5 Steps to a 5 AP Chemistry 2019

368 ❯ STEP 5. Build Your Test-Taking Confidence

Question 5

150

Boiling points of hydrogen compounds of the p-block elements

234

Period

5

Group 16

Group 15

Group 17

Group 14

100

50

0

°C

–50

–100

–150

–200

(a) The Group 14 line is the dotted line. The dotted line should always be below the other lines because

the hydrogen compounds of the Group 14 elements are all nonpolar, while all the other hydrogen

compounds on the graph are polar. The dotted line should not go up at the end because CH 4 is the

only Period 2 hydrogen compound on the graph that is not capable of hydrogen bonding.

Give yourself 1 point if your line remains below all the other lines. Give yourself a second point if

your line does not move up at the left end.

(b) The Period 2 compounds on the graph are, from top to bottom, H 2 O, HF, NH 3 , and CH 4. The first

three of these compounds are the only compounds plotted that are capable of hydrogen bonding.

Hydrogen bonding in stronger than any of the other intermolecular forces present in the compounds

on the graph, which means their boiling points should be higher than those of compounds without

hydrogen bonding.

Give yourself 1 point for this explanation.

(c) Li is a Period 2 element; therefore, it would appear on the left side of the graph and it would have

a much higher boiling point than the other compounds because it is an ionic compound and ionic

compounds, because of their high lattice energies, have very high boiling points.

You get 1 point if you give the correct approximate position and mention ionic bonding.

A total of 4 points is possible.

21-Moore_PE02_p341-370.indd 368 31/05/18 1:54 pm