Lecture 4, 'Physical properties & molecular interactions', relates a molecule's physical properties to its structure. The nature, strength an extent of molecular interactions are also a focus of this material. This lecture references Chapter 4 of Daley & Daley.
Lecture 4 slide sets: Physical properties & interactions
(Click on the links below to download each slide set for the lecture.)Key concepts
1. Molecules have physical properties including physical state, color, odor, refractive index, solubility,
density, melting and boiling points. Physical state can be changed without changing the identity of the
2. Intermolecular interactions determine the bulk properties of molecules and facilitate formation of
macromolecular structures and biochemical interactions.
3. The physical chemistry of water is dominated by hydrogen bonding. Ice has more hydrogen bonds than
liquid water and is less dense (a very odd thing).
4. Like dissolves like.
5. Surfactants are amphipathic molecules: they have both polar and non-polar ends.
a. Energy is added or removed to change physical states.
b. The liquid crystal state is more ordered than a liquid but has less intermolecular contact than a solid.
c. Melting point (temperature at which solids melt to liquids) are increased by increasing: 1) molecular
symmetry; 2) molecular polarity; 3) hydrogen bonding; and 4) molecular weight.
d. Types of intermolecular interactions include: 1) van der Waals forces; 2) dipolar interactions; 3)
hydrogen bonding; and 4) salt bridges.
e. Solubility, the ability of a solute molecule to dissolve in solvent depends on: 1) degree of intermolecular
interactions; 2) MW of the solute; 3) temperature; 4) matched polarity.
f. Surfactants include soaps, bile salts, fatty acids, some phosopholipids.
g. Micelles are spherical aggregations of surfactant in water: the polar tails cluster inside the sphere and
the polar ends of the surfactant interact with water in aqueous solutions.
h. Emulsions are larger spherical structures that contain completely non-polar (and hydrophobic) lipids in
their cores near the surfactant’s non-polar tails.
i. Solids have high density, lots of molecular interaction, low energy levels and no motion. Liquids have
less density, constant but lower molecular interactions, higher energy levels and significant motion.
Gases have very low density, nearly no molecular interactions, very high energy levels and motion.
ii. Arene groups are common components of liquid crystals because they stack together in adjacent
iii. Spherical molecules have significantly higher melting points than linear (elongated) molecules with a
similar number of carbons and similar MW. Why? Spherical molecules have a more stable shape that
undergoes less rotation and spheres pack easily into solid crystalline forms.
iv. Hydrocarbons with an even number of carbons have slightly higher melting points because of
increased chain overlap.
v. Cyclic hydrocarbons have higher melting points then linear molecules with the same number of
carbons because rings stack up and adjacent columns line up and interact with one another.
vi. Molecules that interact via dipolar or hydrogen bonding also have higher melting and boiling points.
vii. Van der Waals attraction is a weak attractive force that occurs when two molecules come within a
critical distance of one another. At that distance the electrons of one molecule are attracted to the
protons of the other. Attractions are temporary and easily affected by the environment. All molecules,
polar and non-polar, experience van der Waals attractions.
viii. Dipolar interactions are slightly stronger attractive forces that occur between polar molecules with
permanent dipoles. The molecules line up with the dipolar positive end of one adjacent to the dipolar
negative end of the next: nose-to-tail.
ix. Hydrogen bonds are a special case of dipolar bonds. Polar hydrogens (attached to O, F or N) bind to a
free electron pair on an O, F or N atom. While hydrogen bonds are weak, aqueous solutions have so
many that hydrogen bonds that the cumulative effect is very powerful.
x. Single-phase solutions are uniform and homogeneous while multiple phase solutions are layered.
Reactions generally prefer single-phase solutions.
xi. Increasing the number of intermolecular interactions between solutes and solvents increases
xii. Increasing MW of the solute decreases solubility.
xiii. Increasing temperature increases solubility.
xiv. Polar solvents dissolve polar solutes while non-polar solvents dissolve non-polar solutes. Water is a
very polar solvent.
xv. Water forms ordered cages of solvation around ionic compounds.
xvi. Soap scum is a precipitate formed when metal ions combine with surfactants like sodium dodecanyl
xvii. Surfactants lower suface tension are vital to lung function. A natural phospholipid (DPPC) coats the
inner surface of the lung and allows oxygen to gain access to the surface of alveoli.
xviii. Hydrophilic (water-loving) molecules are polar or charged and are soluble in water.
xix. Hydrophobic (water-hating) molecules are non-polar and are not soluble in water.
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