P - Block Elements
Elements and Compounds of Elements of Group-16
- VI A group elements is comprised of O, S, Se, Te and Po. These elements are known as chalcogens or ore forming elements . The p-block elements are characterised by ns2 np4 general electronic configuration.
- In this group only sulphur has tendency of catenation.
- Oxygen shows −2 general oxidation states due to high electronegativity. It also exhibits −1 state in H2O2, zero in O2, +2 in OF2 and +1 in O2F2.
- Unlike oxygen, other elements have tendency to show +2, +4 and +6 oxidation states. These elements show co-ordination number 4 and 6 in their compounds.
- All of these form hydrides of general formula H2R order of acidic character of hydride is H2O < H2S < H2Se < H2Te
- Bond angle in hydrides shows the following order. They have v-shape structure
H2O(104.5°) > H2S(92.2°) > H2Se(91.0°) > H2Te(90°).
- Reducing nature has order H2O < H2S < H2Se < H2Te < H2Po.
- The increasing order of acidic nature of trioxide is TeO3 < SeO3 < SO3.
- The order of strength of oxyacids of higher oxidation state (+6) is : H2TeO4 < H2SeO4 < H2SO4
- The order of strength of oxyacids of lower oxidation state (+4) is : H2TeO3 < H2SeO3 < H2SO3
- Only oxygen shows paramagnetic behaviour in the group, due to presence of 2 unpaired electrons in the O2 molecule. This is only explained by molecular orbital theory.
- Ozone is a highly unstable triatomic allotropic form of oxygen. The angle between the 3 oxygen atoms is 116.8°.
- SO2 acts both as an oxidising and reducing agent.
- When SO3 gas is passed in concentrated H2SO4, the fuming H2SO4 or Oleum (H2S2O7) is obtained.
- Sulphuric acid (H2SO4), oil of vitriol is manufactured by two processes, chamber process and contact process.
- The catalyst used in the lead chamber process is oxide of nitrogen and the catalyst used in contact process are platinum, ferric oxide or vanadium pentoxide.
- Sulphuric acid is a good dehydrating as well as oxidising agent.
- Sodium thiosulphate or hypo (Na2S2O3.5H2O) is antichlor. So it is used to remove excess of chlorine in textile industry. It also finds a use in photography as a developer, because it dissolves AgBr.
- SO2 bleaches by reduction in presence of moisture. It's bleaching action is temporary. During the bleaching action, SO2 is oxidised to H2SO4.
- Sulphurous acid series : Oxidation state of S
Sulphurous acid, H2SO3 : +4
Thio sulphurous acid, H2S2O2 : +4, −2
Hydro sulphurous acid, H2S2O4 : +3, +3
Pyro sulphurous acid, H2S2O5 : +5, +3
- Sulphuric acid series : Oxidation state of S
Sulphuric acid, H2SO4 : +6
Thio sulphuric acid, H2S2O3 : +6, −2
Pyro sulphuric acid (or) Disulphuric acid, H2S2O7 : +6, +6
- Peroxo acid series : Oxidation state of S
Per mono sulphuric acid (Caro's acid), H2SO5 : +6
Per disulphuric acid (Marshall's acid), H2S2O8 : +6, +6
- Ozone can be prepared by silent electric discharge on pure dry oxygen gas.
- Formation of ozone from oxygen is reversible and endothermic process.
- Ozone is heavier than air, poisonous and slightly soluble in water. It is highly soluble in turpentine oil or glacial acetic acid.
- Ozone is a powerful oxidizing agent, next to F2. Ozone oxidizes black lead sulphide to white lead sulphate, HCl to Cl2 and iodide to I2.
- Ozone acts as bleaching agent. Ozone bleaches by oxidation. Ozone is dry bleach for oils, ivory etc.
- Sulphuric acid (H2SO4) is also called as 'oil of vitriol' or 'king of chemicals'.
- Sulphuric acid is commercially prepared by contact process. Three main steps are : formation of SO2 by burning of 'S' (or) iron pyrites in oxygen, catalytic oxidation of SO2 into SO3 and absorption of SO3 into H2SO4.
- Pure SO3 obtained is absorbed in 98% conc. H2SO4 to form oleum also called pyro sulphuric acid (H2S2O7).
- Oleum is diluted with H2O to obtain H2SO4 of desired concentration.
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