Water

WATER

Human body has about 65% water and some plants have as much as 95% water. It is a crucial compound for the survival of all life forms.

Physical Properties of Water

• It is a colourless and tasteless liquid.
• The presence of extensive hydrogen bonding between water molecules leads to high freezing point, high boiling point, high heat of vaporisation and high heat of fusion in comparison to H₂S and H₂Se.
• Water has a higher specific heat, thermal conductivity, surface tension, dipole moment and dielectric constant, etc.

STRUCTURE OF WATER

CHEMICAL PROPERTIES OF WATER

(1) Amphoteric Nature:

            H₂O(l) + NH₃(aq) ⇌ OH^-(aq) + NH₄⁺(aq)

            H₂O(l) + H₂S(aq) ⇌ H₃O⁺(aq) + HS^-(aq)

The auto-protolysis (self-ionization) of water takes place as follows:

            H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH^-(aq)

(2) Redox Reactions: Water is easily reduced to hydrogen by highly electropositive metals.

            2H₂O(l) + 2Na(s) → 2NaOH(aq) + H₂(g)

With fluorine also it is oxidised to O₂.

            2F₂(g) + 2H₂O(l) ® 4H⁺ (aq) + 4F^–(aq) + O₂(g)

(3) Hydrolysis Reaction: Due to high dielectric constant, it has a very strong hydrating tendency..

                      P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)

                      SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(aq)

(4) Hydrates Formation:

(i) coordinated water e.g. [Cr(H₂O)₆]Cl₃

(ii) interstitial water e.g., BaCl₂.2H₂O

(iii) hydrogen-bonded water eg. [Cu(H₂O)₄]SO₄.H₂O

HARD AND SOFT WATER

• Temporary hardness,
• Permanent hardness.

Temporary Hardness due to the presence of magnesium and calcium bicarbonates. It can be removed by:

(i) Boiling:

                 Mg(HCO₃)₂ ⎯ heating® Mg(OH)₂ ↓ + 2CO₂ ↑

                  Ca(HCO₃)₂ ⎯ heating® CaCO₃ ↓ +H₂O+ CO₂ ↑

(ii) Clark’s method:

               Ca(HCO₃)₂ + Ca(OH)₂ ® 2CaCO₃ ↓ +2H₂O

               Mg(HCO₃)₂ +2Ca(OH)₂ ®2CaCO₃¯ +Mg(OH)₂¯ +2H₂O

Permanent Hardness is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. Permanent hardness cannot be removed by boiling. It can be removed by the following methods.

(i) Treatment with washing soda (sodium carbonate):

                 MCl₂ + Na₂CO₃ ® MCO₃↓ + 2NaCl (M=Mg, Ca)

                 MSO₄ + Na₂CO₃ ® MCO₃↓ + Na₂SO₄

(ii) Calgon’s method: Sodium hexametaphosphate (Na₆P₆O₁₈), commercially called ‘calgon’, when added to hard water, the following reactions take place.                                                                    

                          Na₆P₆O₁₈ ® 2Na⁺ +Na₄P₆O₁₈^-2   (M = Mg, Ca)

M⁺² +Na₄P₆O₁₈^-2 ® Na₂MP₆O₁₈^-2 +2Na⁺ the complex anion keeps the Mg²⁺ and Ca²⁺ ions in solution

(iii) Ion-exchange method: sodium aluminium silicate (NaAlSiO₄) can be written as   NaZ. When this is added in hard water, exchange reactions take place.          

                          2NaZ(s) +M⁺²(aq) ® MZ₂(s) +2Na⁺(aq) (M=Mg, Ca)

Permutit/zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating with an aqueous sodium chloride solution.                                                                                

                            MZ₂(s) +2NaCl(aq) →2NaZ(s) + MCl₂

Hydrogen Peroxide (H₂O₂)

PREPARATION

(i) Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.

                BaO₂.8H₂O(s) + H₂SO₄(aq) ® BaSO₄(s) + H₂O₂(aq) + 8H₂O(l)

(ii) Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.

2HSO₄^-(aq) - electrolysis ®  HO₃SOOSO₃H(aq) -hydrolysis ® 2HSO₄^-(aq) + 2H⁺(aq) + H₂O₂(aq)

(iii) Industrially it is prepared by the auto-oxidation of 2-alklylanthraquinols.

In this case 1% H₂O₂ is formed. It is extracted with water and concentrated to ~30% (by mass) by distillation under reduced pressure. It can be further concentrated to ~85% by careful distillation under low pressure. The remaining water can be frozen out to obtain pure H₂O₂

PHYSICAL PROPERTIES

In the pure state H₂O₂ is an almost colourless (very pale blue) liquid. H₂O₂ is miscible with water in all proportions and forms a hydrate H₂O₂.H₂O (M.P. 221K). A 30% solution of H₂O₂ is marketed as ‘100 volume’ hydrogen peroxide. It means that one millilitre of 30% H₂O₂ solution will give 100 V of oxygen at STP. Commercially, it is marketed as 10 V, which means it contains 3% H₂O₂.

STRUCTURE

Hydrogen peroxide has a non-planar structure. The molecular dimensions in the gas phase and solid phase are shown:

CHEMICAL PROPERTIES

It acts as an oxidising as well as reducing agent in both acidic and alkaline media.  Simple reactions are described below.

(i) Oxidising action in acidic medium

             2Fe⁺²(aq) + 2H⁺(aq) +H₂O₂(aq) ® Fe⁺³(aq) + 2H₂O

              PbS(s) + 4H₂O₂(aq) ®  PbSO₄(s) + 4H₂O

(ii) Reducing action in acidic medium

             2MnO₄^-(aq) + 5H₂O₂(l) + 6H⁺ ® 2Mn²⁺(aq) + 5O₂(g) + 8H₂O

              HOCl + H₂O₂ ®H₃O⁺ + Cl^- + O₂

(iii) Oxidising action in basic medium

            2Fe⁺² + H₂O₂ ® Fe⁺³ + 2OH^-

            Mn⁺² + H₂O₂ ® Mn⁺⁴ + 2OH^-

(iv) Reducing action in basic medium

             I₂ + H₂O₂ + 2OH^- ® 2I^- + 2H₂O + O₂

        2MnO₄^- + 3H₂O₂ ® 2MnO₂ +3O₂ + 2H₂O +2OH^-

STORAGE

H₂O₂ decomposes slowly on exposure to light.

                   2H₂O₂(l) ®  2H₂O(l) + O₂(g)

In the presence of metal surfaces or traces of alkali (present in glass containers), the above reaction is catalysed. It is, therefore, stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabiliser. It is kept away from dust because dust can induce explosive decomposition of the compound.

Uses

Its wide scale use has led to tremendous increase in the industrial production of H₂O₂. Some of the uses are listed below: 

(i) In daily life it is used as hair bleach and as a mild disinfectant. As an antiseptic it is sold as perhydrol.

(ii) It is used to manufacture chemicals like sodium perborate and per-carbonate, which are used in high quality detergents.

(iii) It is used in the synthesis of hydroquinone, tartaric acid and certain food products and pharmaceuticals (cephalosporin) etc.

(iv) It is employed in the industries as a bleaching agent for textiles, paper pulp, leather, oils, fats, etc.

(v) Nowadays it is also used in Environmental (Green) Chemistry. For example, in pollution control treatment of domestic and industrial effluents, oxidation of cyanides, restoration of aerobic conditions to sewage wastes, etc.

Heavy Water, D₂O

It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries. 

It is used for the preparation of other deuterium compounds, for example:

                     CaC₂ +D₂O ® C₂D₂ + Ca(OD)₂

                     SO₃ + D₂O ® D₂SO₄

                     Al₄C₃ + 12D₂O ® 3CD₄ + 4Al(OD)₃