SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 13

Some Representative Groups and Families

These skills are usually tested on the SAT Subject Test in Chemistry. You should be able to …

• Describe the properties, both physical and chemical, of the major members of each group and family, and the common compounds formed by the sulfur, halogen, and nitrogen families and by major metals and their alloys.

• Write equations for major reactions involving these elements.

This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter.

In the following section, a brief description is given of some of the important and representative groups of elements usually discussed in most first-year chemistry courses.

SULFUR FAMILY

Since we discussed oxygen in Chapter 5, the most important element in this family left to discuss is sulfur.

Sulfur is found free in the volcanic regions of Japan, Mexico, and Sicily. It is removed from the rock mixtures by heating in retorts or furnaces.

Sulfuric Acid

Important properties of sulfuric acid. Sulfuric acid ionizes in two steps:

H2SO4(l) + H2O(l)  H3O+(aq) + HSO4(aq)          Ka1 is very large.
HSO4(aq) + H2O(l)  H3O+(aq) + SO42−(aq)        Ka2 is very small.

to form a strong acid solution. The ionization is more extensive in a dilute solution. Most hydronium ions are formed in the first step. Salts formed with the HSO4(bisulfate ion) are called acid salts; the SO42− (sulfate ion) forms normal salts. Sulfuric acid reacts like other acids, as shown below:

With active metals:

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) (for dilute H2SO4)

With bases:

2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)

With metal oxides:

MgO(s) + H2SO4(aq) → MgSO4(aq) + H2O(l)

With carbonates:

CaCO3(s) + H2SO4(aq) → CaSO4(aq) + H2O(l) + CO2(g)

Sulfuric acid has other particular characteristics.

As an oxidizing agent:

Cu(s) + 2H2SO4(aq)(concentrated) → CuSO4(aq) + SO2(g) + 2H2O(l)

As a dehydrating agent with carbohydrates:

C12H22O11(sugar)  12C(s)+11H2O(l)

Other Important Compounds of Sulfur

Hydrogen sulfide is a colorless gas having an odor of rotten eggs. It is fairly soluble in water and is poisonous in rather small concentrations. It can be prepared by reacting ferrous sulfide with an acid, such as dilute HCl:

FeS(s) + 2HCl(aq) → FeCl2(aq) + H2S(g)

Hydrogen sulfide burns in excess oxygen to form compounds of water and sulfur dioxide. If insufficient oxygen is available, some free sulfur will form. It is only a weak acid in a water solution. Hydrogen sulfide is used widely in qualitative laboratory tests since many metallic sulfides precipitate with recognizable colors. These sulfides are sometimes used as paint pigments. Some common sulfides and their colors are:

ZnS—White

CdS—Bright yellow

As2S3—Lemon yellow

Sb2S3—Orange

CuS—Black

HgS—Black

PbS—Brown-black

Another important compound of sulfur is sulfur dioxide. It is a colorless gas with a suffocating odor.

The structure of sulfur dioxide is a good example of resonance structures. Its molecule is depicted in Figure 39.

Figure 39. Sulfur Dioxide Molecule

You will notice in Figure 39 that the covalent bonds between sulfur and oxygen are shown in one drawing as single bonds and in the other as double bonds. This signifies that the bonds between the sulfur and oxygens have been shown by experimentation to be neither single nor double bonds, but “hybrids” of the two. Sulfur trioxide, shown below, also has resonance structures.

TIP 

Resonance is a hybrid of the two structures shown.

“—” indicates a covalent bond