Acids: Concept, properties and uses

Acid is a water-soluble compounds having a sour taste and capable of turning litmus red and reacting with a base to form a salt. A corrosive liquid with a pH less than 7; a solution of in water of a substance that releases hydrogen ions.

In other word, it is a compound which contains replaceable hydrogen atom or atoms, a part or whole of which can be replaced by means of a metal or a positive radical. For Example: HCl, HNO₃, H₂SO₄ etc are acids because they contain replaceable hydrogen atom or atoms.

Theory related to the Acids

Arrhenius’s Ionic theory:  This theory was introduced in 1887 by the Swedish scientist Svante Arrhenius, that acids are substances that dissociate in water to yield electrically charged atoms or molecules, called ions, one of which is a hydrogen ion (H+), and that bases ionize in water to yield hydroxide ions (OH⁻). It is now known that the hydrogen ion cannot exist alone in water solution; rather, it exists in a combined state with a water molecule, as the hydronium ion (H₃O+). In practice the hydronium ion is still customarily referred to as the hydrogen ion.

Bronsted and Lowry theory: This theory is also called proton theory of acids and bases, a theory, introduced independently in 1923 by the Danish chemist Johannes Nicolaus Bronsted and the English chemist Thomas Martin Lowry, stating that any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge; it is represented by the symbol H+ because it constitutes the nucleus of a hydrogen atom.

Lewis’s electronic theory: In 1923 G. N. Lewis gave this theory in which he suggested that the reaction between H⁺ and OH^- ions. In the Brnsted model, the OH^- ion is the active species in this reaction   it accepts an H⁺ ion to form a covalent bond. In the Lewis model, the H⁺ ion is the active species it accepts a pair of electrons from the OH^- ion to form a covalent bond.

In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H⁺ ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor. A Lewis base is any substance, such as the OH^- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor.

Characteristics of Acids

• They taste sour.
• They turn blue litmus and methyl orange.
• They react with base and alkali to form salt and water.
• Strong acids like HCl, HNO₃, and H₂SO₄ etc. are good conductor of electricity in their aqueous solution.

Types of Acids

• Oxy Acids: These types of acids contain both hydrogen and oxygen. For example: HNO₃, H₂SO₄ etc.
• Hydra Acids: In this acid, only hydrogen is presents and oxygen is absent. For example:  HCl, HBr, HCN etc.

Uses of Acids

• Steel used in construction is treated with acid before being painted. Dilute sulphuric or hydrochloric acid will remove any surface rust which would otherwise spread under the painted surface. Rust remover which is used to repair cars is dilute phosphoric acid - H₃PO₄.
• Acids are used to make fertilizers.
• Lime scale removers contain weak acids. Lime scale is the name given to the calcium carbonate that forms in kettles and pipes. The formation of lime scale is also called furring. To remove lime scale you can try using lemon juice (citric acid) or vinegar (ethanoic acid).
• Baking powder used in cooking contains tartaric acid.
• It is used in petroleum exploration, in the preparation of various types’ explosives, colours, and medicines and in the construction of accumulated batteries.

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