1. 11.1 Intermolecular Forces Keeping Matter Together
Nature’s Forces

2. Phases of Matter: Terminology
Energy is required for phase
change to occur.
Solid-Liquid-Gas
Triangle

3. Heating Cooling Curve From Steam to Ice and Vice-versa 4.184 J g °
0.50 cal
1.84 J
g °
0.43 cal J
g °
1 cal
g o
6.01 kJ
mol
80 cal g
40.67 kJ mol
540 cal g
How much energy of 1g H2O 100°C to 0°C ?
=720cal

4. Intermolecular Forces
At the molecular level:
Molecules or matter is held together by attractive force “glue” called intermolecular forces
Energy added (K.E. increase)

5. Keeping Matter together
Intramolecular Forces -
Force which keeps integrity of molecule together, i.e., bonds or electrostatic bonding.
Intermolecular Forces -
Attractive force between molecules. Responsible for keeping matter in solid or liquid phase

6. The Forces be with You 2 Basic types of Intramolecular Force
Ion - ion - Electrostatic attraction
Covalent Bonds - Mutual sharing of electrons
4 Basic types of Intermolecular Force*
1. Ion - dipole : Ion is attracted to polar molecule
2. dipole - dipole: Polar molecules attracted to each other.
3. dipole - induce dipole: Polar molecules attracted to nonpolar molecules.
4. induce dipole -induce dipole (Van der Waal’s forces – also called London dispersion forces) nonpolar molecules attraction for each other due to electron cloud distortion.
* plus one

7. Relative Strength Interaction Example Energy
ion- ion Na+ Cl kJ/mol
Covalent Bonds H - H kJ/mol
ion-dipole (I-D) Na+ H2O kJ/mol
dipole - dipole (D-D) ICl ICl kJ/mol
dipole - induce dipole (D-ID) HCl O kJ/mol
Van der Waal N2 N kJ/mol
• H-Bond ( kJ/mol

8. Ion - Ion Covalent Bonds
Ion - Ion: Electrostatic attraction between ions
Covalent Bonds: Bond between atoms as a result of electrons sharing.
Bond Energy: = 926 kJ/mol
Bond Energy: = 159 kJ/mol

9. Ion - Dipole Ion - Dipole: Charge and size dependent.
Most important for larger charge and small ionic radius.
Cation Ion Radius DHHyd (kJ/mol) Li Na K RB Cs

10. Dipole - Dipole
Dipole - Dipole: A permanent attractive intermolecular force resulting from the interaction of the positive end of one molecule with the negative end of another.
Occurs between identical or different polar molecules.
NonPolar Polar
M(g/mol) bp (°C) M (g/mol) bp(°C)
N CO
SiH PH
GeH AsH
Br ICl

11. Induce dipole - induced dipole: Vander Waal’s forces
Van der Waal (Induced dipole-Induced dipole):
Intermolecular force responsible for keeping nonpolar molecules (species) together.
Polarisability - The ease of which an e- cloud can be distorted. Larger the atomic size, the greater the number of electrons, the greater the polarizability.
Boiling Point of the Halogens and Noble Gases
Halogen B.pt (K) Noble Gas B.pt (K)
F He 4.6
Cl Ne 27.3
Br Ar 87.5
I Kr 120.9
Xe 166.1

12. Boiling point versus polarisability
Graphs for noble gases and for series of nonpolar molecules; both show a family smooth increase of boiling point with atomic weight (larger degree of polarisability) due to increasing Van der Waal forces
How about H2Te, (-20°C) H2Se, H2S, H2O
Molar Mass Kr He
SiH4
GeH4
SnH4
CH4
-250°C
-100°C
0°C
Temperature
-200°C Ar Ne Xe

13. Boiling point Hydrogen compounds
Graphs for family of hydrogen containing compounds and their boiling point. In general there is an increase in the boiling point except for H2O, HF and NH3. Why? There must be some other force that operate on these compounds which increases their intermolecular forces.
Molar Mass (Period)
H2Te
H2Se
H2O
SiH4
SbH3
GeG4
CH4
-100°C
0°C
100°C
Temperature HI
SnH4 HF
NH3
PH3
AsH3
HCl
HBr
H2S

14. A Special Type of Bonding H-Bonding
H-Bonding: A special glue above and beyond dipole-dipole intermolecular forces.
H-bonding is a strong type of intermolecular force (bond) between hydrogen and very electronegative elements ( kJ/mol).
N-H O-H F-H sometimes (Cl-H)
Bichemical structural Integrity.
Water possesses H-bond: Responsible for water’s unique properties.

15. Example: H-bonding a) C2H6 d) H3CCOOH No Yes b) CH3OH e) H3CCH2OH
Which of the following substances exhibits H-bonding? Draw the H bonds between two molecules of the substances where appropriate.
a) C2H6 d) H3CCOOH
No Yes
b) CH3OH e) H3CCH2OH
Yes Yes
c) H3CCONH2 f) H3CCOCH3
Yes No

16. Biological Integrity
H-bonding is responsible for the structural integrity of Biological molecules.
• Protein structures • DNA and RNA

17. H2O: Nature of Water
Water is a liquid at room temperature as a direct consequence of hydrogen bonding between adjacent water molecules.
(Most other molecules with comparable Molar mass is a gas at room temperature)
Pure water is a liquid between 0°C and 100°C.

18. Example
Identify the dominant intermolecular forces for each of the following substances, and select the substance with the higher boiling point in each pair;
a) MgCl2 or PCl3 b) H3CNH2 or CH3F
ion-dipole dipole-dipole H-bond dipole-dipole
VdW VdW dipole-dipole VdW
Higher Bpt VdW
Higher Bpt
b) CH3OH or CH3CH2OH e) Hexane or cyclohexane
H-bond H-bond VdW VdW
dipole-dipole dipole-dipole Higher Bpt
VdW VdW More surface area
Higher MWt.

19. Overview: Recognizing Intermolecular Forces
Flowchart for recognizing the major types of intermolecular forces. Van der Waal’s forces occur in all instances. The strength of other forces generally increases proceeding from left to right

20. Summary of Nature’s Forces
Bonding forces are relatively strong because they involve larger charges that are closer together. Ionic ( kJ/mol) Metallic ( kJ/mol)
Intermolecular forces are relatively weak because they typically involve smaller charges that are farther apart. H-bond (10-40 kJ/mol) LDF ( kJ/mol)