Heavy Water|Class-11

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Heavy Water
Heavy Water

Discovery:

Heavy water or deuterium oxide (D2O)was discovered by American Chemist Harold C. Urey in 1932.

Preparation of heavy water:

Heavy water is mainly prepared from ordinary water by two processes:

By prolonged electrolysis :

Principle:

This method is based on the principle that, when ordinary water is subjected to electrolysis, protium or hydrogen is liberated more rapidly than deuterium because-

As H+ion is smaller than D+ion, H+ion possesses greater mobility than D+ion.

H+ion possesses lower discharge potentialthan D+ion.

O-H bond is weaker than O-D bond.

Hydrogen atoms combine more rapidly to form H2 than do deuterium atoms to form D2.

Procedure:

The electrolytic cell (designed by Brown, Degget, and Urey) consists of a cylindrical steel cell (which acts as a cathode). A perforated cylindrical sheet of nickel is used as an anode and nearly 0.5(N)NaOH solution is used as an electrolyte. The electrolysis is carried out in different stages. Almost 2.9 x 104 L of ordinary water is to be electrolyzed to obtain 1L of 99% pure D2O.

By fractional distillation:

The boiling points of ordinary water and heavy water are 373K and 374.42K respectively. Due to apparently small differences in boiling points, the fractional distillation of ordinary water is carried out in a long (almost 12-13m) fractionating column, and the process is carried out several times. Ordinary water distilled first leaving behind D2O in the vessel.

Physical properties:

PropertiesD2O (Heavy water)
Molar Mass20.0276
AppearanceVery pale blue
Freezing point276.97  K
Boiling point374.55  K
Dipole moment1.87D
Density at STP (g/mL)1.1056
Dielectric constant78.06
Temp. of maximum density11.6 °C
Surface tension (at 25 °C, N/m)0.07187
Heat of fusion (kJ/mol)6.132
Heat of formation (kJ/mol)-294.6
Heat of vaporisation (kJ/mol)41.521
pH (at 25 °C)7.43
Ionisation constant (mol2L-2)1.95 x 10-15

The difference in physical properties between H2O and D2O is due to the greater nuclear mass of deuterium and a greater degree of association of D2O.

Chemical Properties:

The chemical properties of heavy water are almost similar to that of ordinary water but as O-D bond is stronger than O-H bond, the reaction rate of heavy water is apparently slow than that of ordinary water.

Reaction with alkali and alkaline earth metals:

Heavy water reacts with alkali and alkaline earth metals to produce deuteroxides and dideuterium.

2Na + 2D2O→2NaOD + D2

Ca + 2D2O→Ca(OD)2 + D2

Reaction with metal oxides:

Heavy water reacts with alkali and alkaline earth metals oxides to produce  deuteroxides.

Na2O + 2D2O→2NaOD

CaO  + D2O→Ca(OD)2

Reaction with non-metal oxides:

Heavy water reacts with non-metallic oxides to produce corresponding deuteroacids.

CO2 + D2O→D2CO3 (Deuterocarbonic acid)

SO2 + D2O→D2SO3 (Deuterosulphurous acid)

P4O10 + 6D2O→4D3PO4 (Deuterophosphoric acid)

N2O5 + D2O→2DNO3 (Deuteronitric acid)

Reaction with carbides, nitrides, phosphides, arsenides:

CaC2 + 2D2O→Ca(OD)2 + C2D2(Deuteroacetylene)

Al4C3 + 12D2O→4Al(OD)3 + 3CD4(Deuteromethane)

Ca3N2 + 6D2O→3Ca(OD)2 + 2ND3 (Deuteroammonia)

AlN + 3D2O→Al(OD)3 + ND3

Ca3P2 + 6D2O→3Ca(OD)2 + 2PD3(Deuterophosphine)

Na3As + 3D2O→3NaOD + AsD3 (Deuteroarsine)

Electrolysis:

When heavy water is electrolysed, dideuterium is obtained at the cathode.

2D2O→2D2 + O2

Exchange reactions:

When compounds having active hydrogen are treated with D2O, hydrogen is exchanged by deuterium completely or partially.

NH4Cl + 4D2O→ND4Cl + 4HDO

NaOH + D2O→NaOD + HDO

HCl + D2O→DCl + HDO

CHCl3 + D2O→CDCl3 + HDO

Deuterolysis:

Like hydrolysis, D2O undergoes deuterolysis with salts.

Fe2(SO4)3 + 6D2O→2Fe(OD)3 + 3D2SO4

Na2CO3 + 2D2O→2NaOD + D2CO3

CuSO4 + 2D2O→Cu(OD)2 + D2SO4

AlCl3 + 3D2O→Al(OD)3 + 3DCl

Formation of deuterates:

Heavy water like ordinary water combines with salts as heavy hydrates of crystallization called deuterates. For example:CuSO4.5D2O , Na2SO4.10D2O, MgSO4.7D2O, CoCl2.6D2O, NiCl2.6D2O etc.

Physiological effects:

Heavy water is injurious to human beings, animals, and plants as it slows down the reactions occurring in them. Taking a bath in D2O causes serious dehydration due to its hygroscopic nature.

Uses:

  1. Heavy water is used as a moderator and coolant for nuclear reactors.
  2. Heavy water is used to produce deuterium.
  3. Heavy water is used as a tracer compound for studying various reaction mechanisms.
  4. Deuterium oxide is used for collecting Fourier transform infrared spectroscopy spectra of proteins instead of water in solution.
  5. Deuterium oxide is also used in nuclear magnetic resonance spectroscopy.