Paul Flowers; Edward J. Neth; William R. Robinson, PhD; Klaus Theopold; Richard Langley

18.10 Occurrence, Preparation, and Properties of Sulfur

Learning Objectives

By the end of this section, you will be able to:

Describe the properties, preparation, and uses of sulfur

Sulfur exists in nature as elemental deposits as well as sulfides of iron, zinc, lead, and copper, and sulfates of sodium, calcium, barium, and magnesium. Hydrogen sulfide is often a component of natural gas and occurs in many volcanic gases, like those shown in Figure 18.58. Sulfur is a constituent of many proteins and is essential for life.

A lake is shown surrounded by rocky, mountainous peaks. A white vapor rises from the ground near the lake. — Figure 18.58 Volcanic gases contain hydrogen sulfide. (credit: Daniel Julie/Wikimedia Commons)

The Frasch process, illustrated in Figure 18.59, is important in the mining of free sulfur from enormous underground deposits in Texas and Louisiana. Superheated water (170 °C and 10 atm pressure) is forced down the outermost of three concentric pipes to the underground deposit. The hot water melts the sulfur. The innermost pipe conducts compressed air into the liquid sulfur. The air forces the liquid sulfur, mixed with air, to flow up through the outlet pipe. Transferring the mixture to large settling vats allows the solid sulfur to separate upon cooling. This sulfur is 99.5% to 99.9% pure and requires no purification for most uses.

A diagram is shown in which a vertical tube is embedded on the lower end into a multilayered solid. The upper layer is labeled, “Soil,” and the lower layer is labeled, “Solid sulfur deposit.” A thin tube, beginning at the top of the diagram, leads into the vertical tube to the bottom and is labeled, “Compressed air.” Left-facing and then down-facing arrows are drawn on this inner tube. These arrows then turn upward at the bottom of the tube and are drawn upward to indicate the flow of, “Liquid sulfur,” from the bottom of the diagram to the top outside the inner tube. These arrows lead to a chamber at the top right of the diagram with arrows facing to the left labeled, “Sulfur, water and air.” A horizontal tube in the center right of the diagram leads to the outer tube and arrows drawn downward lead back to the bottom of the diagram. This tube is labeled, “Superheated water.” — Figure 18.59 The Frasch process is used to mine sulfur from underground deposits.

Larger amounts of sulfur also come from hydrogen sulfide recovered during the purification of natural gas.

Sulfur exists in several allotropic forms. The stable form at room temperature contains eight-membered rings, and so the true formula is S₈. However, chemists commonly use S to simplify the coefficients in chemical equations; we will follow this practice in this book.

Like oxygen, which is also a member of group 16, sulfur exhibits a distinctly nonmetallic behavior. It oxidizes metals, giving a variety of binary sulfides in which sulfur exhibits a negative oxidation state (2−). Elemental sulfur oxidizes less electronegative nonmetals, and more electronegative nonmetals, such as oxygen and the halogens, will oxidize it. Other strong oxidizing agents also oxidize sulfur. For example, concentrated nitric acid oxidizes sulfur to the sulfate ion, with the concurrent formation of nitrogen(IV) oxide:

S (s) + 6 {HNO}_{3} (a q) ⟶ 2 H_{3} O^{+} (a q) + {SO}_{4}^{2−} (a q) + 6 {NO}_{2} (g)

The chemistry of sulfur with an oxidation state of 2− is similar to that of oxygen. Unlike oxygen, however, sulfur forms many compounds in which it exhibits positive oxidation states.