Study of Compounds: Hydrogen Chloride — ICSE Class 10 Chemistry Important Questions

Answer: (a) concentrated sulphuric acid.

Sodium chloride is heated with concentrated (not dilute) sulphuric acid: $NaCl+H_2SO_4\rightarrow NaHSO_4+HCl$NaCl+H₂SO₄→ NaHSO₄+HCl. Nitric acid is unsuitable as it is an oxidising agent.

Answer: (b) concentrated sulphuric acid.

$HCl$HCl is dried using concentrated $H_2SO_4$H₂SO₄, which does not react with it. Quicklime, sodium hydroxide and slaked lime are all basic and would react with the acidic gas.

Answer: (a) dense white fumes.

$HCl$HCl reacts with ammonia to form solid ammonium chloride: $NH_3+HCl\rightarrow NH_4Cl$NH₃+HCl→ NH₄Cl, seen as dense white fumes. This is a confirmatory test for $HCl$HCl.

Answer: (a) $3:1$3:1.

Aqua regia is $3$3 parts concentrated $HCl$HCl to $1$1 part concentrated $HNO_3$HNO₃ by volume; the nascent chlorine it liberates dissolves noble metals like gold and platinum.

Answer: (a) Because $HCl$HCl is denser than air it is collected by downward delivery, and because it is highly soluble it cannot be collected over water. Both facts (density and solubility) justify the method, so R correctly explains A.

(i) Below $200^\circ\text{C}$200° C:
$NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl\uparrow$NaCl + H₂SO₄ → NaHSO₄ + HCl↑

(ii) Above $200^\circ\text{C}$200° C (about $500^\circ\text{C}$500° C):
$NaCl + NaHSO_4 \rightarrow Na_2SO_4 + HCl\uparrow$NaCl + NaHSO₄ → Na₂SO₄ + HCl↑

The overall reaction is $2NaCl+H_2SO_4\rightarrow Na_2SO_4+2HCl$2NaCl+H₂SO₄→ Na₂SO₄+2HCl.

(i) Sodium sulphite: a colourless gas with the smell of burning sulphur (sulphur dioxide) is given off.
$Na_2SO_3 + 2HCl \rightarrow 2NaCl + H_2O + SO_2\uparrow$Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂↑

(ii) Iron(II) sulphide: a colourless gas with a rotten-egg smell (hydrogen sulphide) is given off.
$FeS + 2HCl \rightarrow FeCl_2 + H_2S\uparrow$FeS + 2HCl → FeCl₂ + H₂S↑

A dry round-bottomed flask is filled with hydrogen chloride gas and fitted with a stopper carrying a jet tube and a dropper containing water. The flask is held inverted over a trough of water containing a little blue litmus.

A few drops of water are squeezed from the dropper into the flask. The gas dissolves rapidly and almost completely, causing a sudden fall in pressure inside the flask. The higher outside atmospheric pressure then pushes water up through the jet, which enters as a fountain. The water turns red, showing the solution is acidic.

This proves that hydrogen chloride is extremely soluble in water and that its aqueous solution is acidic.

(i) Silver nitrate: a white precipitate of silver chloride forms.
$AgNO_3 + HCl \rightarrow AgCl\downarrow + HNO_3$AgNO₃ + HCl → AgCl↓ + HNO₃
On adding ammonium hydroxide, the white precipitate dissolves (forming a soluble complex), confirming a chloride.

(ii) Lead nitrate: a white precipitate of lead chloride forms.
$Pb(NO_3)_2 + 2HCl \rightarrow PbCl_2\downarrow + 2HNO_3$Pb(NO₃)₂ + 2HCl → PbCl₂↓ + 2HNO₃
This precipitate is soluble in hot water and reappears on cooling.

(i) With zinc (active metal): hydrogen gas is evolved.
$Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$Zn + 2HCl → ZnCl₂ + H₂↑

(ii) With sodium carbonate: carbon dioxide is released with effervescence.
$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\uparrow$Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑

(iii) With sodium hydroxide (neutralisation):
$NaOH + HCl \rightarrow NaCl + H_2O$NaOH + HCl → NaCl + H₂O

(a) Reactants: sodium chloride and concentrated sulphuric acid.
$NaCl + H_2SO_4 \xrightarrow{<200^\circ C} NaHSO_4 + HCl\uparrow$NaCl + H₂SO₄ → (<200° C) NaHSO₄ + HCl↑

(b) The temperature is kept below $200^\circ\text{C}$200° C because (1) the higher temperature needed for the second stage wastes fuel, (2) the glass apparatus may crack at the higher temperature, and (3) the sodium sulphate formed sticks to the flask as a hard crust that is difficult to remove. Concentrated sulphuric acid is used because hydrogen chloride is extremely soluble in water: with dilute acid, the gas would dissolve in the water present instead of being given off.

(c) The gas is dried by bubbling it through concentrated sulphuric acid and collected by downward delivery (upward displacement of air), since it is denser than air and too soluble to collect over water.

(d) When dissolving the very soluble gas in water, an inverted funnel is placed just touching the water surface. This provides a large surface for rapid absorption and prevents sucking back: if water rises, it enters the wide funnel and falls back instead of being drawn up into the hot apparatus.

(a) Hydrogen chloride is a colourless gas with a sharp, pungent, choking smell; it is denser than air and fumes strongly in moist air.

(b) Hydrogen chloride shows acidic behaviour only when it produces $H^+$H^+ (hydrogen) ions, which requires water. Dry $HCl$HCl gas has no free ions, so it does not affect dry litmus; dissolved in water it ionises, $HCl \rightarrow H^+ + Cl^-$HCl → H^+ + Cl^-, and the $H^+$H^+ ions turn blue litmus red.

(c) Aqua regia is a mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio $3:1$3:1 by volume. It liberates nascent chlorine and dissolves noble metals such as gold and platinum.

(d) The reaction with marble ($CaCO_3$CaCO₃) needs ionisation to provide $H^+$H^+ ions, which occurs only in the aqueous acid. Dry $HCl$HCl gas has no free hydrogen ions, so it does not react with marble chips, whereas hydrochloric acid does: $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑.

(i) $NH_3 + HCl \rightarrow NH_4Cl$NH₃ + HCl → NH₄Cl (dense white fumes of ammonium chloride).

(ii) The white precipitate is silver chloride, $AgCl$AgCl.
$AgNO_3 + HCl \rightarrow AgCl\downarrow + HNO_3$AgNO₃ + HCl → AgCl↓ + HNO₃

(iii) The silver chloride precipitate is insoluble in hot water but readily dissolves in ammonium hydroxide, whereas lead chloride ($PbCl_2$PbCl₂) is insoluble in ammonium hydroxide but dissolves in hot water and reappears on cooling. This difference distinguishes them.

(iv) Hydrochloric acid is used in the manufacture of ammonium chloride (and chlorine), in the pickling (cleaning) of metals before galvanising, and in preparing glucose from starch (any one).