1. INTERMOLECULAR FORCES
Physical state
Inter- vs Intra-
Types of IM forces
Dipole-dipole forces
Hydrogen bonding
Ion-ion forces
Ion-dipole forces
Van der Waal’s forces
Liquid properties

2. Physical state
Gas
Liquid
Solid
Widely separated, disordered molecules in continuous motion
Closely spaced, disordered molecules in continuous motion
Ordered molecules in contact and in relatively fixed position
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3. At room temperature (25 C) and pressure (1 atm), some substances are:
Solid: Au, NaCl, Mg
Liquids: H2O, Hg
Gases: O2, N2, H2
Physical state depends on balance between kinetic energies of molecules and IM attractive forces.

4. Solids and Liquids Many properties of liquids & solids are similar
Solids & liquids have much larger densities than gases
44 g solid CO2 or liquid CO2: volume ~30 mL
44 g gaseous CO2: volume ~ mL
Going from liquid to gaseous state involves much more energy:
H2O(s)  H2O(l) energy required ~ 6 kJ/mol
H2O(l)  H2O(g) energy required ~41 kJ/mol

5. Intra vs Inter CO2 Within (intra) or between (inter) molecules
Intermolecular force weaker than Intramolecular force

6. Intermolecular forces
Intramolecular forces
Nonpolar covalent
Polar covalent
Ionic
Intermolecular forces
SOLID > LIQUID > GAS
strong moderate weak

7. Types of Intermolecular (IM) forces
The stronger the force, the more likely it is to play a dominating role in determining structure and properties of a substance.
-Dipole-Dipole
-Hydrogen Bonding
-Ion-ion
-Ion-dipole
-London dispersion forces

8. Polarity
If charge separation exists in molecule, it’s polar & the extent of polarity is measured by its dipole moment (μ)
Influences melting/boiling point, solubility, & chemical reactivity

9. Polar Molecules
Molecules can have polar bonds but not be polar molecules
e.g. CCl4
To be a polar molecule it must:
(1) contain at least one polar covalent bond
(2) not be totally symmetrical

10. Dipole-dipole Present in polar molecules
Molecules orientate themselves so +ve and-ve ends are close together
Strength increases as polarity of molecule increases
e.g. CH3Cl, HCl, CH3CN

11. Hydrogen bonding Present in molecules with -OH, -NH, -FH bonds
Strong dipole attraction
H is unique and sufficiently small that a nearby large electronegative atom’s lone pair of electrons can approach very close to it
e.g. H2O, HF, NH3, CH3OH

12. Effects of Hydrogen Bonding
Molecules with -OH, -NH, -HF have unusually high melting/boiling pts and high solubility in H2O

13. H2O
-has a high boiling point
- takes a lot of energy to break these attractions
if no hydrogen bonding, water would boil at –75 °C
-has a high specific heat i.e cal/g °C
higher than most liquids (e.g. alcohol = 0.5 cal/g °C)
-has a high heat of vaporization i.e. 41 kJ/mol (Hvap)

14. Ion-ion Strongest IM force Present in ionic compounds
Stronger force if ion has a higher charge density
e.g. NaCl, CuO

15. Ion-dipole
Between an ion and the partial charge on the end of a polar molecule
Strength increases as charge density of ionic compound increases
Hydrated compounds
-Water molecules coordinate around ion species by electrostatic attraction
-Hydration of ion can be permanent
e.g. CuSO45H2O

16. London dispersion forces
Also known as van der Waals forces
Present in all molecules, weak IM force
Instantaneous dipole
At some point in time electrons concentrate in one region of atom/molecule
Atom/molecule which had symmetrical distribution of charge now becomes polar
Induced dipole
Instantaneous dipole induces a dipole on neighbouring atom/molecule

17. Higher molar mass means more electrons present  more polarizability
In longer chain molecules, electrons more easily displaced than in compact molecules  more polarizability
n-pentane
Bp = 36.1 °C
neopentane
Bp = 9.5 °C

18. Summary Type of interaction Energy (kJ/mol) Interacting Species
Ion-Ion
Ions only
Ion-Dipole
Ion and Polar molecule
Hydrogen-bonding
H-X (X = N, O, F)
Dipole-Dipole
5 - 25
Two polar molecules
London
All types of molecules

19. Why does NH3 have a higher boiling point than CH4 ?
Practice examples
Why does NH3 have a higher boiling point than CH4 ?
 Why does KCl have a higher melting point than I2 ?

20. Arrange the following in order of increasing boiling points and
Practice example
Arrange the following in order of increasing boiling points and
identify the IM forces present in each compound:
CH3OH, CO2, RbF, CH3Br

21. Liquid properties Viscosity Resistance to flow
Measured by timing how long it takes a certain amount of liquid to flow through a thin tube under gravitational force.
Strong IM forces impede movement
Generally decreases as temperature increases

22. Surface tension Resistance to an increase in surface area
Molecules within bulk of liquid have strong attractive forces versus those on the surface
Gives surface properties of thin, invisible, elastic membrane

23. Capillary action Spontaneous rising of liquid in a narrow tube
Due to adhesive forces
between liquid and tube