Group 16 (the oxygen group) consists of O, S, Se, Te, and Po. These important elements can form divalent anions and have oxidation numbers of +4 and +6.
Basic Knowledge
Oxygen: 21% of the atmosphere and the most abundant element in the Earth's crust (about 46% by mass). Its allotropes include O2 (oxygen) and O3 (ozone). O3 is a pale-blue gas with a distinctive odor and is a strong oxidizing agent; the ozone layer in the stratosphere absorbs ultraviolet radiation. In the laboratory, it is produced by 2H2O2 → 2H2O + O2 (with MnO2 as a catalyst).
Sulfur: Its allotropes include orthorhombic sulfur (stable at room temperature, an S8 ring), monoclinic sulfur (stable at high temperatures), and plastic sulfur (amorphous). Its compounds include H2S (rotten-egg odor and reducing agent), SO2 (irritating odor, reducing agent and bleach), and SO3 (raw material for concentrated sulfuric acid). Concentrated sulfuric acid is manufactured by the contact process: S → SO2 → SO3 → H2SO4; V2O5 is used as a catalyst in the SO2→SO3 step.
Ozone O3 (strong oxidizing agent, pale-blue gas, oxidation number 0)
Hydrogen peroxide H2O2 (weak acid, acts as both an oxidizing and reducing agent)
Hydrogen sulfide H2S (rotten-egg odor, strong reducing agent, highly toxic)
Sulfur dioxide SO2 (bleaching and reducing agent; unlike HClO, it reduces the pigment itself)
Contact process (S→SO2→SO3→H2SO4, V2O5 catalyst)
Properties of concentrated sulfuric acid (hygroscopic, dehydrating, oxidizing, nonvolatile)
Further Study (Principles and Applications)
Concentrated sulfuric acid is called the mother acid of the chemical industry, with annual global production exceeding 200 million tons. It has extensive uses in fertilizers (ammonium sulfate and superphosphate), dyes, pharmaceuticals, and lead-acid batteries for automobiles.
The four properties of concentrated sulfuric acid are: ① hygroscopicity (drying agent), ② dehydrating action (carbonizes sucrose and causes dehydration in organic synthesis), ③ oxidizing action (hot concentrated sulfuric acid dissolves Cu and Ag: Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O), and ④ nonvolatility (liberates HCl from NaCl+H2SO4).
In environmental chemistry, SOx is a major cause of acid rain. It is released into the atmosphere by volcanic gases and fossil-fuel combustion, and sulfuric acid rain forms through SO2 + H2O → H2SO3 and 2SO2 + O2 → 2SO3. Desulfurization equipment (the limestone-slurry method) is essential.
- Allotropes of oxygen = O2 and O3
- Allotropes of sulfur = orthorhombic, monoclinic, and plastic sulfur
- Four properties of concentrated sulfuric acid = hygroscopicity, dehydration, oxidation, and nonvolatility
- Catalyst in the contact process = V2O5
- SO2 is both a bleach and a reducing agent (its bleaching principle differs from Cl2)
- H2S is a strong reducing agent and is used in systematic analysis through sulfide precipitation with metal ions
- SOx compounds cause acid rain
Points to Note (Common Confusions and Frequent Mistakes)
① Dilute sulfuric acid is an ordinary strong acid; only hot concentrated sulfuric acid has oxidizing properties. Cold concentrated sulfuric acid can be handled with Fe and Al because it forms a passive state, but they react when heated. ② SO2 bleaching is reduction bleaching (it reduces pigments), while Cl2 causes oxidation bleaching. ③ H2S is highly toxic (fatal concentrations are several hundred ppm); do not feel safe because of its rotten-egg odor at low concentrations. ④ H2O2 normally acts as an oxidizing agent, but it acts as a reducing agent toward strong oxidizing agents such as KMnO4.
Practice
- Write the three reaction equations for manufacturing concentrated sulfuric acid by the contact process.
- Write the reaction equation for hot concentrated sulfuric acid dissolving copper.
- State the difference between the bleaching principles of SO2 and Cl2.