Chemistry 102 Summary July 3rd
Intramoelcular forces:
1. Forces that hold atoms together in molecules.
2. Are forces within the molecule (bonds).
3. Are very strong when compared to intermolecular forces.
Intermolecular forces:
1. Forces that hold molecules together in the liquid and solid phases.
2. Are forces between molecules.
3. Are weak when compared to intramolecular forces yet account for
many of the observed properties of matter.
-The strength of intermolecular forces at a certain temperature
determines the phase of the substance.
Example: NaCl vs. H2O vs. N2 at room temperature.
NaCl:
H2O:
N2:
-
The strength of intermolecular forces are related to some physical
properties of a substance, all of which are related to phase
changes: boiling point, melting point, molar heat of vaporization
(ΔHvap), molar heat of fusion (ΔHfus), and molar heat of sublimation
(ΔHsub).
Molar Heat of Vaporization (ΔHvap): Amount of energy required to boil one
mole of a substance.
Molar Heat of Fusion (ΔHfus): Amount of energy required to melt one mole
of a substance.
Molar Heat of Sublimation (ΔHsub): Amount of energy required to sublime
one mole of a substance.
Question: What is the relationship between boiling point, melting point, ,
molar heat of vaporization (ΔHvap), molar heat of fusion (ΔHfus), and molar
heat of sublimation (ΔHsub) and the strength of intermolecular forces of a
molecule?
Answer:
Vapor Pressure: The equilibrium pressure of a vapor (gas) over a liquid at a
specific temperature.
- A certain number of molecules have enough kinetic energy to
escape into the gas phase, some condense, when this process is
consistent then this represents the equilibrium vapor pressure.
Question: How is vapor pressure related to temperature?
Answer:
Question: How is vapor pressure related to the strength of intermolecular
forces?
Answer:
Types of Intermolecular Forces
1. Electrostatic forces
• Due to attractions of oppositely charged species (ions).
• Present in ionic molecules.
• Extremely strong; no comparison in strength to the covalent
intermolecular forces.
2. Dipole-dipole forces
• Due to permanent dipole-dipole interactions.
• Strength increases with increasing polarity.
• Present in covalent molecules with a permanent dipole moment,
i.e. only present in polar molecules.
3. Hydrogen bonding
• Due to attraction of N, O or F atom in a molecule for an electron
deficient hydrogen atom in a covalent molecule (special kind of
dipole-dipole force).
• Only occurs in molecules with a hydrogen atom bound directly to
N, O or F atom (the three most electronegative atoms), i.e.
molecule must have N-H, O-H or F-H covalent bond in order to form
hydrogen bonding interactions.
• Strongest covalent intermolecular force; compounds which can
form hydrogen bonding interactions have irregularly strong
intermolecular forces for covalent compounds.
4. London Dispersion Forces (LD)
• Due to instantaneous dipole interactions.
• Strength increases with increasing molar mass (size); shape is also a
factor.
• Present in all covalent molecules (atoms), i.e. present in polar and
nonpolar molecules.
• Generally considered to be the weakest intermolecular force
(although London Dispersion forces are quite strong for very large
molecules).
Questions;
1. Rank the halogens F2, Cl2, Br2 and I2 in order of increasing boiling
point.
2. Compare the boiling points of butane and acetone, can acetone
hydrogen bond:
CH3-CH2-CH2-CH3
Butane
O
H3C C CH3
Acetone
3. In Figure 10.4 why does the boiling point of non –H bonding hydrides
increase steadily from smaller to larger?
4. Match the following compounds to the following curves.
5. Order the following compounds in terms of increasing boiling point
and vapor pressure:
(a) CH4 (16 g/mol), CH3-O-CH3 (46 g/mol), CH3CH2OH (46 g/mol),
CH3CH2CH3 (44 g/mol).
(b) HCl (36.5 g/mol), HF (20 g/mol), LiF (26 g/mol), Ar (40 g/mol)
Other caveats to point out:
1. Shape also affects the magnitude of London Dispersion forces. The
more elongated the molecule, the more surface area in contact
with neighboring molecules, the stronger the London Dispersion
forces.
2. I will not hold you responsible for surface tension and viscosity.
3. IMF are also related to solubility, like dissolves like. Polar substances
and soluble ionic compounds dissolve in polar solvents like water
and nonpolar substances dissolve in nonpolar solvents like hexane
(C6H14).