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}3 and \text{Pb(NO}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}2\text{SO}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}2(g)+\text{Cl}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}2\text{SO}4

  • Below 200^\circ\text C: \text{NaCl}+\text{H}2\text{SO}4\;\longrightarrow\;\text{NaHSO}_4+\text{HCl}.
  • Above 200^\circ\text C: 2\text{NaCl}+\text{H}2\text{SO}4\;\longrightarrow\;\text{Na}2\text{SO}4+2\text{HCl}.
  • Similar: \text{CuCl}2+\text{H}2\text{SO}4\;\longrightarrow\;\text{CuSO}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}2\text{SO}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}2\text{SO}4 only (P2O5 & CaO react => \text{POCl}3 / \text{CaCl}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}4\text{OH} → \text{NH}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}2\text{SO}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{\text{HCl}}}{M{\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}3(g)+\text{HCl}(g)\;\longrightarrow\;\text{NH}4\text{Cl}(s) dense white cloud.

Hydrochloric Acid (Aqueous HCl)

  • Formed when HCl gas dissolves & ionises: \text{HCl}+\text{H}2\text O\;\rightleftharpoons\;\text{H}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}2\text{SO}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}2+\text{H}2 (similarly Mg, Zn, Fe, etc.).

• Action on basic oxides & hydroxides (neutralisation)

  • General: \text{MO/M(OH)}n+n\text{HCl}\rightarrow\text{MCl}n+n\text{H}_2\text O.
  • E.g. \text{Fe}2\text O3+6\text{HCl}\rightarrow2\text{FeCl}3+3\text H2\text O.

• With salts of weaker acids

  • Carbonates / hydrogencarbonates: \text{Na}2\text{CO}3+2\text{HCl}\rightarrow2\text{NaCl}+\text H2\text O+\text{CO}2 (effervescence).
  • Sulphites / hydrogensulphites: \text{K}2\text{SO}3+2\text{HCl}\rightarrow2\text{KCl}+\text H2\text O+\text{SO}2 (pungent gas).
  • Thiosulphates: \text{Na}2\text S2\text O3+2\text{HCl}\rightarrow2\text{NaCl}+\text H2\text O+\text{SO}_2+\text S (yellow S ppt ⇒ distinguishes from sulphites).
  • Metal sulphides: \text{FeS}+2\text{HCl}\rightarrow\text{FeCl}2+\text{H}2\text S (rotten-egg odour).

• Precipitation tests

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

• Oxidation / Reduction

  • Conc HCl + strong oxidiser → Cl$_2$ (greenish-yellow) liberated.
    • \text{MnO}2+4\text{HCl}{(conc)}\rightarrow\text{MnCl}2+2\text{H}2\text O+\text{Cl}_2.
    • \text{PbO}2,\;\text{Pb}3\text O4,\;\text{CaOCl}2 behave similarly.

• Aqua regia

  • Conventional mix 3 parts conc HCl : 1 part conc HNO$_3$.
  • Reaction: 3\text{HCl}+\text{HNO}3\rightarrow\text{NOCl}+2\text{H}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}4\text{OH} ⇒ dense white fumes \text{NH}4\text{Cl}.
  3. With \text{AgNO}3 solution ⇒ curdy white \text{AgCl} ppt (behaviour with NH$4$OH and light as above).
  4. Conc HCl + \text{MnO}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$_2$.