JEE Chemistry Hydrogen Notes Set 02

Water and Hardness

Water has a bent structure with H-O-H angle of about 104.5 degrees. Strong hydrogen bonding explains its high boiling point and high heat of vaporisation. Hard water does not lather with soap, because Ca2+ and Mg2+ ions precipitate the soap as scum.
Temporary hardness is caused by bicarbonates of Ca and Mg and can be removed by boiling or by Clark's method (adding slaked lime). Permanent hardness is caused by chlorides and sulphates of Ca and Mg; it is removed by washing soda, by calgon, by permutit (sodium aluminium silicate) or by ion exchange resins.

Softening Methods

Washing soda (Na2CO3) precipitates Ca2+ and Mg2+ as carbonates. Calgon (sodium hexametaphosphate, Na2[Na4(PO3)6]) holds the ions in a soluble complex, so they are not removed but become inactive. In the permutit process, the Na+ in sodium zeolite is exchanged for Ca2+ and Mg2+, and the zeolite is regenerated with brine. Cation exchange resins (RCOOH type) swap H+ for Ca2+, and anion exchange resins swap OH- for Cl- and SO4 2-, giving demineralised water.

Heavy Water

Heavy water, D2O, is made by electrolysis of water or fractional distillation. Compared with ordinary water, it has higher density (1.1 g/mL), higher boiling point (about 374.6 K), lower dielectric constant, and is a poorer solvent for ionic compounds; its reactions are slower. It is used as a neutron moderator in nuclear reactors and as a tracer in studying reaction mechanisms.

Hydrogen Peroxide

Preparation: acidifying BaO2 with dilute H2SO4 (BaO2 + H2SO4 gives BaSO4 + H2O2), or by the auto-oxidation of 2-ethylanthraquinol in industry.
Structure: non-planar, half-open book shaped molecule with an O-O bond; dipole moment about 2.1 D, which shows it is not linear. O2F2 has a similar structure.
Oxidation number of oxygen is -1, so H2O2 can act both as oxidant (going to -2 in water) and reductant (going to 0 in O2). It decomposes in a disproportionation reaction, 2H2O2 gives 2H2O + O2, which is catalysed by light and metals.
Uses: bleaching of paper, wool and hair; antiseptic; restoring black lead sulphide paintings (PbS + 4H2O2 gives PbSO4 + 4H2O); and as a propellant. Pure H2O2 is a pale blue syrupy liquid, boiling at about 423 K, and is stored in wax-lined or plastic bottles.