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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Heavy water: Preparation, Principle, Properties, Important reactions, Uses

Heavy water: Preparation, Principle, Properties, Important reactions, Uses
It is also called as deuterium oxide. The oxide of heavy hydrogen (deuterium) is called heavy water. Heavy water was discovered by Urey in 1932. By experimental data he showed that `ordinary water', H2O contains small proportion of heavy water, D2O (about 1 part in 5000).

Heavy water

 

It is also called as deuterium oxide. The oxide of heavy hydrogen (deuterium) is called heavy water. Heavy water was discovered by Urey in 1932. By experimental data he showed that `ordinary water', H2O contains small proportion of heavy water, D2O (about 1 part in 5000).

 

Preparation: The main source of heavy water is the ordinary water from which it is isolated. Generally it is prepared by exhaustive electolysis.

 

Principle: The heavy water is isolated either by prolonged electrolysis or by fractional distillation of water containing alkali. Taylor, Eyring and First in 1933 formulated the electrolysis of water in seven stages using N/2-NaOH solution and strip nickel electrodes.

The cell consists of a steel cell 18 inches long and 4 inches in diameter. The cell itself serves as the cathode while the anode consists of a cylindrical sheet of nickel with a number of holes punched in it. A large number of such cells are used for electrolysis of water in several stages. The gases obtained from each stage are separately burnt and the water thus formed is returned to the previous stage. The heavy water gradually concentrates in the residue left behind. The process usually consists of five stages.

 

A partial separation of heavy water from ordinary water can be affected by fractional distillation. This method utilises the small difference in boiling points of protium oxide (H2O) and deuterium oxide (D2O).

Comparison of water and heavy water

 

Property          : H2O - D2O

              

Density at : 20oC       - 0.998       1.017

Freezing point :         0oC    - 3.82oC

Boiling point : 100oC         101.42oC

Maximum density :   1.000 (4oC) - 1.1073 (11.6oC)

Specific heat at :  20oC :      1.00   - 1.01

Surface tension at 20oC :    72.8 dynes/cm -    67.8 dynes/cm

Dielectric constant :   82.0   - 80.5

Viscosity at 20oC :   10.09 millipoises  - 12.6 millipoises

 

The solubilities of substances in heavy water also differ from those in ordinary water. Thus sodium chloride is about 15% less soluble in heavy water than in ordinary water.

Physical Properties

 

Heavy water is a colourless, odourless and tasteless mobile liquid. Higher viscosity of heavy water is responsible for lower solubility of ionic solids like NaCl and smaller mobilities of ions.

Chemical Properties

 

The difference in chemical behaviour between H2O and D2O is very slight. However, the reaction velocity in general is slightly less in case of D2O reactions.

Important reactions of heavy water

1.With metals

 

D2O reacts slowly with alkali and alkaline earth metals liberating heavy hydrogen

2 Na + 2 D2O -- > 2 NaOD + D2 (Sodium deuteroxide)

Ca + 2 D2O -- > Ca (OD)2 + D2 (Calcium deuteroxide)

2. With metallic oxides

Metals like sodium and calcium dissolve in D2O and form heavy alkalies.

Na2O + D2O 2 NaOD

CaO + D2O Ca(OD)2

3. With acid anhydrides

D2O forms corresponding acids containing heavy hydrogen.

SO3 + D2O -- > D2 SO4

Deutero sulphuric acid

P2O5 + 3D2O    -- > 2D3 PO4

Deuterophosphoric acid

 

4. Upon electrolysis, heavy water containing dissolved P2O5, decomposes into deuterium and oxygen which are liberated at the cathode and anode respectively.

2D2O   2D2 + O2

 

5. With salt and other compounds they form deuterates.

 

Cu SO4. 5 D2O, Na2 SO4. 10D2O, NiCl2.6D2O

 

7. Exchange reactions

 

When compounds containing hydrogen are treated with D2O, hydrogen undergoes an exchange for deuterium.

NaOH + D2O  ->  NaOD + HOD

NH4 Cl + 4D2O  -- >   ND4 Cl + 4HOD

 

Biological Properties

In general heavy water, retards the growth of living organisms like plants and animals. The tobacco seeds do not grow in heavy water. Also, pure heavy water kills small fish, tadpoles and mice when fed upon it. Certain moulds have been found to develop better in heavy water.

Uses of heavy water

1.                 As a neutron moderator, in nuclear reactors.

 

2.                 It is used as a tracer compound in the study of reactions occurring in living organisms.

 

3.                 It is used for the preparation of deuterium.

 

 

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