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+H_2SO_4→ NaHSO_4+HCl. Nitric acid is unsuitable as it is an oxidising agent.

Answer: (b) concentrated sulphuric acid.

$HCl$HCl is dried using concentrated $H_2SO_4$H_2SO_4, which does not react with it. Quicklime and sodium hydroxide are basic and would react with the acidic gas, and calcium chloride combines with $HCl$HCl.

Answer: (a) dense white fumes.

$HCl$HCl reacts with ammonia to form solid ammonium chloride: $NH_3+HCl\rightarrow NH_4Cl$NH_3+HCl→ NH_4Cl, 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 $HNO_3$HNO_3 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^:
$NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl\uparrow$NaCl + H_2SO_4 → NaHSO_4 + HCl

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

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

Hydrogen chloride is extremely soluble in water (one volume of water dissolves about $450$450 volumes of the gas), so it would dissolve instead of being collected if passed over water.

Since it is also denser than air, it is collected by downward delivery (upward displacement of air), with the gas jar held mouth upward.

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.
$AgNO_3 + HCl \rightarrow AgCl\downarrow + HNO_3$AgNO_3 + HCl → AgCl + HNO_3
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(NO_3)_2 + 2HCl → PbCl_2 + 2HNO_3
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 → ZnCl_2 + H_2

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

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

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

(b) The lower temperature (below $200^\circ\text{C}$200^) stops at sodium hydrogen sulphate and avoids attacking the glass; more strongly the second stage $NaCl+NaHSO_4\rightarrow Na_2SO_4+HCl$NaCl+NaHSO_4→ Na_2SO_4+HCl needs a much higher temperature and the sticky $Na_2SO_4$Na_2SO_4 is hard to remove. Concentrated sulphuric acid is used because dilute acid contains water in which the $HCl$HCl formed would simply dissolve, so no gas would be obtained.

(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 ($CaCO_3$CaCO_3) 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: $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$CaCO_3 + 2HCl → CaCl_2 + H_2O + CO_2.

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

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

(iii) The silver chloride precipitate is insoluble in hot water but readily dissolves in ammonium hydroxide, whereas lead chloride ($PbCl_2$PbCl_2) 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).