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ChemistryEdu Logo Group 17 Elements | Important Compounds - Chlorine and HCl#

In this article, we will discuss two important compounds of group 17 elements: Chlorine and Hydrochloric acid.

A. Chlorine (Cl2)#

Chlorine (Cl2) is a yellow-green gas at room temperature.

Preparation of chlorine:#

  • Chlorine is prepared by heating manganese dioxide with concentrated HCl.
\[MnO_2 + 4HCl → MnCl_2 + Cl_2 + 2H_2O\]
  • Chlorine can also be prepared by using a mixture of common salt and concentrated H2SO4 in place of HCl in above reaction.
\[4NaCl + MnO_2 + 4H_2SO_4 → MnCl_2 + 4NaHSO_4 + 2H_2O + Cl_2\]
  • Chlorine can be prepared by the action of hydrochloric acid on potassium permanganate.
\[2KMnO_4 + 16HCl → 2KCl + 2MnCl_2 + 8H_2O + 5Cl_2\]

Manufacture of Chlorine:#

  • Deacon's Process: Chlorine is manufactured by oxidation of hydrogen chloride gas by atmospheric oxygen in the presence of CuCl2 as a catalyst at 723 K.
\[4HCl + O_2 → 2Cl_2 + 2H_2O\]
\[[CuCl_2\ is\ used\ as\ catalyst]\]
  • Electrolytic Process: Chlorine is obtained by the electrolysis of brine (concentrated NaCl solution). Chlorine is liberated at anode.

Physical Properties of Chlorine:#

  • Chlorine is soluble in water.
  • It is greenish yellow in colour.
  • Chlorine is 2-5 times heavier than air.

Chemical Properties of chlorine:#

  • Reactivity towards metals and non-metals:
\[2Al + 3Cl_2 → 2AlCl_3\]
\[2Na + Cl_2 → 2NaCl\]
\[2Fe + 3Cl_2 → 2FeCl_3\]
\[P_4 + 6Cl_2 → 4PCl_3\]
\[S_8 + 4Cl_2 → 4S_2Cl_2\]
  • Reactivity towards hydrogen: Chlorine reacts with compounds containing hydrogen to form HCl.
\[H_2 + Cl_2 → 2HCl\]
\[H_2S + Cl_2 → 2HCl + S\]
\[C_{10}H_{16} + 8Cl_2 → 16HCl + 10C\]
  • Reactivity towards Ammonia: With excess ammonia, chlorine gives nitrogen and ammonium chloride whereas with excess chlorine, nitogen tetrachloride is formed.
\[8NH_{3_{(excess)}} + 3Cl_2 →6NH_4Cl + N_2\]
\[NH_3 + 3Cl_2 → NCl_3 + 3HCl\]
  • Reactivity towards alkalies: With cold and dilute alkalies, chlorine produces a mixture of chloride and hypochlorite but with hot and concentrated alkalies, it gives chloride and chlorate.
\[2NaOH_{(cold\ and\ dilute)} + Cl_2 → NaCl + NaOCl + H_2O\]
\[6NaOH_{(hot\ and\ conc.)} + 3Cl_2 → 5NaCl + NaClO_3 + 3H_2O\]

With dry slaked lime, chlorine gives bleaching powder.

\[2Ca(OH)_2 + 2Cl_2 → Ca(OCl)_2 + CaCl_2 + 2H_2O\]
  • Reaction with hydrocarbons:

In the presence of sunlight:

\[CH_4 + Cl_2 → CH_3Cl + HCl\]

At room temperature:

\[C_2H_4 + Cl_2 → C_2H_4Cl_2\]
  • Oxidizing and bleaching property of chlorine: Chlorine water on standing loses its yellow colour due to the formation of HCl and HOCl. Hypochlouous acid (HOCl) so formed gives nascent oxygen which is responsible for oxidizing and bleaching properties of chlorine.

(a) Chlorine oxidizes ferrous to ferric and sulphite to sulphate. It oxidizes sulphur dioxide to sulphur trioxide and iodine to iodate. In the presence of H2O, they form sulphuric acid and iodic acid respectively.

\[2FeSO_4 + H_2SO_4 + Cl_2 → Fe_2(SO_4)_3 + 2HCl\]
\[Na_2SO_3 + Cl_2 + H_2O → Na_2SO_4 + 2HCl\]
\[SO_2 + 2H_2O + Cl_2 → H_2SO_4 + 2HCl\]
\[I_2 + 6H_2O + 5Cl_2 → 2HIO_3 + 10HCl\]

(b) Chlorine is a powerful bleaching agent; bleaching action is due to action.

\[Cl_2 H_2O → 2HCl + O\]
\[Coloured\ substance + O → Colourless\ substance\]

Uses of Chlorine:#

  • Chlorine is used in sterilizing drinking water.
  • Chlorine is used in preparation of poisonous gases like phosgene (COCl2), tear gas (CCl3NO2) and mustard gas (ClCH2CH2SCH2CH2Cl2).

B. Hydrogen Chloride (HCl)#

Preparation of Hydrogen Chloride:#

  • Hydrogen Chloride can be prepared by heating sodium chloride with concentrated sulphuric acid.

At 420 K temperature:

\[NaCl + H_2SO_4 → NaHSO_4 + HCl\]

At 823 K temperature:

\[NaHSO_4 + NaCl → Na_2SO_4 + HCl\]

Properties of Hydrogen Chloride:#

  • Solubility: The aqueous solution of hydrogen chloride is known as hydrochloric acid. High value of Ka (107) indicates that it is a strong acid. Hydrogen chloride is extremely soluble in water and ionizes as follows:
\[HCl_{(g)} + H_2O_{(l)} → H_3O^+_{(aq)} + Cl^-_{(aq)}\]
  • Reaction with NH3:
\[NH_3 + HCl → NH_4Cl\]
  • Reaction with salts of weak acids: It decomposes salts of weak acids.
\[Na_2CO_3 + 2HCl → 2NaCl + H_2O + CO_2\]
\[NaHCO_3 + HCl → NaCl + H_2O + CO_2\]
\[Na_2SO_3 + 2HCl → 2NaCl + H_2O + SO_2\]

Aqua-regia:#

An aqua-regia is a mixture of three parts of concentrated HCl and 1 part of concentrated HNO3.

Questions#

Write the balanced chemical equation for the reaction of Cl2 with hot and concentrated NaOH. Is this a disproportionation reaction?

The reaction is:

\[3Cl_2 + 6NaOH → 5NaCl + NaClO_3 + 3H_2O\]

This is a disproportionation reaction because the oxidation state of chlorine is changed from 0 to -1 and +5.

Give the reason for bleaching action of Cl2.

It is due to the oxidizing property of Cl2.

\[Cl_2 H_2O → 2HCl + O\]
\[Coloured\ substance + O → Colourless\ substance\]

When HCl reacts with finely powdered iron, it forms ferrous chloride and not ferric chloride. Why?

\[Fe + 2HCl → FeCl_2 + H_2\]

Liberation of hydrogen prevents the formation of ferric chloride. So, ferric chloride is not formed.