Hydrogen Chloride

Glauber first prepared hydrogen chloride in 1648 by heating sodium chloride with concentrated sulfuric acid. In 1810, Davy clarified that HCl is a compound consisting of hydrogen and chlorine. Hydrochloric acid is commonly known as muriatic acid.

1.0Introduction

Hydrogen chloride, a hydrogen halide with the chemical formula HCl, is a colourless gas at room temperature. When exposed to air, it emits white fumes of hydrochloric acid due to its reaction with water vapour.

Both hydrogen chloride gas and its aqueous solution, hydrochloric acid, are essential in various technological and industrial applications. Hydrochloric acid, commonly called HCl, is the liquid form of hydrogen chloride dissolved in water. When moisture is present, chlorine bleaches vegetable or organic matter. The bleaching effect of chlorine is permanent. 

2.0Structure of HCl

In hydrogen chloride (HCl), the hydrogen atom loses its only electron, becoming a proton (H⁺), while the chlorine atom gains this electron, forming a chloride ion (Cl⁻). This initially creates an electrostatic attraction between the ions, forming an ionic bond. 

However, because the proton is tiny, it distorts the electron cloud of the chloride ion. This distortion causes the bond to shift from purely ionic to polar covalent bonds, where the electrons are shared but unevenly, with chlorine holding them more tightly.

3.0Laboratory Preparation of Hydrogen Chloride (HCl)

Hydrogen chloride gas is typically prepared by heating sodium chloride (NaCl) with concentrated sulfuric acid (H₂SO₄). The process involves two main steps:

• First Reaction (at 420K): When sodium chloride is heated with concentrated sulfuric acid at around 420K, hydrogen chloride gas is produced, along with sodium bisulfate (NaHSO₄):

NaCl+H₂SO₄→NaHSO₄+HCl

In this reaction, sodium chloride reacts with sulfuric acid to produce sodium bisulfate and hydrogen chloride gas.

• Second Reaction (at 823 K): To generate more hydrogen chloride gas, the sodium bisulfate (NaHSO₄) produced in the first reaction is further heated with additional sodium chloride at around 823K, resulting in the formation of sodium sulfate (Na₂SO₄) and additional hydrogen chloride gas:

NaHSO₄+NaCl→Na₂SO₄+HCl

Drying of Hydrogen Chloride Gas

The hydrogen chloride gas produced in these reactions can contain moisture. To obtain dry HCl gas, it is passed through concentrated sulfuric acid, a drying agent. Concentrated sulfuric acid removes water vapour from the gas, producing anhydrous hydrogen chloride.

This dry HCl gas can then be collected and used for laboratory and industrial purposes.

4.0Organic Synthesis of Hydrogen Chloride

In industry, hydrogen chloride is often produced alongside the creation of chlorinated and fluorinated chemicals like Freon, Teflon, and PVC.

• Chlorination Process: Hydrogen atoms in hydrocarbons are replaced by chlorine, producing hydrogen chloride as a byproduct:

R-H + Cl_2​  →  R-Cl  +  HCl

• Fluorination Process: Chlorinated compounds can react with hydrogen fluoride (HF) to replace chlorine with fluorine, generating more hydrogen chloride:

R-Cl + HF → R-F + HCl

This hydrogen chloride is then dissolved in water to make hydrochloric acid or reused in other industrial processes.

5.0Physical Properties of Hydrochloric Acid

• Hydrogen chloride is a colourless gas with a strong, pungent odour.
• It has a noticeable smell in damp air due to its high solubility in water.
• In dilute solutions, it fully dissociates, making hydrochloric acid a strong acid.
• Hydrochloric acid is a hydrogen chloride (HCl) solution dissolved in water.
• Hydrogen chloride gas becomes a colourless liquid when cooled to 189K.
• It freezes into a white solid at 159K.
• Hydrochloric acid does not corrode glass.
• It is highly corrosive to platinum, gold, silver, mercury, tantalum, etc.

6.0Chemical Properties and Reactions of Hydrochloric Acid

1. Solubility and Ionization: Hydrogen chloride (HCl) is highly soluble in water and ionises as follows:

• HCl (g) + H2​O (l)→   H_3​O+(aq )+ Cl⁻(aq)

The aqueous solution formed is known as hydrochloric acid. The high dissociation constant (Ka) indicates that HCl is a strong acid in water.

2. Reactions with Ammonia: HCl reacts with ammonia (NH₃) to form white fumes of ammonium chloride (NH₄Cl):

• NH₃​+HCl→NH_4​Cl

3. Formation of Aqua Regia: When three parts of concentrated HCl and one part of concentrated nitric acid (HNO₃) are mixed, they form aqua regia, which can dissolve noble metals like gold and platinum:

• Au+4HNO₃​+4HCl→AuCl4−​+NO+2H_2​O 
• 3Pt+16HCl+4HNO₃​→3PtCl₆−​+4NO+8H₂​O

4. Decomposition of Salts: Hydrochloric acid decomposes salts of weaker acids such as carbonates, bicarbonates, and sulfites:

• Na_2​CO₃ ​+ 2HCl → 2NaCl + H_2​O + CO_2​ 
• NaHCO₃​ + HCl → NaCl + H₂​O + CO_2​ 
• Na_2​SO₃​ + 2HCl → 2NaCl + H₂​O + SO_2​

7.0Uses of Hydrochloric Acid

Hydrochloric acid has various applications:

1. Chemical Production:

• Used to produce chlorine gas and other chlorides.
• It is essential to make aqua regia to purify noble metals like gold and platinum.

2. Industrial Uses:

• Employed in the tanning industry.
• Used in dyeing and calico printing.
• Utilised in pickling before galvanising, electroplating, and tinning.

3. Laboratory and Medical Applications:

• One of the essential reagents in laboratories.
• Used in several medicines.

4. Starch Hydrolysis: Hydrolyses starch to produce glucose.
5. Hydrochloric Acid Production: Most hydrogen chloride globally is used to manufacture hydrochloric acid.