Hydrogen Chloride

Melting Point : -114 degC
Boiling Point : -85 degC
Formula weight 36.46
Specific gravity or density 1.2
Flash point none

Hydrogen Chloride is a Diatomic Molecule which consists of two atoms. The atoms in diatomic molecules may be similar (e.g. Oxygen Nitrogen Hydrogen etc.) or they may be dissimilar (e.g. Carbon Monoxide, Hydrogen Chloride, etc.).

Hydrochloric Acid is available in two forms

(i) as a solution or (ii) as a fuming gas.:

(i) as a solution of Hydrogen Chloride in water, where the molecule is dissociated in solution into its ions.


		HCl   ==>   H(+)   +   Cl(-)

(ii) as a colourless fuming gas. The polar covalent gas Hydrogen Chloride is very soluble in Water. In aqueous solution, the molecule exists in ionic form, as the positively charged Hydrogen Ion, H(+), and the negatively charged Chloride Ion, Cl(-).


	HCl	+		==>	H(+)	+	Cl(-)	
	Hydrogen			Hydrogen	Chloride	
	Chloride			Ion		Ion

It should be noted that the Hydrogen Ion, H(+), is stabilised by hydration in aqueous solution, as the Hydronium Ion, H3O(+).

	H(+)	+	H2O	==>	H3O(+)	
	Hydrogen			Hydronium	
	Ion				Ion

Properties of Hydrochloric Acid

Hydrogen Chloride gas is very soluble in water, and a solution of the gas forms Hydrochloric Acid.
When 1 kgm. of water is saturated with the gas at 15 degC It increases in weight to 1.75 kgm., and the relative density is 1.231.
It contains about 43 per cent of HCl; the commercial acid contains about 39 per cent and its relative density is 1.20.

Preparation of Hydrogen Chloride

Many Metallic Chlorides liberate Hydrogen Chloride gas, HCl, when warmed with concentrated Sulphuric Acid, H2SO4.

It is prepared industrially by the combustion of Hydrogen, H2, in Chlorine, Cl2.


			H2   +   Cl2   ==>   2 HCl

Hydrogen Chloride may be prepared in the laboratory by heating Concentrated Sulphuric Acid, with Sodium Chloride.


			NaCl   +    H2SO4   ==>   NaHSO4   +   HCl

The Chlorides of Non-Metals (e.g. Phosphorus Trichloride, PCl3) are covalent compounds.
These chlorides fume in air and are readily hydrolysed to yield either an acid or an acidic oxide. For example, Phosphorus Trioxide, reacts with the Moisture, in air to form Phosphorous Acid, and Hydrogen Chloride.


			PCl3   +   3 H2O   ==>   H3PO3   +   3 HCl

Phosphorus Pentachloride, is hydrolysed by hot Water, to give Orthophosphorus Acid, (i.e. Phosphoric(V)Acid) and Hydrogen Chloride.


		PCl5     +    4 H2O    ==>     H3PO4     +    5 HCl

Phosphorus Pentachloride, is also hydrolysed by cold Water, to give Phosphorus Oxychloride.


		PCl5     +     H2O    ==>     POCl3     +     2 HCl

A mixture of Chlorine, Cl2, and Hydrogen, H2, explodes when exposed to sunlight to give Hydrogen Chloride, HCl. In the dark, no reaction occurs, so activation of the reaction by light energy is required.


		Cl2   +   H2   ==>   2 HCl

Chlorine, removes Hydrogen, from the hydrides of non-metals, forming Hydrogen Chloride, and leaving the non-metal element.


		Cl2   +   H2S   ==>   2 HCl   +   S

When Chlorine Water, (i.e. a solution of Chlorine gas, in Water) in a flask inverted in a basin of the same liquid is exposed to bright sunlight, the Chlorine, is decomposed and a solution of Hypochlorous Acid, remains.


		H2O   +   Cl2   ==>   HCl   +   HClO

The Hypochlorous Acid, is not very stable and the solution readily decomcomposes, especially when exposed to sunlight, yielding Oxygen,


		2 HClO   ==>   2 HCl   +   O2

Chlorine, is soluble in water (which solution is called Chlorine Water) and this loses its yellow colour on standing in sunlight, due to the formation of a mixture of Hypochlorous Acid, and Hydrochloric Acid.


		Cl2   +   H2O    ==>    HOCl   +   HCl

Reactions of Hydrochloric Acid

Ferrous Chloride, is formed by reacting Iron, or Ferrous Oxide, with dilute Hydrochloric Acid.


                	FeO   +   2 HCl    ==>    FeCl2    +    H2O

Ethyne, reacts with Hydrogen Chloride, very slowly.


                 	HCCH  + HCl 	==>	H2C=CHCl  +  HCl

			==>	CH3CHCl2

Ethanol, reacts with Hydrogen Chloride, to form Ethyl Chloride, (i.e. chloroethane) and Water. A dehydrating agent (e.g. Zinc Chloride) is used.


				ZnCl2	
	C2H5OH 	+	HCl	=====>	C2H5Cl + H2O	
	Ethanol				Ethyl 	
					Chloride

Substitution of Methanol with Hydrogen Chloride

Methanol, reacts with Hydrogen Chloride, to form Methyl Chloride, (i.e. chloromethane) and Water. A dehydrating agent (e.g. Zinc Chloride,) is used.


				ZnCl2	

	CH3OH 	+	HCl	==>	CH3Cl	+    H2O	
	Ethanol				Methyl 	
					Chloride

Calcium Chloride, is prepared by dissolving limestone chips (i.e. a naturally occurring form of Calcium Carbonate) or marble chips (i.e. also a naturally occurring form of Calcium Carbonate) in Hydrochloric Acid.

Chlorides are the salts of Hydrochloric Acid, and they are also produced in the direct reaction between their constituent elements.

In the laboratory, a mixture of Cuprous Chloride, and Hydrochloric Acid, is used for converting Benzene Diazonium Chloride, to Chlorobenzene, in the Sandmeyer Reaction.

Preparation of Chlorine

The most common laboratory method for preparation of Chlorine, is to heat 100 gm. of Manganese Dioxide, with 300 ml. of concentrated Hydrochloric Acid.


               MnO2   +   4 HCl   ==>   MnCl2   +   2 H2O   +   Cl2

The gas is bubbled through water to remove any traces of hydrochloric gas that may be present and then it is dried by bubbling it through concentrated sulphuric acid. Chlorine, may also be prepared by dropping cold concentrated Hydrochloric Acid, on Potassium Permanganate.


                2KMnO4  +  16HCl ==>  2MnCl2  +  2KCl  +  8H2O  +  5Cl2

The gas is bubbled through Water, to remove any traces of Hydrochloric Acid gas, that may be present and then it is dried by bubbling it through concentrated Sulphuric Acid.

Uses of Hydrochloric Acid

Gravimetric Analysis is the quantitative analysis of materials that depends on weighing. For example, the amount of silver in a solution of silver salts could be measured by adding excess hydrochloric acid to precipitate silver chloride, filtering the precipitate, washing, drying and weighing.

Ionic Chlorides (i.e. salt-like chlorides) are prepared by the action of Hydrochloric Acid,

		(a)	on a Metal, 	
		(b)	on a Metal Oxide, 	
		(c)	on a Metal Hydroxide, or 	
		(d)	on a Metal Carbonate.

For example, Calcium Carbonate, reacts with Hydrochloric Acid, to form Calcium Chloride.


	CaCO3    +    2 HCl    ==>    CaCl2    +    H2O   +   CO2

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