Difference Between Hydrogen Chloride and Hydrochloric Acid

Main Difference – Hydrogen Chloride vs Hydrochloric Acid

Hydrogen chloride and hydrochloric acid are two terms used to name chemical compounds having the same chemical formula: HCl. Hydrogen chloride is the name of HCl compound that can be in any phase of matter: solid, liquid or gas. But at room temperature, it is a colorless gas. Hydrochloric acid is the aqueous hydrogen chloride solution that has acidic properties. Therefore, the main difference between hydrogen chloride and hydrochloric acid is that hydrogen chloride is a colorless gas at room temperature whereas hydrochloric acid is a solution.

Key Areas Covered

1. What is Hydrogen Chloride
      – Definition, Chemical Structure, and Properties
2. What is Hydrochloric Acid
     – Definition, Chemical Properties, and Reactions
3. What is the Difference Between Hydrogen Chloride and Hydrochloric Acid
     – Comparison of Key Differences

Key Terms: Acid, Chlorane, Cubic, Hydrochloric Acid, Hydrogen Chloride, Orthorhombic, Phase Transition, Polar Covalent Bond

What is Hydrogen Chloride

Hydrogen chloride is a chemical compound having the chemical formula HCl. It is a hydrogen halide. Hydrogen chloride is a gas at room temperature and pressure. This gas has a pungent, sharp odor. It forms white colored fumes upon contact with atmospheric water vapor.

Figure 1: Hydrogen Chloride is a Polar Molecule

The melting point of hydrogen chloride is −114.22 °C and the boiling point is −85.05 °C. Hydrogen chloride is a diatomic molecule; hydrogen atom and chlorine atom are bonded to each other via a covalent bond. The bond between the two atoms is a polar covalent bond. Since the chlorine atom is more electronegative than the hydrogen atom, the chlorine atom attracts electrons more than the hydrogen atom, making the bond polar.

Due to its high polarity, hydrogen chloride molecules are well-soluble in water. When hydrogen chloride dissolves in water, it forms hydrochloric acid. Hydrogen chloride is also soluble in other polar solvents. Frozen HCl molecules undergo phase transition at 98.4K temperature. The transition is from orthorhombic to cubic structure (face-centered).

What is Hydrochloric Acid

Hydrochloric acid is a strong acid with the chemical formula HCl. It is very corrosive in its concentrated form. Hydrochloric acid is a colorless solution prepared by dissolving hydrogen chloride (HCl) in water. The molar mass of Hydrochloric Acid is about 36.5 g/mol. The IUPAC name of hydrochloric acid is chlorane.

Hydrochloric Acid is a monoprotic acid. This means hydrochloric acid releases one proton (H⁺) per molecule in its aqueous solution. Therefore, hydrochloric acid fully dissociates in water. Thus, it has a high value for acid dissociation constant (K_a).

Hydrochloric acid has many applications in laboratory scale and industrial scale. One such industrial-scale application is refining metals. This acid is used in refining metals because most of the metals are easily dissolved in it.

Figure 2: Hydrochloric Acid is a Strong Acid. It Can Turn Blue Litmus Red.

Another important application of hydrochloric acid is pickling of steel, that is, the removal of rust (iron oxide) from iron or steel. The reaction that occurs here is given below.

Fe₂O₃    +    Fe     +    6HCl    →    3FeCl₃ +     3H₂O

In addition, hydrochloric acid is a mild reducing agent. It undergoes redox reactions with strong oxidizing agents such as MnO₂.

MnO_2(aq)     +       HCl_(aq)      →    MnCl_2(aq)   +       Cl_2(g) +       H₂O_(l)

Similarities between Hydrogen Chloride and Hydrochloric Acid

• Both have the same chemical formula and same molar mass.
• Both are acidic compounds.
• Both are well-soluble in water.
• Both compounds form white fumes when exposed to atmospheric water vapor.

Difference Between Hydrogen Chloride and Hydrochloric Acid

Definition

Hydrogen Chloride: Hydrogen chloride is a chemical compound having the chemical formula HCl.

Hydrochloric Acid: Hydrochloric acid is a strong acid with the chemical formula HCl.

Nature

Hydrogen Chloride:  Hydrogen chloride is a hydrogen halide compound.

Hydrochloric Acid: Hydrochloric acid is an acidic solution.

Phase

Hydrogen Chloride:  Hydrogen chloride is a colorless gas at room temperature.

Hydrochloric Acid: Hydrochloric acid is an aqueous solution at room temperature.

IUPAC Name

Hydrogen Chloride:  The IUPAC name and the common name of hydrogen chloride is the same.

Hydrochloric Acid: The IUPAC name hydrochloric acid is chlorane.

Conclusion

Hydrogen chloride and hydrochloric acid are chemical compounds having HCl molecules. Basically, hydrochloric acid is an aqueous solution of hydrogen chloride. The main difference between hydrogen chloride and hydrochloric acid is that hydrogen chloride is a colorless gas at room temperature whereas hydrochloric acid is a solution.

Reference:

 1. “Hydrochloric acid.” Wikipedia, Wikimedia Foundation, 6 Jan. 2018, Available here.
2. Lazonby, John. “Hydrogen chloride.” The Essential Chemical Industry, Available here.

Image Courtesy:

1. “Dipolna molekula HCl” By Drago Karlo – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Hydrochloric acid 04” By Walkerma at en.wikipedia – Own work. Transferred from en.wikipedia to Commons by User:Logan using CommonsHelper (Public Domain) via Commons Wikimedia