Group 16- The Calcogens

1. Group 16- The Calcogens - O, S, Se, Nonmetals; Te, metalloid; and Po metal - Have multiple allotropes: Oxygen is a gas as O2 or O3; Sulfur is a Gas as S2 S3 S4 Solid as S5 to S8 are cyclic molecules. Selenium is a solid in either Se8 or polymeric form - Unpleasant odours - Form one monoatomic anion (-2) Po2+ and Po4+ also exist (radio active) - Six valence electrons: [N.G.] ns2 np4 - Large negative ΔHelectronic attraction

2. “oxygen”, typically diatomic oxygen (O2), Phases of matter: Gas: colourless, odourless, tasteless Liquid: pale blue explosive liquid. Solid: pale blue and very explosive. Reactions Strong oxidizing agent. Reacts spontaneously with the metals of groups 1, 2 and 13; as well as some transition metals. Some of these reactions are quite slow unless heat or a catalyst is added. Why? Another way to increase the reactivity of gaseous O2 is to add water. Iron rusts much more quickly in damp air than in dry air! Oxygen Photo by the US EPA

3. - Ozone (O3) - a significant component of “smog”; - essential component of the upper atmosphere. - Ozone is formed from O2 in an endothermic reaction: This reaction requires a large input of energy, i.e., from an electric current or electromagnetic radiation from the sun. - Ozone is unstable, decomposing to oxygen (O2). In order to maintain constant amount of ozone, it must continually be regenerated. - Printer and photo copiers produce ozone Health hazard - Used as in swimming pools Ozone Photo by the US EPA

4. - Ozone absorbs UV protecting organisms significant DNA damage. -Ozone is broken into O2 and oxygen free radical which reacts ozone to form O2. -This isanexothermic process helping to moderate the Earth’s temperature. S. Rowland and M. Molina in 1974 first reported that atmospheric ozone was disappearing wining them The Nobel Prize in Chemistry in 1995. -The “Ozone Hole” in the Antarctic was first reported in 1985 by J. Farman, B. Gardiner, and J. Shanklin. Atmospheric Ozone • The “hole in the ozone layer” is an area in which the ozone has thinnedsubstantially. 70% of the ozone • over the Antarctic and 30% of the ozone over the • Arctic had disappeared by the early 1980s!

5. Where is the ozone going? • Chlorofluorocarbons (CFCs) used as refrigerants – accelerate the • decomposition of ozone to O2. • the CFC absorbs UV radiation to give a free chlorine atom as a “free radical” • This chlorine atom reacts with ozone to form O2 and ClO. • 3) The ClO reacts with a free oxygen atom to form more O2 and regenerate the chlorine atom. • This cycle continues until the chlorine atom finds something to react • with other than ozone. That means that one polluted CFC molecule • can be responsible for the destruction of millions of ozone molecules!

6. Sulfur • Sulfur exists as a wide range of allotropes: • S2 is violet • S3 is blue • S4 is red • S5 is red-orange • S6 is yellow-orange • S7 to S15 are all shades of yellow as are S18, S20 • & S (a polymer) • Some of these allotropes can be further divided into different forms based • on their crystal structure (e.g. -S8 and β-S8 have different densities and • melting points). • Naturally occurring sulfur is -S8, eight sulfur atoms in a puckered ring often called a crown. This form of sulfur is insoluble in water. • If sulfur is heated to 400 oC then rapidly cooled (e.g. by pouring into cold water), we get “plastic sulfur” which can be pulled into threads.

7. The Frasch process • The Frasch process is an extraction technique. • In Texas and Louisiana, sulfur deposits of 60-100 meters thick are 400-800 • meters underground. • Superheated water (160oC liquid, 16 bar) melts the sulfur then hotcompressed • air (20-25 bar) forces the molten sulfur • up a third pipe. • It takes 10-15 kg of water to extract 1 kg of S8 • The sulfur produced is 98-99.5% pure and can be • poured into moulds or distributed as a liquid.

8. The Claus process • The Claus process generates S8 from H2S. • Mandated emission limits for H2S in • the oil and gas industry. Sour gas wells!! • Two-step process. • H2S is burned in the presence of O2. • (exothermic) • 2) SO2 is reacted with additional H2S. • (exothermic but requires a catalyst). • In practice, what is done is to pipe a mixture of H2S and O2 • into a combustion chamber where • most of it reacts to form S8. The • remaining H2S and SO2 produced • are then piped into two sequential • reaction chambers where they • complete the second step.

9. Oxoacids of Sulfur • Oxoacids of sulfur are strong acids. When SO3 is dissolved in water, • sulfuric acid is produced: • - If SO3 is bubbled through concentrated sulfuric acid, one produces • “fuming sulfuric acid” a.k.aOleum: • ySO3.H2O or H2SO4.xSO3 • - Intermediate in production of sulfuric acid. Used in the manufacturing of • TNT. • - When power plants burn sulfur-containing coal or oils, they release sulfur • dioxide which reacts with another pollutant, nitrogen dioxide, to produce • sulfur trioxide and nitrogen monoxide: • SO3 produces dissolves in atmospheric water vapour causing acid rain!

10. Oxoacids of Sulfur • Sulfur oxoanions – Sulfate SO4-2 and Sulfite SO3-2. • - More complicated oxoanions with bridging sulfur • Ex) S2O4-2 S2O6-2 S2O8-2 S4O6-2 • Draw the Lewis structures of sulfur trioxide and sulfite. How do are these two species similar? How do they differ? If you dissolved 1 mole of each in a liter of water, which would give you a Solution with a lower pH?