By Kingsley Idiagbor, B.Sc (Hons), PGDCs, NCE, MNSM
WHAT IS AN ACID?
Acid is any substance which dissolves in water to produce hydroxonium ion as the only positive ions.
The general molecular formula of acids is HA where H is functional hydrogen atom while A is known as acid radical. When the acid is dissolved in water, the functional hydrogen atom, H, combines with the water molecule to form the hydroxonium ion leaving the ion of the acid radical, A- behind. Thus: HA + H2O ------------>H3O++ A-
CLASSES OF ACIDS There are two major categories of acids namely organic or natural acids and mineral or artificial acids. Organic acids occur in abundance in nature. Examples include lactic acid (found in milk);citric acid (in oranges);formic acid (in ants) and acetic or ethanoic acid (in vinegar). Mineral acids on the other hand, can be prepared in the laboratories and industries from mineral elements or inorganic matter. Examples include: hydrochloric acid, HCl; trioxonitrate (V) acid, HNO3and tetraoxosulphate (VI) acid, H2SO4.
WHAT IS THE BASICITY OF AN ACID? By basicity of an acid, we meanthe number of replaceable hydrogen atoms contained in that acid molecule. Therefore, the basicity of HCl and HNO3is one because there is a single hydrogen atom in each of the molecules to be replaced. HCl and HNO3are said to bemonobasic. H2SO4and H2C2O4(oxalic acid) aredibasicsince there are two molecules of hydrogen that are replaceable in both acids. Acids like H3PO4which have more than two replaceable hydrogen molecules are said to bepolybasic.
Note however, that not all the hydrogen atoms in an acid molecule is replaceable by a metal.For example, ethanoic acid has four hydrogen atoms. Only one of them is replaceable by a metal. CH3COOH (Ethanoic acid) therefore ismonobasic
Properties of Acid Physical
Acids turn blue litmus paper red;
Dilute forms of acid have sour taste;
Concentrated acids are corrosive and can cause serious burns
Chemical
Reaction with metals: Metals found above hydrogen in the activity series react with acids to liberate hydrogen gas. 2HCl + Zn------->ZnCl2+ H2
Reaction with bases: Acid reacts with base to form salt and water only. This is termedneutralizationreaction. NaOH + HCl ------>NaCl + H2O Base + Acid------->Salt + Water
Reaction with trioxocarbonates: Carbon(IV)oxide gas,CO2 is evolved when acids react with trioxocarbonates (IV) 2HCl + Na2CO3----->2NaCl + H2O + CO2
General Preparation methods for Acids The following is an outline of the general methods for preparing acids:
Dissolution of acid anhydride in water 2NO2+ H2O----->HNO2+ HNO3
Direct combination of constituent elements H2+ Cl2------>2HCl
Displacement of weak acid or a volatile acid from its salt: This is a method in which a strong acid is employed in displacing a more volatile acid such as trioxonitrate(V)acid from one of its salts.Thus: 2NaNO3+ H2SO4----->Na2SO4+ 2HNO3
USES OF ACIDS Most acids are heavy chemicals in that they are needed in large quantities in the industries and for varied uses. For example, they can be used for the manufacture of the following:
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