Exhaustive Study Guide on Hydrogen Chloride and Hydrochloric Acid

Fundamental Molecular Properties

• Molecular Formula: $\text{HCl}$

• Molecular Mass: $36.5$

• Chemical Bonding: Covalent bond, represented as $H:Cl$ or $H-Cl$ (Lewis dot structure / orbital diagram).

• Synthesis via Direct Combination:

  • Hydrogen combines with chlorine in the presence of diffused sunlight:     $H_2(g) + Cl_2(g) \xrightarrow{\text{Diffused Sunlight}} 2HCl(g)$

  • Reaction Conditions and Variations:

  • In direct sunlight, the reaction is explosive.

  • In total darkness, the reaction is negligible.

  • In the presence of a catalyst such as activated carbon, the reaction occurs even in the dark because activated carbon absorbs hydrogen, which increases its reactivity.

  • A burning jet of hydrogen continues to burn in an atmosphere of chlorine gas to produce hydrogen chloride.

Occurrence and Historical Context

• Historical Milestones:

  • 1648: Johann Rudolf Glauber prepared the acid by heating common salt ($\text{NaCl}$) with concentrated sulphuric acid ($\text{H}_2\text{SO}_4$).

  • Antoine Lavoisier: Named the aqueous solution muriatic acid.

  • 1810: Humphry Davy proved its composition and named it hydrochloric acid.

• Natural Occurrence:

  • Free State: Found in volcanic gases and emissions.

  • Combined State: Present in the gastric juice of mammals at a concentration of $0.2\,\% - 0.4\,\%$, where it aids in digestion.

General Methods of Preparation

• Direct Combination (Synthesis): Combining moist hydrogen gas and chlorine gas under diffused sunlight or using activated carbon in the dark.

• Action of Concentrated Sulphuric Acid on Metallic Chlorides:

  • General reaction formula: $\text{Metal chloride} + \text{Concentrated sulphuric acid} \rightarrow \text{Salt} + \text{Hydrogen chloride}$

  • Reaction with sodium chloride below $200\,^\circ\text{C}$:     $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{<200\,^\circ\text{C}} \text{NaHSO}_4 + \text{HCl}(g)$

  • Reaction with sodium chloride above $200\,^\circ\text{C}$:     $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\text{above } 200\,^\circ\text{C}} \text{Na}_2\text{SO}_4 + 2\text{HCl}(g)$

  • Reaction with copper chloride:     $\text{CuCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + 2\text{HCl}(g)$

Laboratory Preparation of Hydrogen Chloride Gas

• Reactants: Sodium chloride ($\text{NaCl}$) and concentrated sulphuric acid ($\text{H}_2\text{SO}_4$).

• Apparatus and Setup:

  • A flat-bottomed flask or round-bottomed flask fitted with a two-holed rubber stopper.

  • A thistle funnel inserted through one hole, dipping below the level of concentrated sulphuric acid.

  • A delivery tube passing through the second hole into a drying washer bottle containing concentrated sulphuric acid.

  • A gas jar for collection.

• Procedure:

  • Place sodium chloride in the flask and pour concentrated sulphuric acid through the thistle funnel.

  • The reaction proceeds slowly in the cold; gentle heating produces controlled effervescence of $\text{HCl}$ gas.

  • The evolved gas is passed through concentrated sulphuric acid to remove moisture.

  • Collected by downward delivery (upward displacement of air).

• Chemical Equations:

  • Primary temperature condition ($<200\,^\circ\text{C}$):     $\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{<200\,^\circ\text{C}} \text{NaHSO}_4 + \text{HCl}\uparrow$

  • Secondary high-temperature stage ($>200\,^\circ\text{C}$):     $\text{NaHSO}_4 + \text{NaCl} \xrightarrow{\text{above } 200\,^\circ\text{C}} \text{Na}_2\text{SO}_4 + \text{HCl}\uparrow$

  • Overall reaction at high temperature:     $2\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\text{above } 200\,^\circ\text{C}} \text{Na}_2\text{SO}_4 + 2\text{HCl}\uparrow$

  • Note: The formation of $\text{NaHSO}_4$ clearly demonstrates the acidic nature of sodium hydrogen sulphate.

• Reagent Selection Rationale:

  • Sodium Chloride ($\text{NaCl}$): Preferred because it is cheap and readily available compared to other metallic chlorides.

  • Concentrated Nitric Acid ($\text{HNO}_3$) Unsuitability: Concentrated nitric acid cannot replace concentrated sulphuric acid because $\text{HNO}_3$ is volatile and volatilizes along with the escaping $\text{HCl}$ gas.

• Drying and Purification:

  • Suitable Drying Agent: Concentrated sulphuric acid ($\text{H}_2\text{SO}_4$).

  • Unsuitable Drying Agents:

  • Phosphorus Pentoxide ($\text{P}_2\text{O}_5$): Reacts chemically with $\text{HCl}$ gas:       $2\text{P}_2\text{O}_5 + 3\text{HCl} \rightarrow \text{POCl}_3 + 3\text{HPO}_3$

  • Quick Lime ($\text{CaO}$): Basic oxide that reacts chemically with $\text{HCl}$ gas:       $\text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}$

• Collection Methods:

  • Collected by downward delivery (upward displacement of air) because $\text{HCl}$ gas is $1.28$ times heavier than air.

  • Cannot be collected over water due to its extremely high solubility in water.

• Gas Jar Identification and Completion Tests:

  • Visual Observation: Fumes appear above the mouth of the jar when it is completely filled.

  • Ammonium Hydroxide Test: Bring a glass rod dipped in ammonium hydroxide solution ($\text{NH}_4\text{OH}$) near the mouth of the jar. Formation of dense white fumes of ammonium chloride ($\text{NH}_4\text{Cl}$) confirms the jar is full:     $\text{HCl} + \text{NH}_4\text{OH} \rightarrow \text{NH}_4\text{Cl} + \text{H}_2\text{O}$

• Safety Precautions:

  • The lower end of the thistle funnel must be submerged in concentrated sulphuric acid to prevent generated gas from escaping through the funnel.

  • The delivery tube must dip into the concentrated sulphuric acid drying bottle.

  • Temperature must be maintained strictly at or around $200\,^\circ\text{C}$.

• Reasons to Avoid Temperatures Above $200\,^\circ\text{C}$:

  • Glass apparatus may crack at higher temperatures.

  • Fuel is wasted unnecessarily.

  • Sodium sulphate ($\text{Na}_2\text{SO}_4$) forms a hard crust that adheres strongly to the glass vessel, making it difficult to clean.

Physical Properties of Hydrogen Chloride Gas

• Colour: Colourless.

• Smell: Distinctive pungent, choking odor.

• Taste: Sour (acidic).

• Physiological Effect: Highly corrosive; severe irritant to nose, throat, and lungs.

• Density:

  • Relative density is approximately $1.28$ times heavier than air.

  • Vapour density of $\text{HCl}$ is $18.25$ (compared to air at $14.4$).

• Boiling Point: $-83\,^\circ\text{C}$.

• Melting Point: $-113\,^\circ\text{C}$.

• Liquefaction and Solidification: Easily liquefied to a colourless liquid under high pressure ($40\,\text{atm}$) at low temperature ($10\,^\circ\text{C}$).

• Solubility:

  • Extremely soluble in water ($1$ volume of water dissolves approximately $452$ volumes of $\text{HCl}$ gas at room temperature).

  • Being polar covalent, it dissolves in polar solvents and certain non-polar organic solvents like acetone and toluene.

Demonstrations of Density and Solubility

• Density Demonstration Experiment:

  • Setup: Pour $\text{HCl}$ gas into a jar containing a burning candle.

  • Observation: The candle flame is extinguished.

  • Inference: $\text{HCl}$ gas is heavier than air, occupies the lower portion of the jar, and displaces the air upwards. It also proves that $\text{HCl}$ is non-combustible and does not support combustion.

• Fountain Experiment (Solubility Demonstration):

  • Apparatus: A dry round-bottomed flask filled with dry $\text{HCl}$ gas, closed with a double-holed rubber stopper. One hole holds a dropper filled with water; the other holds a long glass jet tube immersed in a beaker of blue litmus solution.

  • Procedure: Squeeze the water dropper into the flask.

  • Observation: Blue litmus solution travels up the jet tube with great force, creating a vivid red fountain inside the flask.

  • Mechanism & Inference:

  • $\text{HCl}$ gas dissolves rapidly in the few drops of water inside the flask.

  • This creates a high vacuum, drastically lowering internal pressure.

  • The higher external atmospheric pressure forces the blue litmus solution up through the jet tube.

  • The change from blue to red demonstrates the acidic nature of $\text{HCl}$ in aqueous form.

  • Atmospheric Fuming: Owing to its high solubility, $\text{HCl}$ gas fumes in moist air, creating tiny droplets of hydrochloric acid.

Chemical Properties of Hydrogen Chloride Gas

• Combustibility: Neither combustible nor a supporter of combustion. Immediately extinguishes a burning splint.

• Thermal Dissociation: Dissociates into hydrogen and chlorine gases when heated above $500\,^\circ\text{C}$:   $2HCl \xrightarrow{>500\,^\circ\text{C}} H_2 + Cl_2$

• Reactions with Electropositive Metals:

  • Reacts with metals positioned above hydrogen in the electrochemical series when heated, forming metallic chlorides and releasing hydrogen gas:     $\text{Metal} + \text{Hydrogen chloride}(g) \rightarrow \text{Metal chloride} + \text{Hydrogen}(g)$

  • Reaction with Sodium:     $2Na + 2HCl \rightarrow 2NaCl + H_2\uparrow$

  • Reaction with Calcium:     $Ca + 2HCl \rightarrow CaCl_2 + H_2\uparrow$

  • Reaction with Magnesium:     $Mg + 2HCl \rightarrow MgCl_2 + H_2\uparrow$

  • Reaction with Zinc:     $Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$

  • Reaction with Iron:     $Fe + 2HCl \rightarrow FeCl_2 + H_2\uparrow$

• Combination with Ammonia:

  • Direct combination of two gaseous compounds producing a solid compound:     $NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$

  • Observed as dense white fumes of solid ammonium chloride.

Preparation and Mechanism of Hydrochloric Acid

• Nature of Hydrochloric Acid: An aqueous solution of hydrogen chloride gas.

• Ionization Process:

  • Pure $\text{HCl}$ is a covalent compound.

  • Upon dissolution in water, its polar nature causes complete ionization:     $HCl + H_2O \rightarrow H_3O^+ + Cl^-$

• Non-Acidic Behavior in Non-Aqueous / Anhydrous States:

  • Dry $\text{HCl}$ gas and liquefied $\text{HCl}$ do not contain free hydronium ions ($H_3O^+$).

  • They do not turn dry blue litmus paper red and do not conduct electricity, confirming their purely covalent nature.

  • A solution of $\text{HCl}$ gas dissolved in toluene (an organic non-polar solvent) does not ionize into $H_3O^+$; hence, it has no effect on blue litmus and is a non-conductor of electricity.

• Laboratory Preparation Method:

  • Prepared by dissolving $\text{HCl}$ gas in water until saturation.

  • Concentrated hydrochloric acid contains roughly $36\,\%$ $\text{HCl}$ by mass.

• Funnel Arrangement Mechanism:

  • Direct Absorption Hazard: Direct absorption of $\text{HCl}$ gas via a narrow tube causes rapid dissolution faster than the gas is generated. Pressure in the delivery tube drops rapidly, creating a vacuum that sucks water back up into the hot generating flask (back-suction). This causes reaction stoppage and explosive risks due to water mixing with hot concentrated $\text{H}_2\text{SO}_4$

  • Funnel Arrangement Setup: An inverted funnel is connected to the gas supply line and placed so its rim barely touches the surface of the water in a trough.

  • Prevention of Back-Suction:

  • As water rises inside the funnel due to initial absorption, the liquid level outside the funnel falls.

  • This drop creates an air gap between the rim of the funnel and the water surface.

  • Atmospheric pressure enters through the air gap, equalizing internal and external pressure and causing the water column inside the funnel to collapse back into the trough.

  • This cyclic mechanism continues automatically until saturation is reached.

  • Benefits of Funnel Arrangement:

  • Effectively prevents or minimizes back-suction.

  • Provides a significantly larger surface area for gas absorption.

• Anti-Suction Safety Device: An empty safety flask is placed between the generating flask and the water trough. Any accidental back-suction delivers liquid into this empty vessel, preventing contact with hot acid in the generator.

Properties and Azeotropic Behavior of Hydrochloric Acid

• Distillation and Constant Boiling Mixture (Azeotrope):

  • Distilling dilute hydrochloric acid causes water loss until the concentration reaches $22.2\,\%$ $\text{HCl}$ by weight ($77.8\,\%$ water by weight).

  • At this specific composition, the solution boils at a constant temperature of $110\,^\circ\text{C}$ without any further change in concentration.

  • Azeotrope Definition: A constant boiling mixture is a liquid solution that boils at a fixed temperature without undergoing any alteration in its chemical composition, as the vapor phase retains the identical ratio of components as the liquid phase.

• Physical Properties of the Acid:

  • Appearance: Clear, colourless liquid.

  • Odor: Pungent, choking odor.

  • Taste: Sour.

  • Skin Contact: Concentrated acid is highly corrosive and causes severe skin blisters.

  • Solubility: Miscible in water in all proportions.

  • Boiling Point: $110\,^\circ\text{C}$ (volatile acid).

• Action on Indicators:

  • Moist Litmus: Blue turns to Red.

  • Methyl Orange: Orange turns to Pink.

  • Phenolphthalein: Remains Colourless.

Chemical Reactions and Decomposition Behavior of Hydrochloric Acid

• Reactions with Active Metals:

  • Reacts with metals above hydrogen in the activity series to yield metal chlorides and hydrogen gas:     $\text{Metal} + \text{Hydrochloric acid} \rightarrow \text{Metallic chloride} + \text{Hydrogen}\uparrow$

  • Magnesium:     $Mg + 2HCl \rightarrow MgCl_2 + H_2\uparrow$

  • Zinc:     $Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$

  • Iron:     $Fe + 2HCl \rightarrow FeCl_2 + H_2\uparrow$

• Reactions with Bases (Oxides and Hydroxides):

  • Neutralization reactions producing metallic salt and water only:     $\text{Base} + \text{Hydrochloric acid} \rightarrow \text{Metallic chloride} + \text{Water}$

  • Magnesium Oxide:     $MgO + 2HCl \rightarrow MgCl_2 + H_2O$

  • Zinc Oxide:     $ZnO + 2HCl \rightarrow ZnCl_2 + H_2O$

  • Ferrous Oxide:     $FeO + 2HCl \rightarrow FeCl_2 + H_2O$

  • Ferric Oxide:     $Fe_2O_3 + 6HCl \rightarrow 2FeCl_3 + 3H_2O$

  • Copper(II) Oxide:     $CuO + 2HCl \rightarrow CuCl_2 + H_2O$

  • Sodium Hydroxide:     $NaOH + HCl \rightarrow NaCl + H_2O$

  • Calcium Hydroxide:     $Ca(OH)_2 + 2HCl \rightarrow CaCl_2 + 2H_2O$

  • Zinc Hydroxide:     $Zn(OH)_2 + 2HCl \rightarrow ZnCl_2 + 2H_2O$

  • Copper(II) Hydroxide:     $Cu(OH)_2 + 2HCl \rightarrow CuCl_2 + 2H_2O$

• Decomposition of Salts of Weaker Acids:

  • Carbonates and Hydrogen Carbonates:

  • Carbonates:       $\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow$       $\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2\uparrow$

  • Hydrogen Carbonates:       $\text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow$       $\text{Ca(HCO}_3)_2 + 2\text{HCl} \rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O} + 2\text{CO}_2\uparrow$

  • Sulphites and Hydrogen Sulphites:

  • Sulphites:       $\text{Na}_2\text{SO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{SO}_2\uparrow$       $\text{K}_2\text{SO}_3 + 2\text{HCl} \rightarrow 2\text{KCl} + \text{H}_2\text{O} + \text{SO}_2\uparrow$       $\text{MgSO}_3 + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O} + \text{SO}_2\uparrow$

  • Hydrogen Sulphites:       $\text{NaHSO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{SO}_2\uparrow$

  • Sulphides:

  • Yields metal chlorides and hydrogen sulphide gas ($\text{H}_2\text{S}$):       $\text{Na}_2\text{S} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{S}\uparrow$       $\text{FeS} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2\text{S}\uparrow$

  • Thiosulphates:

  • Dilute hydrochloric acid decomposes thiosulphates to give sulphur dioxide gas and a yellow precipitate of sulphur:       $\text{Na}_2\text{S}_2\text{O}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{SO}_2\uparrow + S\downarrow$

  • Diagnostic Distinction: This reaction distinguishes thiosulphates from sulphites, as sulphites do not precipitate free yellow sulphur when treated with dilute $\text{HCl}$.

• Precipitation Reactions:

  • Direct reaction with select nitrate solutions to form insoluble precipitate chlorides:

  • Lead Nitrate:       $Pb(NO_3)_2 + 2HCl \rightarrow PbCl_2\downarrow + 2HNO_3$       (The white precipitate of lead chloride dissolves upon heating).

  • Mercury(I) Nitrate:       $Hg_2(NO_3)_2 + 2HCl \rightarrow Hg_2Cl_2\downarrow + 2HNO_3$

  • Silver Nitrate Reaction:

  • Forms a thick curdy white precipitate of silver chloride:       $AgNO_3 + HCl \rightarrow AgCl\downarrow + HNO_3$

  • Solubility Profile of $\text{AgCl}$ Precipitate:

    • Insoluble in nitric acid ($\text{HNO}_3$).

    • Readily soluble in ammonium hydroxide ($\text{NH}_4\text{OH}$), forming a soluble complex salt, diammine silver(I) chloride:         $AgCl + 2NH_4OH \rightarrow [Ag(NH_3)_2]Cl + 2H_2O$

  • Re-precipitation: Adding dilute nitric acid to clear diammine silver(I) chloride solution destroys the complex and re-precipitates white silver chloride:       $[Ag(NH_3)_2]Cl(aq) + \text{Nitric acid} \rightarrow \text{White precipitate of } AgCl\downarrow$

  • Photolytic Sensitivity: Exposure to light causes $\text{AgCl}$ to darken/blacken due to decomposition into metallic silver (fine black powder) and chlorine gas:       $2AgCl \xrightarrow{\text{Light}} 2Ag + Cl_2$

Oxidation Reactions and Aqua Regia

• Oxidation of Concentrated Hydrochloric Acid:

  • Concentrated $\text{HCl}$ is oxidized to chlorine gas by strong oxidizing agents:     $\text{Oxidizing agent} + \text{Concentrated HCl} \rightarrow \text{Metallic chloride} + \text{Water} + \text{Chlorine}$

  • Manganese Dioxide ($\text{MnO}_2$):     $MnO_2 + 4HCl(\text{conc.}) \rightarrow MnCl_2 + 2H_2O + Cl_2\uparrow$

  • Lead Dioxide ($\text{PbO}_2$):     $PbO_2 + 4HCl(\text{conc.}) \rightarrow PbCl_2 + 2H_2O + Cl_2\uparrow$

  • Red Lead ($\text{Pb}_3\text{O}_4$):     $Pb_3O_4 + 8HCl(\text{conc.}) \rightarrow 3PbCl_2 + 4H_2O + Cl_2\uparrow$

  • Bleaching Powder ($\text{CaOCl}_2$):     $CaOCl_2 + 2HCl(\text{dil.}) \rightarrow CaCl_2 + H_2O + Cl_2\uparrow$

  • Potassium Permanganate ($\text{KMnO}_4$):     $2KMnO_4 + 16HCl \rightarrow 2MnCl_2 + 2KCl + 8H_2O + 5Cl_2\uparrow$

• Aqua-Regia:

  • Composition: A fresh mixture containing $3$ parts concentrated hydrochloric acid and $1$ part concentrated nitric acid by volume:     $3HCl(\text{conc.}) + HNO_3(\text{conc.}) \rightarrow NOCl + 2H_2O + 2[Cl]$

  • Products include nitrosyl chloride ($\text{NOCl}$), water, and nascent chlorine ($[Cl]$).

  • Action on Noble Metals: Nascent chlorine reacts with noble metals (gold and platinum) to produce soluble chlorides:

  • Gold:       $Au + 3[Cl] \rightarrow AuCl_3 \quad \text{(Gold(III) chloride)}$

  • Platinum:       $Pt + 4[Cl] \rightarrow PtCl_4 \quad \text{(Platinum(IV) chloride)}$

Applications and Industrial Uses

• General Laboratory Uses: Essential analytical reagent and primary reactant in preparing aqua regia.

• Industrial Applications:

  • Chemical Manufacturing: Production of chlorine gas and inorganic chlorides like ammonium chloride ($\text{NH}_4\text{Cl}$), which is widely used in dry cells.

  • Synthetic Dyes & Pharmaceuticals: Manufacturing dyes, drugs, paints, and photographic chemicals (e.g., silver chloride).

  • Food Industry: Hydrolysis of starch to produce glucose.

  • Pickling Steel: Removes oxide scale/coatings from steel surfaces prior to tin or chromium plating, galvanizing, or painting (used alongside inhibitors).

  • Bone Refining: Purification of bone black by dissolving insoluble calcium phosphate present in raw bones.

  • Rust Removal: Descaling rust from iron plates and sheets.

  • Metallurgical Preparation: Surface cleaning before soldering, electroplating, and galvanizing.

  • Glue Extraction: Extraction of glue from animal bone tissue.

  • Textiles and Leather: Tanning leather and calico printing.

• Medicinal Uses:

  • Administered as dilute hydrochloric acid to treat patients suffering from achlorhydria or deficient gastric juice secretion.

  • Kills ingested microorganisms and pathogens entering the alimentary canal with food.

Analytical Tests for Identification

• Organoleptic Identification: Recognized by its unique, suffocating, pungent odor.

• Ammonia Vapour Test: A glass rod dipped in liquid ammonia solution held near the gas/acid vapors produces dense white fumes of ammonium chloride:   $NH_3 + HCl \rightarrow NH_4Cl$

• Silver Nitrate Test: Addition of $\text{AgNO}_3$ solution to $\text{HCl}$ gas or hydrochloric acid gives a curdy white precipitate ($\text{AgCl}$) that is insoluble in $\text{HNO}_3$ but soluble in excess $\text{NH}_4\text{OH}$.

• Oxidizing Agent / Chlorine Evolution Test: Heating concentrated $\text{HCl}$ with manganese dioxide ($\text{MnO}_2$) liberates greenish-yellow chlorine gas:   $MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2\uparrow$   The evolved chlorine turns moist starch-iodide paper dark blue-black.