JEE Chemistry The P Block Elements Notes Set 15

Group 16: Oxygen Family

1. Members O, S, Se, Te, Po are chalcogens. First ionisation enthalpy O > S > Se > Te > Po. Electronegativity O > S > Se > Te. Metallic character increases down the group.
2. Oxygen is a diatomic gas with a p-pi to p-pi double bond. The others form single-bonded rings or chains, such as S8.
3. All members show allotropy. Oxygen: O2 and O3. Sulphur: rhombic (alpha, stable at room temperature), monoclinic (beta, stable above 95.5 C) and plastic sulphur; the transition temperature is 95.5 C.
4. Sulphur melts at about 113 C. Viscosity first falls as S8 rings slip over each other, rises sharply above 160 C as rings break and join into long chains, and falls again near 190 C as long chains break.
5. Sulphur is the best catenator of the group (polysulphides, H2Sx).

Sulphur and Hydrogen Sulphide

1. Milk of sulphur is obtained by boiling sulphur with lime and acidifying. Colloidal sulphur comes from thiosulphate with acid or from H2S with SO2.
2. Sulphur burns in fluorine to give SF6 and in air to give SO2.
3. H2S is made by \( \ce{FeS + 2HCl -> FeCl2 + H2S} \). It cannot be dried with conc. H2SO4 because it is oxidised to sulphur. P2O5 or fused CaCl2 is used.
4. H2S is a strong reducing agent: it reduces Cl2, I2, H2O2, SO2, Fe(III), acidic KMnO4 and K2Cr2O7. Pb acetate paper turns black and alkaline nitroprusside turns violet.
5. Alkali metal sulphides are soluble; Al2S3 and Cr2S3 are completely hydrolysed by water.

Sulphur Dioxide, Sulphuric Acid and Thiosulphate

1. SO2 is made industrially by roasting FeS2 or ZnS in air. In the lab, copper with hot conc. H2SO4 gives SO2: \( \ce{Cu + 2H2SO4 -> CuSO4 + SO2 + 2H2O} \).
2. SO2 is a reducing agent (it reduces Fe(III) to Fe(II), acidic dichromate and permanganate) and also an oxidising agent towards H2S: \( \ce{SO2 + 2H2S -> 3S + 2H2O} \).
3. H2SO4 is a strong dehydrating and oxidising agent. Metals above hydrogen give H2 with dilute acid, but Cu, Ag, Hg give SO2 with hot conc. acid. Carbon and sulphur are oxidised to CO2 and SO2.
4. P2O5 is a stronger dehydrating agent than H2SO4. Conc. H2SO4 with conc. HNO3 is a nitrating mixture.
5. Sodium thiosulphate, \( \ce{Na2S2O3.5H2O} \), is oxidised by iodine to tetrathionate, \( \ce{2S2O3^2- + I2 -> S4O6^2- + 2I-} \), and gives S and SO2 on acidification. It is used as antichlor and in photography.

Ozone

1. Ozone is made by passing a silent electric discharge through pure dry O2. It is a pale blue gas with a characteristic smell and is a powerful oxidising agent: \( \ce{O3 -> O2 + [O]} \).
2. It is absorbed by turpentine oil. It oxidises KI to iodine: \( \ce{O3 + 2KI + H2O -> 2KOH + I2 + O2} \) (this is used for estimating ozone with thiosulphate). It makes mercury lose its meniscus (tailing of mercury).
3. Uses: sterilising water, bleaching, air purification and as an oxidant for organic compounds.