Intermolecular Forces Flashcards
Five types of molecular structures
Linear, trigonal planar, tetrahedral, (trigonal) pyramidal, and bent
Linear: number of electron domains, lone pairs, and bond angle
1 or 2 electron domains, 0 lone pairs, 180 degrees
Trigonal planar: electron domains, lone pairs, angle
3 electron domains, 0 lone pairs, 120 degrees
Tetrahedral: electron domains, lone pairs, bond angle
4 electron domains, 0 lone pairs, 109.5 degrees
Trigonal pyramidal: electron domains, lone pairs, angle
4 electron domains (but only three atoms bonded to central atom), 1 lone pair, 107 degree angle
Bent: electron domains, lone pairs, bond angle
Four electron domains (two atoms bonded to central atom), two lone pairs, 105 degrees
How is the electronic shape different from the actual shape?
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.
VSEPR Theory stands for
Valence shell electron pair repulsion theory
What is VSEPR Theory?
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.
Do double and triple bonds have a greater repulsion than single bonds?
No, for VSEPR Theory, they count as 1 electron domain total.
What is the order of electron domains in terms of repulsion?
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.
Can you determine by electronegativity alone if a molecule is polar?
No- you need to know the shape and the dipoles.
What are London dispersion/Van der Waal forces?
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.
Dipole-dipole forces
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.
Hydrogen bonding
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.
How do you represent hydrogen bonds?
Draw the molecules, show all charges and lone pairs, and represent the hydrogen bond(s) with a dashed line.
What is the bond angle for a hydrogen bond between molecules?
180 degrees.
Why are London dispersion forces stronger as you go down the periodic table?
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.
Why can a molecule with individual polar bonds not necessarily have a dipole?
Sometimes, all of the dipoles pull with the same strength on opposite sides; if the molecule is symmetrical, the forces will cancel out.
Intermolecular forces v. intramolecular forces
Intermolecular forces occur between two or more molecules (ex. hydrogen bonds) while intramolecular forces occur within one molecule (ex. covalent bonds).
Generally, what is the relationship between atomic mass and boiling point? Why?
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.
When a substance is boiled, which forces are broken: Intra or intermolecular forces?
Intermolecular forces are broken, but intramolecular forces remain intact.
How can you determine solubility for a substance?
Like dissolves like, so a polar solute will typically dissolve in a polar solvent. A nonpolar solute will typically dissolve in a nonpolar solvent.
How do intermolecular forces relate to solubility of a polar substance?
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.
