Hydrogen Chloride & Hydrochloric Acid — Comprehensive Notes

Syllabus Blueprint

• Preparation of hydrogen chloride (HCl) from sodium chloride.
• Density experiment ("heavier-than-air" pour‐down test).
• Solubility demonstration (fountain experiment).
• Laboratory manufacture of hydrochloric acid and anti-suction safety.
• Reaction of HCl (gas & aqueous) with ammonia, metals, oxides, hydroxides, carbonates, hydrogen carbonates, sulphides, sulphites, thiosulphates.
• Precipitation tests with $\text{AgNO}<em>3$ and $\text{Pb(NO}</em>3)_2$.
• Complete physical / chemical properties, equations, apparatus diagrams, precautions, uses and analytical tests.

Molecular Formula & Structure

• Formula : HCl  Molar mass : 36.5 g mol$^{-1}$.
• Bonding: single polar covalent bond (electronegativity difference yet electrons shared).
  • Lewis / dot: H : Cl or H – Cl.
  • Orbital overlap: $1s^1(\text H)+3p^5(\text Cl)\;\longrightarrow\;\sigma$ bond.

Historical Highlights & Natural Occurrence

• 1648 – Glauber first prepared acid (heating NaCl with conc $\text{H}<em>2\text{SO}</em>4$).
• Lavoisier coined name “muriatic acid”; Sir H. Davy (1810) renamed hydrochloric acid.
• Free HCl gas in volcanic exhalations.
• $0.2{-}0.4\%$ HCl present in gastric juice of mammals (aids digestion).

General Preparations of Hydrogen Chloride Gas

• Direct synthesis

• $\text{H}<em>2(g)+\text{Cl}</em>2(g)\xrightarrow[\text{diffused sunlight}]{}2\text{HCl}(g)$.
• Explosive in bright sunlight; negligible in dark; proceeds in dark with activated carbon catalyst (adsorbs H$_2$ ↔ increases local concentration).
• A burning jet of H$2$ will burn inside Cl$2$ generating HCl white cloud.

• From metallic chlorides + conc $\text{H}<em>2\text{SO}</em>4$

• Below $200^\circ\text C$: $\text{NaCl}+\text{H}<em>2\text{SO}</em>4\;\longrightarrow\;\text{NaHSO}_4+\text{HCl}$.
• Above $200^\circ\text C$: $2\text{NaCl}+\text{H}<em>2\text{SO}</em>4\;\longrightarrow\;\text{Na}<em>2\text{SO}</em>4+2\text{HCl}$.
• Similar: $\text{CuCl}<em>2+\text{H}</em>2\text{SO}<em>4\;\longrightarrow\;\text{CuSO}</em>4+2\text{HCl}$.

Laboratory Preparation of Hydrogen Chloride Gas

• Reactants

• Cheap, readily-available common salt $\,(\text{NaCl})$ (preferred to other chlorides).
• Conc $\text{H}<em>2\text{SO}</em>4$ (conc HNO$_3$ avoided—volatile; would distil over with HCl).

• Apparatus & Key Features

• Flask (flat or round bottom) fitted with thistle funnel + delivery tube through drying tower.
• Heat mixture gently to maintain $\approx 200^\circ\text C$.
• Drying agent : conc $\text{H}<em>2\text{SO}</em>4$ only (P$<em>2$O$</em>5$ & CaO react => $\text{POCl}<em>3$ / $\text{CaCl}</em>2$ formation).
• Collection : downward delivery (upward displacement of air) – gas is $1.28$ × heavier than air; never over water (very soluble).
• Identification: dense white fumes over jar mouth; rod dipped in $\text{NH}<em>4\text{OH}$ → $\text{NH}</em>4\text{Cl}$ white smoke.

• Precautions

• Thistle funnel tip below acid surface.
• Delivery tube must dip into drying agent, not into acid.
• Do not over-heat (>$200^\circ\text C$): glass may crack, fuel wasted, hard $\text{Na}<em>2\text{SO}</em>4$ crust forms.

Demonstrations & Experiments

• Density test ("candle jar")

• Pour HCl gas into a jar containing a lit candle; lower layer fills with HCl, candle extinguishes ⇒ heavier-than-air & non-supporter of combustion.

• Fountain experiment (solubility)

• Dry round-bottom flask full of HCl sealed by stopper with (i) long jet tube dipping into beaker of blue litmus solution, (ii) dropper with water.
• Press dropper → few drops enter, dissolve HCl → pressure falls → external solution rushes in creating red fountain (blue litmus→red due to acid). Shows:
  • Very high solubility (452 vol gas / 1 vol water @ RT).
  • Acidic nature.

Physical Properties of Hydrogen Chloride Gas

• Colourless, pungent, choking.
• Sour taste (if dissolved—corrosive; gas irritant to eyes, nose, lungs).
• Density: $\dfrac{M<em>{\text{HCl}}}{M</em>{\text{air}}}=\dfrac{36.5}{29} \approx1.28$ (V.D. 18.25 vs 14.4 for air).
• m.p. $-114^\circ\text C$ b.p. $-85^\circ\text C$.
• Liquefies at $40\,\text{atm},\;10^\circ\text C$.
• Soluble in polar water and non-polar solvents (acetone, toluene) owing to dipole-induced-dipole interactions.

Chemical Behaviour of Hydrogen Chloride Gas

• Non-combustible; extinguishes flame.
• Thermal dissociation 2\text{HCl}\xrightarrow{>500^\circ\text C}}\text{H}2+\text{Cl}2.
• With active metals above H in activity series
  • $\text{2Na}+2\text{HCl}\;\longrightarrow\;2\text{NaCl}+\text{H}_2$ (similar Ca, Mg, Zn, Fe etc.).
• With ammonia
  • $\text{NH}<em>3(g)+\text{HCl}(g)\;\longrightarrow\;\text{NH}</em>4\text{Cl}(s)$ dense white cloud.

Hydrochloric Acid (Aqueous HCl)

• Formed when HCl gas dissolves & ionises: $\text{HCl}+\text{H}<em>2\text O\;\rightleftharpoons\;\text{H}</em>3\text O^++\text{Cl}^-$.
• Dry or liquefied HCl contains no free $\text H^+$ → does not redden litmus or conduct electricity; aqueous solution does.
• Also dissolves in toluene without ionisation → again no acidic tests.

Laboratory Preparation of Hydrochloric Acid Solution

• Inverted funnel arrangement set just touching water in trough.
• Process
  • At first contact HCl dissolves rapidly → water rises inside funnel → back suction lowers external level → air gap forms → pressures balance → water falls → cycle repeats until saturation (~$36\%$ HCl by mass).
• Safety: Anti-suction empty flask placed between generator & trough; prevents hot conc $\text{H}<em>2\text{SO}</em>4$ receiving back-flow of water.
• On distillation acid concentrates to azeotrope $22.2\%$ HCl + $77.8\%$ H$_2$O, boils constant at $110^\circ\text C$.

Physical Properties of Hydrochloric Acid

• Colourless, pungent.
• Violently corrosive when concentrated (causes skin blisters).
• Miscible with water in all proportions; b.p. $110^\circ\text C$ (for azeotrope).

Acidic & Chemical Properties of Hydrochloric Acid

• Indicator action

• Blue litmus → red; methyl orange (orange) → pink/red; phenolphthalein remains colourless.

• Action on metals (above H)

• $\text{Ca}+2\text{HCl}\rightarrow\text{CaCl}<em>2+\text{H}</em>2$ (similarly Mg, Zn, Fe, etc.).

• Action on basic oxides & hydroxides (neutralisation)

• General: $\text{MO/M(OH)}<em>n+n\text{HCl}\rightarrow\text{MCl}</em>n+n\text{H}_2\text O$.
• E.g. $\text{Fe}<em>2\text O</em>3+6\text{HCl}\rightarrow2\text{FeCl}<em>3+3\text H</em>2\text O$.

• With salts of weaker acids

• Carbonates / hydrogencarbonates: $\text{Na}<em>2\text{CO}</em>3+2\text{HCl}\rightarrow2\text{NaCl}+\text H<em>2\text O+\text{CO}</em>2$ (effervescence).
• Sulphites / hydrogensulphites: $\text{K}<em>2\text{SO}</em>3+2\text{HCl}\rightarrow2\text{KCl}+\text H<em>2\text O+\text{SO}</em>2$ (pungent gas).
• Thiosulphates: $\text{Na}<em>2\text S</em>2\text O<em>3+2\text{HCl}\rightarrow2\text{NaCl}+\text H</em>2\text O+\text{SO}_2+\text S$ (yellow S ppt ⇒ distinguishes from sulphites).
• Metal sulphides: $\text{FeS}+2\text{HCl}\rightarrow\text{FeCl}<em>2+\text{H}</em>2\text S$ (rotten-egg odour).

• Precipitation tests

• $\text{AgNO}<em>3+\text{HCl}\rightarrow\text{AgCl}(\text{curdy white})+\text{HNO}</em>3$.
  • AgCl insoluble in HNO$3$, dissolves in excess $\text{NH}</em>4\text{OH}$ forming $[\text{Ag(NH}<em>3)</em>2]^+$; re-precipitates with dilute HNO$_3$.
• $\text{Pb(NO}<em>3)</em>2+2\text{HCl}\rightarrow\text{PbCl}<em>2(\text{white ppt})+2\text{HNO}</em>3$ (ppt dissolves on heating).
• $\text{Hg}<em>2(\text{NO}</em>3)<em>2+2\text{HCl}\rightarrow\text{Hg}</em>2\text{Cl}<em>2(\text{white})+2\text{HNO}</em>3$.

• Oxidation / Reduction

• Conc HCl + strong oxidiser → Cl$_2$ (greenish-yellow) liberated.
  • $\text{MnO}<em>2+4\text{HCl}</em>{(conc)}\rightarrow\text{MnCl}<em>2+2\text{H}</em>2\text O+\text{Cl}_2$.
  • $\text{PbO}<em>2,\;\text{Pb}</em>3\text O<em>4,\;\text{CaOCl}</em>2$ behave similarly.

• Aqua regia

• Conventional mix 3 parts conc HCl : 1 part conc HNO$_3$.
• Reaction: $3\text{HCl}+\text{HNO}<em>3\rightarrow\text{NOCl}+2\text{H}</em>2\text O+2[\text{Cl}]$ (nascent Cl).
• Dissolves Au, Pt:
  • $\text{Au}+3[\text Cl]\rightarrow\text{AuCl}_3$.
  • $\text{Pt}+4[\text Cl]\rightarrow\text{PtCl}_4$.

Characteristic Tests for HCl / HCl Gas

1. Pungent choking odour.
2. Moist glass rod dipped in $\text{NH}<em>4\text{OH}$ ⇒ dense white fumes $\text{NH}</em>4\text{Cl}$.
3. With $\text{AgNO}<em>3$ solution ⇒ curdy white $\text{AgCl}$ ppt (behaviour with NH$4$OH and light as above).
4. Conc HCl + $\text{MnO}<em>2$ on warming ⇒ Cl$2$ (turns moist starch-iodide paper blue-black).

Uses of Hydrochloric Acid

• Essential laboratory reagent (acidification, preparation of chlorides, gas-generating reactions).
• Industrial pickling of steel & other metals (removal of rust/scale).
• Manufacture of chlorine, PVC, dyes, fertilizers, leather processing, glucose from starch etc.
• Regeneration of ion-exchange resins in deionised water plants.
• Food industry (purified dil. HCl) & medicine (adjusting gastric acidity; destroying microorganisms entering alimentary canal).

Summary of Safety & Environmental Aspects

• Corrosive; inhalation causes severe irritation—operate in fume hood.
• Dry gas rapidly forms mist/fumes in moist air → eye/respiratory hazard.
• Neutralise spills with weak base (e.g., NaHCO$_3$) before disposal.
• Aqua regia requires rigorous ventilation; generates toxic NOCl & Cl$_2$.