5 Steps to a 5 AP Chemistry 2019

AP Chemistry Practice Exam 1 ❮ 333

Question 6

Acetylene, C 2 H 2 , will react with a limited amount of bromine vapor, Br 2 , to form 1,2-dibromoethene,

C 2 H 2 Br 2. The product of the reaction is contaminated with unreacted acetylene and 1,1,2,2-tetrabromo-

ethane, C 2 H 2 Br 4. The key reactions are:

C 2 H 2 (g) + Br 2 (g) → C 2 H 2 Br 2 (g)

C 2 H 2 (g) + 2 Br 2 (g) → C 2 H 2 Br 4 (g)

(a) The left box below shows the Lewis electron-dot diagram for acetylene. The box on the right has an

incomplete Lewis electron-dot diagram for 1,2-dibromoethene. Complete the diagram on the right by

adding all the electron pairs.

H C CH

Br Br

HCC H

(b) In the box below, draw the complete Lewis electron-dot structure for 1,1,2,2-tetrabromoethane.

(c) Arrange the compounds C 2 H 2 , C 2 H 2 Br 2 , and C 2 H 2 Br 4 in order of decreasing H–C–C bond angle.

Why did you place these compounds in the order you predicted? Your explanation should include

approximate bond angles.

Question 7

A student in Denver, Colorado, used the following reaction to generate oxygen gas in a gas generator:

2 KClO 3 (s) → 2 KCl(s) + 3 O 2 (g)

The gas generator was connected by a tube to an inverted flask, filled with water, in a water bath. The gas

displaced all the water. The volume of the sample was 500.0 mL at 26°C, and the pressure in the room

was 626 mm Hg. The vapor pressure of water at 26°C is 25 mm Hg.

A second student conducted a similar experiment under identical conditions beginning with the fol-

lowing reaction:

Mg 3 N 2 (s) + 6 H 2 O(l) → 3 Mg(OH) 2 (s) + 2 NH 3 (g)

The second student’s results were dramatically different from those of the first student (not as good), indi-

cating that there was something drastically different.

(a) How many grams of oxygen are in the flask? Show all calculations.

(b) How many hydrogen atoms are in the flask after it is filled with oxygen?

(c) Why is the most likely reason why the second student did not get good results? Explain your

reasoning.

STOP. End of AP Chemistry Practice Exam 1, Section II (Free Response).

20-Moore_PE01_p307-340.indd 333 31/05/18 1:58 pm