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NEET Chemistry Notes Hydrogen – Hard and soft water

NEET Chemistry Notes Hydrogen – Hard and soft water 

Hard and soft water 

Hard and Soft Water

The water which lathers with soap is soft, if not, is hard. Hardness of water is of two types:

Temporary hardness
Permanent hardness

  • Temporary Hardness of Water

It is due to the presence of magnesium and calcium hydrogen carbonates. It can be removed either by boiling, through which the soluble Mg(HC03)2 and Ca(HC03)2  is converted into Mg(OH)2, CaC03. These precipitates can be removed by filtration or by Clark’s method, which involves the addition of calculated amount of lime to hard water.
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  •  Permanent Hardness of Water

It is due to the presence of soluble salts of : magnesium and calcium in the form of chlorides and sulphates. It can be removed by the following  methods.

  •  Calgon’s Method

Sodium hexa meta phosphate (Na6P6018), commercially called calgon, when added to hard water, the calcium and magnesium ions present in hard water react with calgon and form soluble complex salts which does not cause any hindrance in other information.
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  • Ion-exchange Method or Permutit Process

It is also used to remove permanent hardness.
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Where, NaZ is sodium aluminium silicate (NaAlSi04). This is also known as sodium zeolite. It is regenerated for further use by treating with an aqueous NaCl solution (brine).

  • Synthetic Resins Method

Now a days hard water is softened by using synthetic cation exchangers. Cation exchange resins contain large organic molecules with —S03H group and are water insoluble. Ion exchange resin (RS03H) is changed to Na by treating it with NaCl.
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The resin can be regenerated by adding aqueous NaCl solution.

Heavy Water (D20)

  •  It has quite similar physical and chemical properties to those of H20. Dielectric constant of D20 is lower than that of H20 and rate of reactions are much slower than H20.
  • It is used as a moderator in nuclear reactions, as trace compound for studying reaction mechanism, for the preparation of deuterium.

Hydrogen Peroxide (H202)

It is a compound with an oxygen-oxygen single bond. It is also a strong oxidiser.

Preparation

  •  By the reaction oif sulphuric acid or phosphoric acid on hydrated barium peroxide (Ba02)

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In both cases, BaS04 is removed by filteration and hence, more or less a pure H202 solution is obtained by this method.

  •  By adding calculated quantity of sodium peroxide to a 20% ice cold sulphuric acid solution.
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    Sodium sulphate is removed by cooling when crystals of Na2S04 10 H20 separated out.
  • Merck’s process H202 can be obtained by passing a current of C02 through a cold pasty solution of barium peroxide in water
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Industrial Preparation of H202

H202can be prepared by the following methods:

  • By the electrolysis of 50% sulphuric acid The electrolysis of 50% sulphuric acid, to give perdisulphuric acid (H2S2O2) which on distillation yields 30% solution of hydrogen peroxide.
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  • By the auto-oxidation of 2-ethylanthraquinol In this process, air is passed through 10% solution of 2-ethylanthraquinol in a mixture of benzene and a higher alcohol
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    The resulting 2-ethylanthraquinone is then reduced by hydrogen in the presence of palladium as a catalyst. Thus, the continuity of the process is maintained and process needs only H2, atmospheric 02 and water as the major raw materials.

Strength of Hydrogen Peroxide Solution

H202 is commercially represented in terms of volume strength.
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% strength =NEET Chemistry Notes Hydrogen - Hard and soft water 11 X volume strength

Properties

  • Pure H202 is a pale blue syrupy liquid.
  • It freezes at – 0.5°C and has a density of 1.4 in pure state.
  •  It is diamagnetic in nature.
  •  Its dipole moment is 2.1 D.
  •  Due to auto oxidation property, it cannot be used as a polar solvent.

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Reaction

  • Oxidising action in acidic medium
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  •  Reducing action in acidic medium
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  •  Oxidising action in basic medium
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  •  Reducing action in basic medium
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  •  Many reactions of H202 are radical reactions, therefore a mixture of H202 and Fe (II) is a source of hydroxyl radicals for organic reactions.

Uses of H202

  •  Aqueous solution of H202 is used as germicide, antiseptic, preservative for milk and wine, bleaching agent for soft materials. 30% H202 is called perhydrol. Its volume strength is 100 and molarity is 8.8.
  • It is used as an antichlor and in refreshing old oil paintings which became black.

Structure of H202

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