Study of Compounds — Hydrogen Chloride

The labelled diagram for the laboratory preparation of hydrogen chloride gas is shown below:

(a) Conc. H₂SO₄ is used for the preparation of hydrogen chloride gas in the laboratory.

As conc. H₂SO₄ is non-volatile and has a high boiling point, therefore, it displaces the volatile hydrogen chloride from the salt sodium chloride. Hence, conc. H₂SO₄ is used as a reactant in the laboratory preparation of HCl from sodium chloride.

(b) $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_4 + \text{HCl [g.]}$

(c) Conc. H₂SO₄

Reason — Drying agent used for drying should only remove the moisture and not react with it, hence, conc. sulphuric acid is used as the drying agent.

(d) Phosphorus pentoxide and calcium oxide are good drying agents, but they cannot be used to dry hydrogen chloride gas since they react with it.

2P₂O₅ + 3HCl ⟶ POCl₃ + 3HPO₃

CaO + 2HCl ⟶ CaCl₂ + H₂O

(e) Direct absorption of hydrogen chloride gas in water is not feasible because it causes back suction in the delivery tube.

1. Hydrogen chloride gas is extremely soluble in water.
2. If a delivery tube through which HCl gas is passed is directly immersed in water, the rate of absorption of HCl gas is high and hence a partial vacuum is created in the tube.
3. The pressure outside being higher causes the water to be pushed up into the delivery tube and damages the apparatus. This is called back-suction.

(f) A special funnel arrangement is used to dissolve HCl gas in water as it provides a large surface area for absorption while preventing back-suction. The rim of the funnel is positioned so that it just touches the water in the trough. If back-suction occurs, the water rises up the funnel and the air gap between the rim of the funnel and the water's surface equalizes the pressure inside and outside.

(a) Hydrochloric acid (HCl)

(b) The reactants used are sodium chloride and sulphuric acid.

(c) The empty flask acts as an Anti-Suction device. If back suction takes place then the water gets collected in it and will not reach the generating flask.

(d) The drying agent used is Concentrated Sulphuric acid. It is chosen as drying agent because it does not react with HCl.

(e) The role of the inverted funnel in the arrangement is -

1. It prevents or minimizes back suction of water.
2. It provides a large surface area for absorption of HCl gas.

(i) Using an active metal below magnesium

Fe + 2HCl ⟶ FeCl₂ + H₂↑

(ii) Using an oxidising agent not containing lead.

MnO₂ + 4HCl ⟶ MnCl₂ + 2H₂O + Cl₂↑

(i) Action on metals — Hydrochloric acid reacts with metals above hydrogen in the activity series forming metallic chlorides and evolving hydrogen.

Ca + 2HCl ⟶ CaCl₂ + H₂ ↑

(ii) Action on oxides — Hydrochloric acid reacts with oxides to form salt and water only.

CuO + 2HCl ⟶ CuCl₂ + H₂O

(iii) With salts of weaker acids — Hydrochloric acid decomposes salts of weaker acids.

Na₂CO₃ + 2HCl ⟶ 2NaCl + H₂O + CO₂ ↑

(a) Pb(NO₃)₂ + 2HCl ⟶ PbCl₂ + 2HNO₃
Hg₂(NO₃)₂ + 2HCl ⟶ Hg₂Cl₂ + 2HNO₃

(b) FeO + 2HCl ⟶ FeCl₂ + H₂O

(c) CaCO₃ + 2HCl ⟶ CaCl₂ + H₂O + CO₂ ↑

(d) Na₂S + 2HCl ⟶ 2NaCl + H₂S ↑

The balanced chemical reactions are:

A : $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_4 + \text{HCl [g]}$ ↑

B : $\text{Zn} + 2\text{HCl ( dil.)} \xrightarrow{\Delta} \text{ZnCl}_2 + \text{H}_2$ ↑

C : AgNO₃ + HCl ⟶ AgCl (white ppt.) + HNO₃

D : $\text{MnO}_2 + \underset{\text{Conc.}}{\text{4HCl}} \xrightarrow{\Delta} \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$ ↑

E : NaHCO₃ + HCl ⟶ NaCl + H₂O + CO₂ ↑

(a) Below diagrams show the special funnel arrangement used for the absorption of HCl gas in water:

(b) The reasons are :

1. Prevents or minimizes back-suction of water.
2. Provides a large surface area for the absorption of the HCl gas.

(c) The equations are:

(A) $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_4 + \text{HCl [g]}$ ↑

(B) $\text{2NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\gt 200 \degree\text{C}} \text{Na}_2\text{SO}_4 + \text{2HCl [g}]$ ↑

(i) Dry HCl (Hydrogen chloride) gas is the gas Y

(ii) High solubility of HCl in water

(iii) Ammonia (NH₃) gas also demonstrates high solubility in water.

P — $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_4 + \text{HCl [g]}$ ↑

Q — Fe + 2HCl (dil.) ⟶ FeCl₂ + H₂ [g]

R — NH₄OH + HCl (dil.)⟶ NH₄Cl + H₂O

S — $\text{PbO}_2 + 4\text{HCl} \xrightarrow{\Delta} \text{PbCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$ ↑

(a) When HCl gas is passed through lead nitrate, white precipitate of lead chloride is formed.

Pb(NO₃)₂ + 2HCl ⟶ PbCl₂ ↓ + 2HNO₃

(b) Hydrochloric acid, when reacted with silver nitrate solution it gives a thick curdy white precipitate of silver chloride. This white precipitate is insoluble in nitric acid.

AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

(c) On addition of ammonium hydroxide, the resultant of part (b) i.e., thick curdy white precipitate of silver chloride dissolves and forms a complex salt called diammine silver (I) chloride.

AgCl + 2NH₄OH ⟶ [Ag(NH₃)₂Cl)] + 2H₂O

(a) On reaction with HCl, lead nitrate solution gives white precipitate which can be dissolved in hot water.

Pb(NO₃)₂ + 2HCl ⟶ PbCl₂ ↓ + 2HNO₃

Whereas, silver nitrate solution gives thick curdy white precipitate which is insoluble in hot water.

AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

(b) On reaction with HCl, potassium sulphite gives SO₂ gas with pungent odour

K₂SO₃ + 2HCl ⟶ 2KCl + H₂O + SO₂ ↑

However, potassium sulphide gives H₂S gas with rotten eggs odour.

K₂S + 2HCl ⟶ 2KCl + H₂S ↑

(a) The equation for the laboratory preparation of hydrogen chloride gas :

$\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_4 + \text{HCl [g]}$ ↑

(b) Concentrated sulphuric acid is used as the drying agent used in the above preparation.

Reason — Drying agent used for drying should only remove the moisture and not react with it, hence, conc. sulphuric acid is used as the drying agent. Other drying agents like phosphorous pentoxide (P₂O₅) and quick lime (CaO) cannot be used since they react with hydrogen chloride gas.

(c) Temperature is maintained at nearly 200°C as at higher temperatures above 200°C the glass apparatus may crack, fuel is wasted, sodium sulphate formed, forms a hard crust which sticks to the glass and is difficult to remove.

(i) H₂S gas is evolved when dilute hydrochloric acid reacts with Iron [II] sulphide.

FeS + 2HCl ⟶ FeCl₂ + H₂S ↑

(ii) SO₂ is evolved when dilute hydrochloric acid reacts with sodium sulphite.

Na₂SO₃ + 2HCl ⟶ 2NaCl + H₂O + SO₂ [g] ↑

Polar covalent

Reason — Hydrogen chloride (HCl) is a polar covalent molecule because it is formed by sharing of electrons between hydrogen and chlorine atoms. However, chlorine is more electronegative than hydrogen, so it pulls the shared electrons closer to itself. This creates a partial negative charge on chlorine and a partial positive charge on hydrogen, making the molecule polar.

Conc. sulphuric acid.

Reason — Drying agent used for drying should only remove the moisture and not react with it, hence, conc. sulphuric acid is used as the drying agent.

HCl is highly soluble in water

Reason — Fountain experiment is performed to show the great solubility of HCl gas in water.

Only R

Reason — Toluene is an organic solvent and non-polar in nature. When HCl dissolves in toluene, it does not ionize to form hydronium [H₃O⁺] ions. Since there are no free hydrogen ions, the solution does not show acidic properties and also cannot react with sodium carbonate to release carbon dioxide gas. Therefore, only R is correct — the solution does not show any acidic property. b

Both A and R are true and R is the correct explanation of A.

Explanation— HCl is produced, when moist hydrogen gas combines with chlorine in presence of diffused sunlight. Hence, assertion (A) is true.

${\text{H}_2} + {\text{Cl}_2} \xrightarrow[\text{sunlight}]{\text{diffused}} {2\text{HCl}}$

The formation HCl is a explosive reaction in direct sunlight but it is negligible in dark. So, it is carried out in diffused sunlight. Hence, reason (R) is true.

Therefore, Reason (R) thus provides a valid explanation for why this reaction is occurring in diffused sunlight. Hence, Reason (R) is the correct explanation of assertion

A is true but R is false.

Explanation— The principle of fountain experiment is to show the great solubility of the HCl gas. Hence the assertion (A) is true.

The blue litmus solution turns red due to the acidic nature of hydrogen chloride gas in the Fountain experiment. Hence reason (R) is false.

Both A and R are true and R is the correct explanation of A.

Explanation— Hydrogen chloride gas is collected by the downward delivery (upward displacement of air). Hence, the assertion (A) is true.
HCl is collected by downward delivery because it is 1.28 times heavier than air. Hence, reason (R) is true and it is the correct explanation of assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation— Hydrogen chloride (HCl) gas fumes in moist air because it is highly soluble in water. In the presence of moisture, HCl gas dissolves in water vapour in the air to form tiny droplets of hydrochloric acid, which appear as white fumes. So, both the assertion and the reason are true, and the reason correctly explains the assertion.

Both A and R are true and R is the correct explanation of A.

Explanation— Hydrogen chloride (HCl) is a polar covalent compound, which means it can dissolve in polar solvents like water due to attraction between opposite charges. Interestingly, it can also physically dissolve in some non-polar organic solvents like toluene, but without ionizing. The reason HCl can dissolve in both is due to its polar covalent nature. Hence, both the Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

A is true but R is false.

Explanation— HCl gas dissociates into hydrogen and chlorine, when heated to 500°C and hydrogen chloride reacts with metals that come before hydrogen in the electrochemical series to form corresponding chlorides of those metals. Hence, assertion (A) is true.
When concentrated Hydrochloric acid is oxidised by strong oxidising agent like lead oxide lead chloride is formed. Hence, reason (R) is false.

Pb₃O₄ + 8HCl (conc.) ⟶ 3PbCl₂ + 4H₂O + Cl₂ ↑

Both A and R are true but R is not the correct explanation of A.

Explanation— Hydrogen chloride has sour (acidic) taste. Hence, the assertion (A) is true.
HCl is highly soluble in water and it can be proved by performing Fountain experiment. Hence, the reason (R) is true.

However, the high solubility of HCl in water doesn't relate to its taste, Hence, the reason (R) is not the correct explanation of assertion (A).

(a) NH₄OH + HCl (dil.)⟶ NH₄Cl + H₂O

(b) NaHSO₃ + HCl (dil.) ⟶ NaCl + H₂O + SO₂

(c) Pb(NO₃)₂ + 2HCl (dil.) ⟶ PbCl₂+ 2HNO₃

(d) Pb₃O₄ + 8HCl (conc.) ⟶ 3PbCl₂ + 4H₂O + Cl₂ ↑

Aqua regia : 1 part of conc. HNO₃ and 3 parts of conc. HCl

Nitric acid present in aqua regia oxidizes HCl to chlorine.

(a) When Sodium carbonate is treated with dil. HCl, odourless carbon dioxide gas is produced.

Na₂CO₃ + 2HCl ⟶ 2NaCl + H₂O + CO₂ ↑

However, when sodium sulphite is treated with dil.HCl, sulphur dioxide gas with a suffocating odour (burning smell) is produced.

Na₂SO₃ + 2HCl ⟶ 2NaCl + H₂O + SO₂ ↑

(b) When sodium thiosulphate is treated with dil. HCl it produces sulphur dioxide gas and yellow sulphur precipitates.

Na₂S₂O₃ + 2HCl ⟶ 2NaCl + S + H₂O + SO₂ ↑

However, S is not produced when sodium sulphite is treated with dil.HCl.

Na₂SO₃ + 2HCl ⟶ 2NaCl + H₂O + SO₂ ↑

It shows that hydrochloric acid (HCl) is oxidized to chlorine (Cl₂) by strong oxidizing agents such as MnO₂, PbO₂ and red lead.

(a) Silver nitrate solution
AgNO₃ + HCl ⟶ AgCl (white ppt.) + HNO₃

(b) Magnesium foil
Mg + 2HCl ⟶ MgCl₂ + H₂

(c) Caustic soda solution
NaOH + HCl ⟶ NaCl + H₂O

(d) Zinc carbonate
ZnCO₃ + 2HCl [dil.] ⟶ ZnCl₂ + H₂O + CO₂ [g.]

(e) Manganese (IV) oxide
$\text{MnO}_2 + \underset{\text{Conc.}}{\text{4HCl}} \xrightarrow{\Delta} \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$ ↑

(f) Copper oxide
CuO + 2HCl ⟶ CuCl₂ + H₂O.

The balanced equation are :

(a) Iron
Fe + 2HCl (dil.) ⟶ FeCl₂ + H₂ ↑

(b) Sodium hydrogen carbonate
NaHCO₃ + HCl ⟶ NaCl + H₂O + CO₂ ↑

(c) Iron (II) sulphide
FeS + 2HCl ⟶ FeCl₂ + H₂S ↑

(d) Magnesium sulphite
MgSO₃ + 2HCl ⟶ MgCl₂ + H₂O + SO₂ ↑

When silver nitrate soln. is added to dil. HCl, curdy white ppt. of silver chloride is formed whereas, there is no reaction when silver nitrate soln. is added to dil. HNO₃.

AgNO₃ + HCl ⟶ AgCl (white ppt.) + HNO₃

AgNO₃ + HNO₃ ⟶ no reaction

(a) A colourless gas having smell of rotten eggs is given off. The gas evolved is H₂S
CuS + 2HCl ⟶ CuCl₂ + H₂S ↑

(b) Curdy white precipitate of silver chloride [AgCl] is obtained, which is soluble in excess of NH₄OH.
AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃
AgCl + 2NH₄OH ⟶ [Ag(NH₃)₂Cl)] + 2H₂O

(c) White precipitate of PbCl₂ is formed which is soluble in hot water.
Pb(NO₃)₂ + 2HCl ⟶ PbCl₂ ↓ + 2HNO₃

(d) Hydrogen gas is evolved with bubbles and it burns with a pop sound
Zn + 2HCl ⟶ ZnCl₂ + H₂ ↑

(e) Effervescence of CO₂ seen which turns lime water milky.
Na₂CO₃ + 2HCl ⟶ 2NaCl + H₂O + CO₂ ↑

(f) When concentrated hydrochloric acid is added to manganese dioxide (MnO₂), a greenish-yellow gas (chlorine gas) is evolved with effervescence.
MnO₂ + 4HCl (conc.) $\xrightarrow{\Delta}$ MnCl₂ + 2H₂O + Cl₂↑

(a) Na₂S₂O₃ + 2HCl ⟶ 2NaCl + S + H₂O + SO₂ ↑

(b) Ca(HCO₃)₂ + 2HCl ⟶ CaCl₂ + 2H₂O + 2CO₂ ↑

(c) 2KMnO₄ + 16HCl $\xrightarrow{\Delta}$ 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂↑

(d) Na₂S + 2HCl ⟶ 2NaCl + H₂S [g.] ↑

(e) NaHCO₃ + HCl ⟶ NaCl + H₂O + CO₂ ↑

(a) $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_4 + \text{HCl [g]}$ ↑

(b) Fe + 2HCl (dil.) ⟶ FeCl₂ + H₂ [g]

(c) NH₄OH + HCl (dil.)⟶ NH₄Cl + H₂O

(d) $\text{PbO}_2 + 4\text{HCl} \xrightarrow{\Delta} \text{PbCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$ ↑

(a) Anhydrous HCl is a gas and does not contain any ions, hence it is a poor conductor of electricity. On the other hand, aqueous HCl (HCl dissolved in water) forms ions in solution, and thus becomes an excellent conductor of electricity.

HCl + H₂O ⟶ H₃O⁺ + Cl^-

(b) When hydrogen chloride gas is exposed to air, it gives white fumes, due to the formation of hydrochloric acid on dissolving in atmospheric water vapour.

(c) A solution of hydrogen chloride in water gives ions,

HCl + H₂O ⟶ H₃O⁺ + Cl^-

These ions are responsible for conducting electricity and turning blue litmus red, due to the presence of hydronium [H₃O⁺] ions which make aqueous solution of HCl acidic in nature.

Toluene is an organic solvent and HCl is a covalent compound. A solution of HCl in toluene contains only molecules and not ions due to which it does not conduct electricity. The absence of hydronium [H₃O⁺] ions make the solution neutral and it does not effect litmus paper.

(d) When a glass rod dipped in ammonium hydroxide (NH₄OH) is brought near the mouth of a bottle full of HCl gas, then it leads to the formation of dense white fumes due to the production of ammonium chloride.

NH₄OH + HCl ⟶ NH₄Cl + H₂O

(e) Dry hydrogen chloride gas does not contain any ions. Due to the absence of hydronium [H₃O⁺] ions, it is neutral and does not affect a dry strip of blue litmus paper. On the other hand, in the presence of a drop of water, HCl dissolves in it and dissociates into hydronium [H₃O⁺] ions and chloride ions [Cl^-]

HCl + H₂O ⟶ H₃O⁺ + Cl^-

The presence of hydronium [H₃O⁺] ions makes the aqueous solution acidic and blue litmus turns red.

(f) Hydrogen chloride gas is not collected over water since it is highly soluble in water.

(g) Hydrochloric acid forms a constant boiling mixture at 110°C, 22.2% by weight. On boiling further, the mixture evolves out the vapours of both acid and water in the same proportion as in the liquid.

Hence, dilute HCl cannot be concentrated beyond 22.2% by boiling.

(h) Quicklime [CaO] is alkaline in nature and it reacts with HCl forming the respective chloride. Hence, it can't be used as a drying agent.

(a) Quicklime is not used to dry HCl gas because CaO is alkaline.

(b) When sodium chloride is heated with concentrated sulphuric acid below 200°C, one of the product formed is sodium hydrogen sulphate.

(a) Copper oxide (CuO)

(b) Hydrogen chloride and Ammonia

(c) Hydrogen and Oxygen

(d) Silver chloride

(e) Aqua Regia

(f) Fountain experiment

(g) Oxygen gas

(h) Dilute hydrochloric acid

(i) Hydrogen chloride

(j) Chlorine

Explanation

(a) Copper oxide (CuO) is black, reacts with HCl to form blue-coloured copper(II) chloride solution.
CuO + 2HCl ⟶ CuCl₂ + H₂O

(b) NH₃ (g) + HCl (g) ⟶ NH₄Cl (white solid)

(c) 2H₂ (g) + O₂ (g) ⟶ 2H₂O (liquid water)

(d) AgCl dissolves in excess NH₄OH due to formation of [Ag(NH₃)₂]⁺ complex.
AgCl + 2NH₄OH ⟶ [Ag(NH₃)₂]⁺Cl^- + 2H₂O

(e) Aqua regia (1 part of conc. HNO₃ and 3 parts of conc. HCl) dissolves gold.

(f) Fountain experiment vividly shows HCl gas dissolving in water, creating a fountain effect due to pressure drop.

(g) When chlorine water is exposed to sunlight, chlorine reacts with water to form hypochlorous acid. This acid then breaks apart to give nascent oxygen (O) and hydrochloric acid (HCl). The nascent oxygen combines to form Oxygen gas:
Cl₂ + H₂ $\xrightarrow{\text{sunlight}}$ HClO + HCl
HClO $\xrightarrow{\text{sunlight}}$ HCl + [O] (nascent oxygen)
[O] + [O] ⟶ O₂

(h) AgNO₃ + HCl ⟶ AgCl (white ppt.) + HNO₃ which dissolves in excess NH₄OH.

(i) NH₃ [g] + HCl [g] ⟶ NH₄Cl [s]

(j) H₂ + Cl₂ ⟶ 2HCl; HCl is strongly acidic in water.

(i) Sulphur dioxide gas

(ii) Chlorine gas
$\text{MnO}_2 + 4\text{HCl} \xrightarrow{\Delta} \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2$ ↑

A → Silver nitrate
B → Hydrochloric acid
C → Silver chloride

Explanation — Solution A is silver nitrate which reacts with acid B i.e., Hydrochloric acid to give a white precipitate C of AgCl which is insoluble in nitric acid but soluble in ammonium hydroxide.

AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

AgNO₃ + HNO₃ ⟶ no reaction

AgCl + 2NH₄OH ⟶ [Ag(NH₃)₂Cl)] + 2H₂O