Chem 110 Recitation Week 1 Intermolecular Forces

Chem 110 Recitation – Week 1 – Intermolecular Forces

Firs, the difference between intra and intermolecular forces:

Intra – within the molecule; inter – between different molecules

1. What types of forces, intramolecular or intermolecular,

a. prevent ice cubes from adopting the sap of their container?

Intermolecular

b. are overcome when ice melts?

Intermolecular

c. are overcome when liquid water is vaporized?

Intermolecular

d. are overcome when gaseous water is converted to hydrogen gas and oxygen gas?

Intramolecular

Indicators of the strength of IMFs (Intermolecular Forces):

Boiling point (vaporization)

Freezing point (condensation)

Melting point

For liquids:Surface tension, capillary action, viscosity, vapor pressure

To determine how these are affected, you will need to concern yourself both with the intermolecular forces, but also with the size and shape of the molecule.

Types of IMFs you should worry about:

Dispersion forces (also sometimes referred to as London forces)

Fleeting little forces that occur between nonpolar molecules

Dipolar forces

Occur when the positive pole of one molecular interacts with the negative pole of another

Hydrogen Bonding forces (remember, hydrogen likes to have FON)

Bonding between a hydrogen on an oxygen, nitrogen or fluorine and another oxygen, nitrogen or fluorine

Things you should know how to do:

1. Name the forces present in a sample.

a. What is the strongest IMF in a sample of pure CH3OCH3?

Dipole forces

b. Name all of the IMFs present in a sample of HCl.

Dispersion, Dipole

c. What is the strongest IMF in a pure sample of any noble gas?

Dispersion

d. Which of the following would exhibit dipole-dipole IMFs?

CCl4, Cl2, N2, NCl3, CH4

NCl3, (all others are nonpolar based on shaped or electronegativity difference)

2. Be able to compare intermolecular forces between different molecules.

a.Which of the following has the strongest IMFs?

CH3CH3, CO2, H2O

H2O has H-bonding, which is stronger than CO2 or CH3CH3, which both have dispersion.

b.Which, if any, of the following would exhibit hydrogen bonding?

CH3F, NH3, CH3CH2OCH2CH3

Only NH3 (it’s the only with with a H attached to the correct molecule)
c. Which isomer of butane, C4H10, will have the highest viscosity?

(a) or (b)

More viscous means it sticks together better, so (b) would be more viscous.

d. Rank the compounds NH3, CH4, and SiH4 in order of increasing boiling point.

CH4 < SiH4 < NH3 (the first two both have dispersion, but Si weighs more, and the last has H-bonding, so it is held together better)

3. Be able to determine how intermolecular forces affect physical and chemical properties.

a. TRUE or FALSE Liquids with high surface tension tend to have weak intermolecular forces. FALSE

b. Of the gases Ne, N2, O2, CH4, SiH4, which one would you expect to be easiest to liquefy?

I’m going to go ahead and read this as the one that wants to stay a liquid longer, or the one with the highest boiling point. That would be SiH4.

c. Which of the following would probably have the lowest boiling point?

CH4, SiH4, PH3, AsH3, NH3

CH4 (it has the weakest forces)

d. Which of the following substances would be expected to have the highest melting point?

CaO, Br2, CO2, HCl, H2O

CaO (this is an ionic solid, which tend to have very high melting points)

e. Which forces oppose vaporization of each substance?

Hexane, water, SiCl4

Hexane – dispersion; water – H-bonding; SiCl4 – dispersion