Intermolecular Forces
Now that we understand how bonds keep atoms together, the next question is, what force keeps molecules together? Do they bond to each other as well? Let’s take a look.Â
How IMFs are related to melting and boiling points.
*Correction: The bp (boiling point) of "Ethanol" is 173˚ F or 78˚ C.

All these molecules have similar molecular weights; however they have vastly different boiling points. Why is that?
Intermolecular forces (IMF’s) keep molecules stuck together. Questions about boiling / melting points of molecules are always referring to the strength of IMF’s between molecules.
- Hydrogen bonding
Hydrogens attached to small, highly electronegative atoms can hydrogen bond. Namely N, O and F.

How hydrogen bonding works.
- Dipole-dipole
We like to call this the net dipole force because it’s only present when the molecule has a net dipole.

How dipole-dipole forces work.
Van der Waals Forces
Also known as London Dispersion Forces or Dipole-Induced Dipole Forces.
All molecules possess Van der Waals Forces. They increase with the following properties:
Size (the heavier, the stronger the force)
Shape:Ring > Straight Chains > Branched Chains

This picture is trying to illustrate how rings have greater surface area for interactions than straight or branched chains.
How Van der Waals forces work.
Practice Questions: Now that we have learned about factors that influence the strength of IMF's let's apply them!
PRACTICE:Which of the following pairs of molecules would have the highest boiling point?
PRACTICE:Which of the following pairs of molecules would have the highest boiling point?
PRACTICE:Which of the following pairs of molecules would have the highest boiling point?
PRACTICE:Which of the following pairs of molecules would have the highest boiling point?
- Predict which compound in each pair has the higher boiling point. Explain your prediction. c. CH3CH2CH2CH2CH3...
- Predict which compound in each pair has the higher boiling point. Explain your prediction. a. CH3CH2OCH3 or C...
- Which of the following pure compounds can form hydrogen bonds? Which can form hydrogen bonds with water? Which...
- Draw the hydrogen bonding that takes place between a. two molecules of ethanol. b. two molecules of propylamin...
- For each pair of compounds, circle the compound you expect to have the higher boiling point. Explain your reas...
- Predict which compound in each pair has the higher boiling point. Explain your prediction. e. or f. or
- Draw the hydrogen bonding that takes place between c. a molecule of dimethyl ether and two molecules of water....
- Diethyl ether and butan-1-ol are isomers, and they have similar solubilities in water. Their boiling points ar...
- N-Methylpyrrolidine has a boiling point of 81 °C, and piperidine has a boiling point of 106 °C. b. Tetrahydro...
- List each set of compounds in order of increasing boiling point. a. hexane, octane, and decane
- Rank the following compounds from highest boiling to lowest boiling:
- Explain the following: a. 1-Hexanol has a higher boiling point than 3-hexanol.
- Explain why c. H2O (100 °C) has a higher boiling point than HF 120 °C2. d. HF 120 °C2 has a higher boiling p...
- a. Which of the following compounds forms hydrogen bonds between its molecules? 1. CH3CH2OCH2CH2OH 2. CH3CH2N...
- In each pair of compounds, which compound has the higher boiling point? Explain your reasoning. c. 2,2,5-trim...
- In each pair of compounds, which compound has the higher boiling point? Explain your reasoning. b. nonane or ...
- In each pair of compounds, which compound has the higher boiling point? Explain your reasoning. a. octane or ...
- List each set of compounds in order of increasing boiling point. b. Octane, (CH3)3C—C(CH3)3 and CH3CH2C(CH3)2...
- Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning. (b)
- (•••) Ionic compounds like sodium acetate have a high melting point. Despite this, they are highly soluble in ...
- (•••) Despite methylcyclohexane (98.2 amu) having a higher molecular weight than toluene (92.1 amu), toluene m...
- Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning. (a) e...
- Draw a picture of the hydrogen bonding in methanol.
- Which has g. the higher boiling point: 1-bromopentane or 1-chloropentane?
- Which has h. the higher boiling point: diethyl ether or butyl alcohol?
- For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if ...
- For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if ...
- For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if ...
- For each pair of compounds, predict which one has the higher molecular dipole moment, and explain your reasoni...
- For each pair of compounds, predict the one with a higher boiling point. Which compounds have zero dipole mome...
- For each pair of compounds, predict the one with a higher boiling point. Which compounds have zero dipole mome...
- For each pair of compounds, predict the one with a higher boiling point. Which compounds have zero dipole mome...
- Rank the boiling points of the following molecules. Explain your ranking ( 1 = highest bp; 3 = lowest bp ).
- a. Which compound would you expect to have a greater dipole moment, methyl acetate or butanone? b. Which woul...
- Imidazole boils at 257 °C, whereas N-methylimidazole boils at 199 °C. Explain the difference in boiling points...
- Rank the following isomers in order of increasing boiling point, and explain the reasons for your order of ran...
- (c) Peroxyacetic acid (bp = 105 °C) has a lower boiling point than acetic acid (bp = 118 °C), even though pero...
- Identify the hydrogen bond donors and hydrogen bond acceptors in the following molecules. (a)
- Identify the hydrogen bond donors and hydrogen bond acceptors in the following molecules. (d)
- In each pair, which would you expect to have the higher melting point? (b)