1. Liquids and Solids
Intramolecular forces › Intermolecular forces
break down water into H & O atoms › boiling water (liquid to “gas”)
Intramolecular forces:
“forces” holding atoms or ions together
covalent bond vs. ionic bond
Intermolecular forces:
“forces” holding molecules together
stronger the forces the more difficult to
separate the molecules
thus Higher the m.p. or b.p.
Solid ↔ Liquid ↔ Gas
2. Intermolecular forces
In order of increasing strength
1. London dispersion forces (LDF)
attractive “forces” among nonpolar molecules
induced by temporary dipole
↑ Molar Mass or branching the ↑ LDF ↑ mp
4. Intermolecular forces
3. Hydrogen Bond
Special type of dipole-dipole interaction that occurs between the H
atom in a polar bond with another F/O/N
The force of attraction between a partially
positive H atom that is covalently bonded
to a F/O/N and a partially negatively
charged F/O/N
6. Example
• Which of the following has the higher boiling point?
C
H
H C
H
H
H
O H C
H
H C
H
H
H
O H
7. Example
Arrange the following molecules in order of intermolecular forces
from weakest to strongest?
N2 H2O H2S CH3CH2CH3
8. Example
Which of the following compounds exhibit only London dispersion
forces?
NH3 CH4 H2 CH3CH2Cl
9. Phase Transformation.
Endothermic Melting Evaporation
Solid → → → → → → Liquid → → → → → →Gas
Requires energy to overcome the relatively strong intermolecular forces in the liquid
Exothermic Freezing Condensation
Solid ← ← ← ← ← ← Liquid ← ← ← ← ← ←Gas
10. Vaporization/Evaporation
Molecules of a liquid can escape the liquid’s surface and form a gas
Endothermic process
Requires energy to overcome the relatively strong intermolecular forces in the liquid
11. Vapor Pressure
• Evaporation liquid molecules
escape into the “air” above
• Condensation: vapor molecules
convert/return to liquid
• Equilibrium: When no further
change
is visible, the opposing
processes balance each
other
12. Vapor Pressure
• Equilibrium vapor pressure: Pressure of the vapor present at
equilibrium
• The system is at equilibrium when no net change occurs in the
amount of liquid or vapor because the two opposite processes
exactly balance each other
Variation in cooking time based on altitude
13. Vapor Pressure
Which of the following would be expected to have the highest
vapor pressure at room temperature?
a) CH3CH2CH2OH
b) CH3CH2CH2NH2
c) CH3CH2CH2CH3
→d) CH3CH2CH3
14. Crystalline Solids vs. Amorphous
• Solids that posses rigid and long range order; the
atoms/molecules/ions occupy specific position
• Substances with a regular arrangement
of their components form crystalline solids
• Extensive 3-D network
• Highly ordered
17. Ionic Crystal
• Crystalline solid composed of oppositely charged ions
held together via electrostatic attraction (attraction
among opposite charged ions)
• Most are water soluble
• Electrically conductive
• High melting point
18. Molecular Crystal
• Crystals made up of small molecules held together by intermolecular
forces; LDF
• Low m.p.
• Soft
• Dissolve in nonpolar solvent
• Non-conductor of electricity
19. Atomic Crystal
• Crystals composed of atoms of the same kind or
an ALLOY if the metal atoms are of different kind
• Variety of m.p.
• Insoluble in all common solvents
• Malleable
• Excellent conductor of electricity
• Good heat conductors
20. Alloy
Metals form alloys of two types
• Substitutional: Different atoms are substituted in the host metal atoms
• Interstitial: Small atoms are embedded into the “holes” of the metallic structure
21. Phase Change
What happens when a solid melts or a liquid evaporates?
Answer:
Overcome intermolecular attraction and phase change
How much energy is involved with phase change?
22. Definitions:
• Melting point:
the temperature at which the solid and liquid co-exist together
• Boiling point:
The temperature at which the liquid vapor pressure=external pressure
• Molar heat of fusion:
amount of energy needed to melt 1 mole of a solid
• Molar heat of vaporization:
amount of energy needed to vaporize 1 mole of a substance
24. Example
The unusually high value of the molar heat of vaporization of water
(40.6 kJ/mole) is an important factor in moderating the temperature of
the earth’s surface. Calculate the amount of heat in kJ needed to
evaporate 10.5 kg of liquid water at 100.0 o C
4
2
2
2
1 mol H O
1000 g 40.6 kJ
10.5 kg H O 2.37 10 kJ
1 kg 18.016 g H O mol
´ ´ ´ = ´
25. Example
The molar heat of fusion and the molar heat of vaporization for water
is 6.02 kJ/mol and 40.6 kJ/mol, respectively. The specific heat of liquid
water is 4.18 J/g K and specific heat of ice is 2.03 J/g K
How much heat is required to convert 25.0 g of ice at -5.0◦C to liquid
water at 75.0 ◦C?
