Trends in the Acidic Nature of Hydrogen Halides

A dedicated preparation of the 75 questions on Chemistry in the next IIT-JEE exam can make or break your rank. Unacademy brings the best-detailed topics on different chemistry topics to make it fun and exciting to learn. So, today the Unacademy team brings one of the important concepts for IIT-JEE aspirants, i.e., hydrogen halides.

Halides are compounds that contain halogens. Since halogens are highly electronegative elements, these can survive alone under natural surroundings. While halides are found in animals, plants, and minerals, a couple of salts and acids containing halides are crucial for human life. So, let us go into detail about their occurrence, synthesis, and physical properties.

About Hydrogen Halides and its Main Characteristics:

Hydrogen halides are known as hydrohalic acids when present in aqueous forms. These are diatomic and inorganic compounds that function as Arrhenius acids. It goes with the HX formula as X is one of the halogens like bromine, chlorine, fluorine, astatine, iodine, etc. All these compounds are in gaseous form at standard pressure and temperature. A quick look at the main characteristics of these compounds is:

Properties:

The hydrogen halides do not tend to ionize the gas phase and are diatomic molecules. Hence, it distinguishes these halides from the hydrochloric acids. For example, hydrogen chloride is a gas at room temperature and reacts with water to form reactive acids. The normal distillation can’t regenerate the diatomic molecules of these acids. While working in labs, the names of acids and molecules are not distinguished for effective working. For example, HCl is termed hydrochloric acid but not gaseous hydrogen chloride.

Process of synthesis:

Hydrogen fluoride and hydrogen chloride can be obtained by direct reaction of hydrogen with fluorine and chlorine. When required on an industrial scale, it is produced by treating the halide salts with sulfuric acids. Hydrogen and bromine combine at high temperatures in the presence of the platinum catalysts to form hydrogen bromide. To produce HI, the reaction of iodine needs to be carried out with hydrazine or hydrogen sulfide.

Trends in Acidic nature:

The boiling point of hydrogen halide and melting points are high due to the attractive forces among HX molecules. Hydrogen fluoride has the highest melting and boiling points due to hydrogen bonding. The boiling point of these halides made using hydrogen rises from HCl to HI. The increasing strengths of the intermolecular Van der Waals forces correlate with the increasing number of electrons in the structure and hence, cause a rise in boiling points. The concentrated hydrohalic acid produces white fumes.

The acidic strength of hydrogen halide increases in the order of HF< HCl< HBr< HI. All these acids are hydrogen halides dissolved in water and the H-A bond is the main reason for their strength. This bond strength helps determine the acidity of the acid and the polar nature of the acidic bonds. 

Some of the common reactions of hydrogen halides include:

• Production of chloroethane by hydrochlorination of ethylene:

C2H4+ HCl   →   CH3CH2Cl

• Production of ammonium halides with the reaction of hydrogen halides and ammonia:

HX+ NH3       →     NH4X

The acidity of hydrogen halides:

• Hydrofluoric acid:

Hydrogen fluoride dissolves freely in water but the respective acid, i.e., hydrofluoric acid is a weak acid. The bond enthalpy of the H-F bond is high at 562 kJ/mol. Hence, a large amount of energy is required to break this bond which is not possible at room temperatures. 

• Hydrobromic acid and hydroiodic acid:

Hydrogen bromide and hydrogen iodide dissolve completely in water and form hydrobromic acid and hydriodic acid.  Both these are strong acids. The bond enthalpy for H-Br is +366 kJ/mol while the same for H-I is +299 kJ/mol.

• Hydrogen chloride:

Hydrogen chloride is considered an acid as it can transfer protons to other elements. Hydrogen chloride gas dissolves in water to form hydrochloric acid. Here, a proton from hydrogen chloride is donated to the lone pairs on the water molecule. Hence, a dative covalent bond is formed between transferred protons and oxygen. The chemical reaction is as follows:

H2O+ HCl → H3O+ + Cl-

H3O+ is the hydronium ion or oxonium ion and is also known as hydronium ion. While this is the normal form of protons present in water, it is termed as H+ (aq) only. Since hydrochloric acid is formed by dissolving hydrogen chloride in water, it is fully ionized in solution. Thus, hydrochloric acid is a strong acid and the H-Cl bond enthalpy is +562 KJ/mol.

Main factors determining the acidity of hydrogen halides:

The main factors determining the hydrogen halide acidity can be evaluated with the example of weak hydrofluoric acid. It is a weak acid as its entropy decreases when the hydrogen fluoride reacts with water. This is significant in hydrogen fluoride due to the attraction of the small fluoride ions and nearby hydronium ions. The small fluoride ions are present in significant amounts that put stress on the surrounding water molecules.

Conclusion:

Halides containing hydrogen are colorless at room temperatures and release great energies when dissolved in water. Hydrogen fluoride is the exception that boils at 19 degrees Celsius. The acids thus formed are strong and the acidic strength of hydrogen halide increases down the group. Hydrofluoric acid is an exception to this trend as its strength depends on concentration and homoconjugation. 

The white fumes are visible from the hydrohalic acid solutions. This is due to the tiny droplets of concentrated aqueous solutions. While the reactions of hydrogen with fluorine and chlorine give respective halides, HI is produced least by direct methods due to the least stability. The reaction of iodine with hydrogen sulfide or with hydrazine produces this halide. Hydrogen chloride is a major part of gastric acid while hydrogen fluoride, chloride, and bromine are volcanic gasses.