ICSE Class 10 Chemistry Chapter 08 Study of Compounds Hydrogen Chloride

Study Of Compounds - Hydrogen Chloride

Syllabus

Hydrogen chloride: preparation of hydrogen chloride from sodium chloride; refer to the density and solubility of hydrogen chloride (fountain experiment); reaction with ammonia; acidic properties of its solution.

Preparation of hydrogen chloride from sodium chloride; (the laboratory method of preparation can be learnt in terms of reactants, product, condition, equation, diagram or setting of the apparatus, procedure, observation, precaution, collection of the gas and identification).

Simple experiment to show the density of the gas (Hydrogen Chloride) - heavier than air.

Solubility of hydrogen chloride (fountain experiment); (setting of the apparatus, procedure, observation, inference) - method of preparation of hydrochloric acid by dissolving the gas in water - the special arrangement and the mechanism by which the back suction is avoided should be learnt.

Reaction with ammonia

Acidic properties of its solution - (reaction with metals, their oxides, hydroxides and carbonates to give their chlorides, decomposition of carbonates, hydrogen carbonates, sulphides, sulphites, thiosulphates and nitrates).

Hydrogen Chloride

Molecular formula: HCl

Molecular mass: 36.5

Bond: Covalent

H-Cl or H - Cl

Dot diagram or Lewis structure

Hydrogen + Chlorine - Hydrogen chloride

\[H_2(g) + Cl_2(g) \xrightarrow{\text{Diffused Sunlight}} 2HCl(g)\]

Note: The reaction is explosive in direct sunlight but it is negligible in the dark.

However, in the presence of a catalyst such as activated carbon, the reaction takes place even in the dark. (Activated carbon absorbs hydrogen which increases reactivity).

(ii) Burning jet of hydrogen also burns in chlorine forming hydrogen chloride.

Occurrence

Glauber prepared the acid in 1648 by heating common salt (NaCl) with concentrated sulphuric acid. Lavoisier named it muriatic acid. Davy in 1810 named it as hydrochloric acid.

Hydrogen chloride gas occurs in free state in volcanic emissions.

Hydrochloric acid (0.2 - 0.4 percent) is present in gastric juice of mammals and it helps in digestion.

General Preparation Of Hydrogen Chloride Gas

By Synthesis (Direct Combination)

(i) Moist hydrogen gas combines with chlorine in the presence of diffused sunlight.

By Heating Metallic Chloride With Conc. Sulphuric Acid

Laboratory Preparation Of Hydrogen Chloride Gas

In the laboratory, hydrogen chloride gas can be prepared by the action of concentrated sulphuric acid on sodium chloride.

Reactants: Sodium chloride and concentrated sulphuric acid.

Procedure: Set up the apparatus as shown in Fig. 8.1.

Place some common salt in a flask and pour concentrated sulphuric acid through the thistle funnel. The reaction proceeds slowly in the cold. On heating the mixture gently there is effervescence and HCl gas is evolved in a controlled manner.

The gas is dried by passing through washer bottle containing concentrated sulphuric acid.

It is collected by upward displacement of air in a gas jar.

Reaction

\[NaCl + H_2SO_4 \xrightarrow{< 200°C} NaHSO_4 + HCl \uparrow\]

Though it is a reversible reaction, yet it goes to completion as hydrogen chloride continuously escapes as a gas.

The reaction can occur up to the stage of formation of sodium sulphate on heating above 200°C.

NaHSO_4 + NaCl \xrightarrow{\text{above } 200°C} Na_2SO_4 + HCl \uparrow

or

\[2NaCl + H_2SO_4 \xrightarrow{\text{above } 200°C} Na_2SO_4 + 2HCl\] (Conc.)

Note: (i) Sodium chloride is cheap and therefore it is preferred for preparation of HCl over other metal chlorides.

(ii) Conc. nitric acid is not used during the preparation of HCl because it is volatile and may volatilize out alongside hydrogen chloride.

Purification Of HCl Gas

It is dried by passing through conc. sulphuric acid.

The other drying agents like phosphorus pentoxide (P2O5) and quick lime (CaO) cannot be used, since they react with hydrogen chloride.

\[2P_2O_5 + 3HCl \rightarrow POCl_3 + 3HPO_3\]

\[CaO + 2HCl \rightarrow CaCl_2 + H_2O\]

Collection

Hydrogen chloride gas is collected by the downward delivery of jar (by displacement of air) as it is 1.28 times heavier than air.

It is not collected over water, since it is highly soluble in water.

Identification

When the jar is completely filled with hydrogen chloride, fumes appear above the jar's mouth.

When hydrogen chloride gas is exposed to air, it gives white fumes, due to the formation of hydrochloric acid on reacting with atmospheric water vapour.

In order to know whether the gas jar is full, bring a rod dipped in ammonium hydroxide near its mouth.

\[HCl + NH_4OH \rightarrow NH_4Cl + H_2O\]

Dense white fumes of ammonium chloride will be produced proving thereby that the jar is full of hydrogen chloride gas.

Precautions

(a) The lower end of the thistle funnel must be dipped in conc. sulphuric acid.

(b) Delivery tube should be dipped in drying agent i.e., conc. H2SO4.

(c) Temperature is maintained at nearly 200°C

At higher temperature i.e., above 200°C:

(i) the apparatus which is made of glass may crack.

(ii) fuel is wasted.

(iii) sodium sulphate formed, forms a hard crust which sticks to the glass and is difficult to remove.

Physical Properties Of Hydrogen Chloride Gas

Experiment To Demonstrate Density - To Show That HCl Gas Is Heavier Than Air

It can be proved by pouring the gas in a jar with a burning candle as shown in Fig. 8.2. The candle gets extinguished because HCl gas being heavier occupies the lower portion of the jar and forces the air out of it.

Experiment To Demonstrate Solubility - To Show That HCl Gas Is Highly Soluble

The great solubility of the gas can be demonstrated by means of Fountain Experiment as illustrated below.

Fountain Experiment

Take a dry round bottomed flask filled with dry HCl gas. Arrange the apparatus as shown in Fig. 8.3. On the mouth of the flask, fix a rubber stopper with two holes. Through one hole pass a long jet tube, and through other hole pass a dropper with few drops of water. Put the jet tube in the beaker containing blue litmus solution.

Press the dropper. It is seen that the blue litmus solution enters the jet tube with a great force, forming a red fountain.

Reason

As the water goes in the flask from the dropper, HCl gas present in the flask dissolves due to its high solubility, thereby lowering the pressure inside.

The outside pressure being higher, pushes the blue litmus solution inside, through the jet tube.

The blue litmus solution turns red due to the acidic nature of hydrogen chloride gas.

Note: Due to the high solubility, hydrogen chloride gas fumes in moist air forming tiny droplets of hydrochloric acid.

Chemical Properties Of Hydrogen Chloride Gas

Combustibility

The gas is neither combustible nor a supporter of combustion. It does not burn, rather it extinguishes a burning splint.

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