in
determining structure of proteins.
 Stickiness of glue and honey is also due to hydrogen bonding.

18. REFERENCES
 Principles of inorganic chemistry by Puri and Sharma.
 Organic chemistry by R L Madan