Intermolecular Forces

Question 1

Five types of molecular structures

Answer 1

Linear, trigonal planar, tetrahedral, (trigonal) pyramidal, and bent

Question 2

Linear: number of electron domains, lone pairs, and bond angle

Answer 2

1 or 2 electron domains, 0 lone pairs, 180 degrees

Question 3

Trigonal planar: electron domains, lone pairs, angle

Answer 3

3 electron domains, 0 lone pairs, 120 degrees

Question 4

Tetrahedral: electron domains, lone pairs, bond angle

Answer 4

4 electron domains, 0 lone pairs, 109.5 degrees

Question 5

Trigonal pyramidal: electron domains, lone pairs, angle

Answer 5

4 electron domains (but only three atoms bonded to central atom), 1 lone pair, 107 degree angle

Question 6

Bent: electron domains, lone pairs, bond angle

Answer 6

Four electron domains (two atoms bonded to central atom), two lone pairs, 105 degrees

Question 7

How is the electronic shape different from the actual shape?

Answer 7

Electronic shape is strictly based on the number of electron domains. For example, NH3 has four electron domains, so it is electronically tetrahedral, but it is actually trigonal pyramidal.

Question 8

VSEPR Theory stands for

Answer 8

Valence shell electron pair repulsion theory

Question 9

What is VSEPR Theory?

Answer 9

That electron pairs are arranged around a central atom in a simple molecule or ionic compound so that they are as mutually repulsive as possible (as far apart as possible). This theory is used to predict molecular shape.

Question 10

Do double and triple bonds have a greater repulsion than single bonds?

Answer 10

No, for VSEPR Theory, they count as 1 electron domain total.

Question 11

What is the order of electron domains in terms of repulsion?

Answer 11

Lone pair to lone pair is strongest repulsion, lone pair to bonding pair is second strongest, and bonding pair to bonding pair has the least repulsion.

Question 12

Can you determine by electronegativity alone if a molecule is polar?

Answer 12

No- you need to know the shape and the dipoles.

Question 13

What are London dispersion/Van der Waal forces?

Answer 13

The weakest kind of intermolecular forces that occur between all atoms and molecules based on the random chance of the buildup of electrons, creating a temporarily polar molecule. Since electrons are constantly moving, a partial charge may occur.

Question 14

Dipole-dipole forces

Answer 14

Acts between polar molecules, AKA molecules with an overall dipole. The positive part of a molecule is attracted to the negative part of another molecule, and vice versa.

Question 15

Hydrogen bonding

Answer 15

Has some dipole character, strongest of the three intermolecular forces, but is 10x weaker than a covalent bond. Acts between polar molecules in which a hydrogen atom is covalently bonded to an extremely electronegative atom, either Fluorine, Oxygen, or Nitrogen, that has a lone pair.

Question 16

How do you represent hydrogen bonds?

Answer 16

Draw the molecules, show all charges and lone pairs, and represent the hydrogen bond(s) with a dashed line.

Question 17

What is the bond angle for a hydrogen bond between molecules?

Answer 17

180 degrees.

Question 18

Why are London dispersion forces stronger as you go down the periodic table?

Answer 18

Because there are more electrons in the heavier elements, so there is a higher chance for them to build up on one side of the atom.

Question 19

Why can a molecule with individual polar bonds not necessarily have a dipole?

Answer 19

Sometimes, all of the dipoles pull with the same strength on opposite sides; if the molecule is symmetrical, the forces will cancel out.

Question 20

Intermolecular forces v. intramolecular forces

Answer 20

Intermolecular forces occur between two or more molecules (ex. hydrogen bonds) while intramolecular forces occur within one molecule (ex. covalent bonds).

Question 21

Generally, what is the relationship between atomic mass and boiling point? Why?

Answer 21

Generally, as you go down a group and increase in atomic mass, boiling point increases because there is a stronger chance that London dispersion forces will take effect; therefore, it becomes harder to boil the substance.

Question 22

When a substance is boiled, which forces are broken: Intra or intermolecular forces?

Answer 22

Intermolecular forces are broken, but intramolecular forces remain intact.

Question 23

How can you determine solubility for a substance?

Answer 23

Like dissolves like, so a polar solute will typically dissolve in a polar solvent. A nonpolar solute will typically dissolve in a nonpolar solvent.

Question 24

How do intermolecular forces relate to solubility of a polar substance?

Answer 24

Ion-dipole forces occur between water and ions in aqueous solutions, and hydrogen bonds occur between water molecules and polar molecules. London dispersion forces occur between non-polar molecules.