ACIDS

A hydrogen-containing compound which gives hydrogen ions (H₃O⁺ or H⁺) when dissolved in water, is called an acid.

Acids can be classified into two groups:

❖ Hydracids: The compounds of hydrogen with highly electronegative

(nonmetallic) elements other than oxygen, are called hydracids.

Examples: HCl, H₂S, HBr, etc. are some typical hydracids.

❖ Oxoacids: The acids which contain an oxygen-containing anion (called

oxoanion), are called oxoacids (earlier called oxyacids). Thus, oxoacids are the

compounds which contain hydrogen, oxygen and another nonmetallic element.

Examples: H₂SO₄, HNO₃, H₃PO₄, H₂CO₃, etc. are some typical oxoacids.

Formula of Hydracids

The formula of a hydracid can be written as follows:

•   Write the symbol of hydrogen (H).
•   Write the symbol or formula of the anion alongwith its valency present in the acid on the right of H.
•   Criss-cross their valency numbers.
•   Divide these numbers (written as subscripts) by a common factor, if required.

For example, the formula of the hydrochloric acid can be written as follows:

Naming Hydracids

Hydracids are obtained by dissolving the corresponding gas in water. These gases are named as covalent compounds.

Name of the gaseous hydracid = Hydrogen + Name of the anion of the second                                               element

For example,

Name of HCl(g) = Hydrogen + Chloride → Hydrogen chloride

Hydracids are named by writing hydro followed by the root of the name of the

second element, then –ic, and finally acid with a space (blank) between ic and acid.

Hydro + Root of the second element + ic + acid

The only exception is HI (aq) in which case o in hydro is dropped while writing its

name.

Some typical hydracids

The gaseous HCl, HBr, H₂S, etc. do not bear the names of acids. They are named as covalent compounds. A compound that dissolves in water to form an acid is called an acid anhydride (acid without water). Only the aqueous solutions of acid anhydrides are named as acids. Therefore,

• HCl(g) is called hydrogen chloride while HCl(aq) is called hydrochloric acid
• HCN(g) is called hydrogen cyanide while HCN(aq) is called hydrocyanic acid.

The distinction is naming the anhydrides and the acids is not critical for oxoacids, because all their anhydrides are different molecules. For example, the anhydride of H₂SO₄ is SO₃, not gaseous H₂SO₄. Thus, H₂SO₄ is always called sulphuric acid, not hydrogen sulphate.