Hydrogen : Water and Heavy Water

WATER AND HEAVY WATER

Properties of Heavy water: – colorless, odorless, tasteless liquid

 

H2O D2O
Molecular weight 18 20
Density gm/mL 0.9982 1.1056
Boiling Pt 1000C 101.450C
Melting Pt. 0oC 3.80C
Temp. of maximum density 40C 11.60C
Viscosity 10.87 14.2
Latent heat of vaporization (cal/gm) 539 557
Dielectric constant 82 80.5
Refractive index 1.3329 1.3284

 

NaCl and BaCl2 have been found to be about 15% less soluble in D2O than H2O.

In chemical properties heavy water is similar to water but slow in reaction.

1. Liberation of Deuterium –

          2D2O + 2Na → 2NaOD + D2

          2D2O \underrightarrow { \quad Electrolysis\quad }    2D2­ + O

2. With acid anhydrides heavy water form acids-

          N2O5 + D2O → DNO3 ( Deutronitric acid)

          SO3 + D2O → D2SO4 (Deutorsulphuric acid)

3. With metallic oxide formation of Deuteroxide

          Na2O + D2 → 2NaOD

          CaO + D2O → Ca(OD)2       

4. With carbides

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

                                          Deutromethane

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

 5. With Nitrides

          Mg3N2 + 6D2O → 3Mg(OD)2 + 2 ND3

6. Deuterolysis

          BiCl3 + D2O \rightleftarrows BiOCl + 2DCl

7. Formation of deutrohydrates : like CuSO4 5D2O

8. Heavy water – exchange reaction-

          NaOH + D2O → NaOD + HDO

          NHCl + D2O → NH3 DCl + HDO

          Mg3N2 + 3D2O → 2 ND3 + 3MgO

9. Direct reaction –

          P4O10 + 6D2O → 4D3PO4

 

Physiological effect of heavy water – It does not support life so well like ordinary water. Some animals die when placed in heavy water. Lewis has shown that tobacco seeds do not germinate in heavy water. According to Taylor it is a bactericide and germicide. 

 

Uses of heavy water –

  • As a moderator and coolant in nuclear reactions
  • In the production of deuterium
  • As a tracer in the study of mechanism of chemical reaction.

 

HARDNESS OF WATER: –

HARD AN SOFT WATER

Water is said to be a soft water if it produces sufficient lather with the soap and water is described as being hard if it forms an insoluble scum before it forms a lather with soap.

The hardness of natural water is generally caused  by presence of the bicarbonates and sulphates of calcium and magnesium but infact all soluble salts that forms a scum with soap cause hardness.

          Ca2+ (aq) + 2C17H35COO (aq) → (C17H35COO)2 Ca

          Mg2+ (aq) + 2C17H35COO (aq) → (C17H35COO)2 Mg

                                      Anion of soap     Insoluble prescipitates

Soap will not produce lather with water until all the calcium and magnesium ions have been separated.

 

 Hardness of water is of two types:

  • Temporary hardness
  • Permanent hardness

 

(a)    Temporary hardness: This is due to the presence of bicarbonates of calcium and magnesium. Rain water dissolves small quantities of CO2 form the atmosphere forming a very dilute solution of carbonic acid. This water attacks calcium and magnesium carbonates in any rocks over which it follows and the soluble bicarbonates are formed.

                    CaCO3 + H­2O + CO2  → Ca(HCO­3)2

Temporary hardness in water is easily removed by boiling as the bicarbonates decompose readily and the insoluble carbonates are precipitated; which can be removed by filtration.

            Ca(HCO3)2 \underrightarrow { \quad Boil\quad } CaCO3 + H2O + CO2

                  Mg(HCO3)2  \underrightarrow { \quad Boil\quad } MgCO3 + H2O + CO2

Temporary hardness can also be removed by Clark’s process, which involves the addition of slaked lime (Ca(OH)2).  

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O

                                                          (Insoluble)

It is essential to add only the calculated amount of Ca(OH)2 because excess will cause artificial hardness.

(B)    Permanent hardness : Permanent hardness is introduced when water passes over rocks containing the sulphates or chlorides of calcium and/or magnesium. This type of hardness cannot be removed by boiling or by the addition of slaked lime. The substances used to remove the hardness of water are known as water softeners.

The various water softness are:

(i)     Washing soda: It removes both the temporary and permanent hardness by converting soluble calcium and magnesium compounds into insoluble carbonates.

CaCl2 + Na2CO3 → CaCO3 + 2NaCl

                   CaSO4 + Na2CO3 → CaCO3 + Na2SO4

                   Ca(HCO3)2 Na2CO3 → CaCO3 + 2NaHCO3

                                                (Insoluble)          (Soluble)

In place of sodium carbonate, caustic soda or sodium phosphate can also be used.        

MgCl2 + 2NaOH → Mg(OH)2 + 2NaCl

            3MgSO4 + 2Na3PO4 → Mg3(PO4)2 + 3Na2SO4   

(ii)    Permutit: Permutit is the technical name given to certain hydrated silicates of aluminium and sodium. It is obtained by fusing sodium carbonate, china clay, silica or quartz. The product is washed with water to remove soluble protion. The remaining crystalline mass Na2Al2Si2O8. xH2O is called as Permutit. The sodium ions of Permutit are exchanged with calcium and magnesium ions when hard water is passed through it.

Na2Al2Si2O8. xH2O + Ca2+ → CaAl2Si2O8. xH2O + 2Na+

                                                          or                or

                                                          Mg2+           MgAl2Si2O8. xH2O

These method is useful for the removal of both temporary and permanent hardness of water.     

(iii)   Calgon: The complex salt of metaphosphoric acid, sodium hexametaphosphate (NaPO3)6 is known as calgon. It is represented as Na2[Na4(PO3)6]. Calcium and magnesium salts present in hard water react with calgon to give complex salts.

2CaSO4 + Na2[Na4(PO3)6 → Na2 [Ca(PO3)6] + 2Na2SO4

            2MgSO4+Na2[Na4[Na4(PO3)6] → Na2[Mg2(PO3)6] + 2Na2SO4

This method is especially used softening water for boiler use.

(iv)   Ion exchange resins: Ion exchange resins are the most popular water softners these days. These resins are synthetic substances.

The cation Exchanger : It consists of granular insoluble organic acid resin having giant molecules with  –SO3H or –COOH groups. The hard water is passed through a bed of cation exchanger, which removes the cations like Na+, Mg2+, Ca2+ and other by exchanging with H+ ions.

2RH + Ca2+ → (R)2 Ca + 2H+

The water coming form cation exchanger is acidic on account of free H+ ions.

The anion exchanger: It contains giant organic molecules with basic groups derived from amines.

The exit water of cation exchanger is then passed through another bed containing anion exchanger. This exchanger removes anions like Cl, by exchanging with OH ions.

R–NH3 OH + Cl → R– NH3Cl + OH

The OH ions neutralize the H+ ions.

                             H+ + OH = H2O

This process gives distilled water and can be used in laboratories. Ion exchange resins remove all soluble minerals from water.

The cation exchanger is regenerated by the action of an acid and the anion exchanger is regenerated by using an alkali solution.

 

Degree of hardness : The degree of hardness is defined as the number of part of calcium carbonate or equivalent to various calcium and magnesium salts present in a million parts of water by mass. It is expressed as ppm.

 

HOW TO CALCULATE THE DEGREE OF HARDNESS:

  1. Calculate the equivalents of salt present in hard water in 1000L.
  2. Consider this equivalents of salt = equivalents of CaCO3
  3. Calculate the weight of CaCO3 using equivalents
  4. Report this weight as degree of hardness in ppm.

 

Example :  one litre of water contains 1 mg CaCl2 and 2 MgCl2 calculate total hardness of water in ppm?

Solution:  In 1000L mass of MgCl2  = 2gm

Equivalents of MgCl2 = mass/(equivalent weight of MgCl2) = \frac { 2 }{ (24+17)/2 } = 0.042

Similarly equivalents of CaCl2 in 1000L water  = 0.018

Total equivalents = 0.042+0.018 = 0.060

Now consider this is equivalents of CaCO3 ,

so weight of CaCO3 =equivalents x equivalent weight of CaCO3 = 0.06 x 50 = 3

Hardness in ppm = 3 ppm