Download Unit 3: Bonding and Nomenclature Content Outline: Intermolecular

Unit 3: Bonding and Nomenclature
Content Outline: Intermolecular Forces (3.2)
I.
Intermolecular Forces (Between molecules; like an “Interstate” goes between states.)
A. These are forces or attractions between molecules of solids or liquids mainly, but can be gases too.
B. They vary in strength; but are generally weaker than chemical bonds, as there is no real physical
interaction like there are in bonds…hence the term forces.
C. These forces usually affect the boiling points of chemicals.
1. Higher boiling points = more interactions/attractions between molecules.
2. Lower boiling points = less interactions/attractions between molecules.
Various strengths of magnetics can be a great visual of “strength” of attraction. A flimsy refrigerator
magnet, cut in half is great for weak Hydrogen bonds.
D. There are 4 basic types of Intermolecular Forces (from strongest  weakest):
1. Network Solid (Please help students “see” the term and strong force.)
A. This is a solid molecule that is a 3D -network of covalently bonded atoms
For example, a diamond and graphite made of Carbon or elemental silicon such as sand and
quartz.
Diamond is the hardest substance, best heat conductor; but very expensive.
Graphite and silicon can be lubricants.
B. They are tough and have very high melting points and boiling points as they have numerous
intermolecular bonds.
2. Hydrogen Bonds (Please help students “see” the term.)
a. These are forces of attraction that usually are associated with a polar molecule with a positively
charged Hydrogen molecule and a negatively charged second molecule, in close proximity,
containing Oxygen, Fluorine, or Nitrogen.
b. These are relatively weak attractions, but probably the most important in terms of life.
i. They hold the two halves of a DNA molecule together.
ii. They give water the ability to move other charged molecules (food) through our blood
vessels.
iii. They allow trees to move water in massive quantities from the roots to the leaves.
c. They usually affect the boiling point of substances, including water.
i. The more Hydrogen bonds present within a liquid, the higher the boiling point or Specific
heat.
α. This is the amount of heat energy needed to raise 1 gram of a substance by 1OC.
b. Water has a specific heat of 4.18 J… so it takes a whole lot of energy to break all for
Hydrogen bonds, at one time, and turn it into a gas.
c. Iron has a specific heat of 0.449J… so it heats up very quickly in that sea of electrons.
This is why we use it to cook food on a stove or fire. Remember, we also use
metals for conducting electricity.
Which would you rather touch after 1 hour in the summer sun, a glass of water or an
iron pan?
ii. The fewer Hydrogen bonds, the lower the boiling point usually.
iii. A single water molecule has 4 simultaneous Hydrogen bonds, so it requires a high
temperature to boil, which is good since water covers 2/3rds of Earth and makes up
90% of your body.
d. Hydrogen bonds are represented by dotted lines, for example H…O.
e. As Hydrogen bonds are fairly weak attractions, they are easy affected/broken by changes in
temperature (high heat breaks them), changes in pH concentrations, or changes in ionic
solutions, such as saline (salt water).
3. Dipole-Dipole Forces (Polar molecules) (Please helps students “see” the term.)
a. These attractions occur between two opposite charged parts of two different molecules.
b. “Dipole” means “two poles” on one single molecule. Like the Earth has two poles – north and
south poles.
c. These forces are represented by a single crossed arrow, such as +-->.
i. The crossed part (+) represents the positive part of the first molecule.
ii. The arrow part --> represents the attraction to the negative part of the second molecule.
d. For linear molecules, orientation of molecules is not an issue; just attraction by opposite
charges.
e. For lattice or 3D folded chains, the molecule orientation is important. Just like in the folding of
proteins – string of folded chain of amino acids.
f. Induced Dipole Moment (Please help students “see” the term.)
i.
This is a temporary moment where a non-polar molecule behaves “like” a polar molecule
temporarily because of electron attraction with another molecule. (“Induce” means “to
make into”)
ii. The most important example of this temporary attraction is Oxygen gas (O2) dissolved in
water.
α. Oxygen gas is non-polar, but when it interacts with water, it can become induced to
act polar and be relatively attracted to the highly positive portion of water (the
Hydrogen atoms).
 Cold water holds onto the Oxygen gas tighter because the molecules are
moving slower.
 Warmer water holds onto the Oxygen gas weaker because the molecules are
moving faster.
 Regardless, the dissolved Oxygen gas in water allows for life forms to live in
the water. They mainly use gills or diffusion to get the Oxygen gas out of the
water.
4. London Dispersion Forces (Non-polar molecules) (Please help students “see” the term.)
a. These were discovered by Fritz London in 1930.
b. They are also known as Van der Waals Interactions associated with enzymes.
c. They are the temporary interactions between molecules due to temporary
“clumping/dispersion” of electrons on one side of an atoms nucleus. This temporary “clumping”
creates a temporary polar “like” molecule. Now it can quickly and briefly interact with another
charged particle/molecule.
Remember, enzymes “speed up” a chemical reaction by quickly and briefly grabbing molecules
and either puts them together with a chemical bond or grabs and breaks the bond. The
temporary polar “like” quality allows for the “grabbing”.
d. Any atom or molecule can have these forces, because every atom has moving electrons.
i. These are the only type of interaction that Noble gases can have with each other or other
atoms… brief and temporary.
e. These can also affect boiling points of substances.
i. High amount of interactions = higher boiling points.
ii. Fewer interactions = lower boiling points.
iii. The number of interactions (and boiling points) increases as atomic or molecular masses
(AMUs) increase (molecule gets larger).
iv. The larger the molecule becomes the stronger the dispersion force due to increases in
polarizability.
f. Polarizability
i. These are the distortions to the electrical cloud because of the electrical charge associated
with the clumping.