Kaplan MCAT General Chemistry Review

Polarity affects melting point just as it does boiling point: More polar molecules melt at a higher temperature because they tend to stick together better. Molecular symmetry also plays a more prominent role than with boiling point, because another consideration is how well molecules pack or “fit together” in the crystal. The more symmetrical a molecule is, the better it packs in the crystal, just as symmetrical puzzle pieces in a jigsaw puzzle fit together better than asymmetrical pieces.

Similar Questions

1) For straight-chain alkanes, which do you suppose have higher melting points: alkanes with an odd number of carbons, or those with an even number of carbons?

2) Which molecule would you expect to melt at a higher temperature, n–pentane or neopentane (2,2–dimethylpropane)? Why?

3) Between phenol (hydroxybenzene) and aniline (aminobenzene), which would melt at a higher temperature and why?

Boiling Points

Key Concepts

Chapter 3

Boiling points

Intermolecular forces

Molecular symmetry

Given the following five molecules, place them in order of increasing boiling point:

1) Look for unusually heavy molecules.

Remember that molecular weight is one of the key determinants of boiling point. Something that is extraordinarily heavy is going to be harder to boil than something that is lighter. In this case, all the molecules are in the same general range of molecular weight, so this factor won’t help us place the molecules in order.

2) Look for highly polar functional groups.

Compounds 3 and 5 are going to have higher-than-usual boiling points. Between compounds 3 and 5, compound 3 will boil at a higher temperature because it has a more polar functional group; also, the alcohol is capable of hydrogen bonding. Compound 3 will have hydrogen bonds that are a stronger version of dipole–dipole interactions.

The other factor that affects boiling point is the presence of polar functional groups. These groups help increase boiling point because they increase the attractions of molecules for each other.

Remember: Hydrogen bonding is the strongest type of intermolecular attraction.

Takeaways

Remember that only two factors affect relative boiling points between substances: molecular weight and intermolecular forces.

3) Look for the effect of dispersion forces.

Compound 4 will boil at a higher temperature than 1, 2, and 5 because it is longer (eight carbons versus five, which increases its London forces); therefore, there are more opportunities for it to attract other molecules of 4.

Although 1, 2, and 5 all have the same surface area, the polar group on 5 gives it a higher boiling point than 1 and 2.

Remember: Dispersion forces are the only kind of intermolecular attractions that cause nonpolar molecules to stick together.

Things to Watch Out For

Be sure to consider the factors in the order presented here. For example, polarity is more important than size.

4) Look for trends in the symmetry of molecules.

In this case, pentane, 1, will boil higher than neopentane, 2. This is because neopentane is more symmetrical and, therefore, a more compact molecule; thus, it has a less effective surface area. You can determine this by imagining a “bubble” around each molecule. Neopentane could very easily fit into a spherically shaped bubble, whereas pentane would require an elongated, elliptical bubble with a greater surface area.

If neopentane has a smaller surface area, then there are fewer opportunities for it to engage in dispersion-type attractions with other molecules of neopentane, making it a lower-boiling-point compound (the actual boiling points are 36.1°C for pentane and 9.4°C for neopentane).

At this point, we’re really splitting hairs. Notice that compounds 1 and 2 are merely constitutional isomers of one another. If two molecules have the same weight and are relatively nonpolar, symmetry is the factor that decides which one will boil at a higher temperature.

Similar Questions

1) How could you easily alter the structure of compound 3 to lower its boiling point?

2) How could you easily alter the structure of compound 5 to raise its boiling point?

5) Put it all together. Order the compounds as specified by the question. The ordering of the boiling points will therefore be as follows:

2 < 1 < 5 < 4 < 3

Rate Law from Experimental Results

Key Concepts

Chapter 5

Kinetics

Reaction mechanisms

Rate law