ACIDS
The word “acid” comes from the Latin word “acidus” which means sour.
Various theories are proposed to explain concept of Acids and Bases. One of old theory proposed by Arrhenius , known as Arrhenius theory define; an acid is a substance which ionizes and gives hydrogen ions (H+) when dissolved in water.
Example: Hydrochloric acid in water releases H+ ions.
HCl + H2O → H+ (aq) + Cl–(aq)
similarly H2SO4 (aq)→ 2H+ (aq) + SO42-(aq)
Note : A hydrogen ion (H+) cannot exist on its own, so it combines with a water molecule to form a hydronium ion (H3O+).
H+ + H2O → H3O+
Example: Hydrochloric acid when dissolved in water liberates a hydrogen ion and a chloride ion. The hydrogen ion combines with water to form a hydronium ion.
HCl + H2O → H3O+ (aq) + Cl–(aq)
Classification of acids:
I : Classification of acids based on source:
Based on the source the acids were classified into two types.
1. Organic acids 2. Inorganic acids
Organic acids:
Acids obtained from organic compounds like curd, lemons, grapes, raw mango, citrus fruits and gooseberry are called organic acids. All organic acids contain carbon and oxygen.
Inorganic acids:
Acids which are synthesized in the laboratory by non-carbonic compounds are called Inorganic acids or mineral acids. They do not contain Carbon but contain other non metals like S, Cl, Br etc.
Following table is the list of some acids which are used in the laboratory.
| Inorganic acid | Chemical formula | Organic Acids | Chemical Formula |
| Sulphuric acid | H2SO4 | Acetic acid | CH3COOH |
| Nitric acid | HNO3 | Benzoic Acid | C6H5COOH |
| Hydrochloric acid | HCl | Formic Acid | HCOOH |
| Perchloric acid | HClO4 | Propanoic Acid | CH3CH2COOH |
II : Classification of acids based on concentration:
The word concentration indicates the quantity of acid in relative to the quinatity of water in the aqueous solution of that acid.
- Concentrated (Conc) acid solution : concentrated acid contains high percentage of acid than water in the solution.
- Dilute (Dil) acid solution : It contains low percentage of acid than water in the solution.
For example , 0.1 molar HCl is dilute acid solution while 1 Molar HCl acid solution in concentrated solution.
Dilution of acid:
Dilution means addition of water. Mixing an acid with water reduces the concentration of hydronium ions (H3O+) of the acid per unit volume. The dilution is exothermic process.
III : Classification of acids based on strength:
Based on ionisation, the acids were classified into strong acids and weak acids.
Strong acids: Acids which ionizes completely into ions are called strong acids.
Example: HCl, H2SO4, HNO3 etc
Weak acids: Acids which ionizes partially into its ions are called weak acids.
Example: CH3COOH, H2CO3 , all organic acids are weak acids.
IV : Classification of acids based on basicity of acids:
Basicity of acid is number of removable H+ ions by one molecule.
Based on basicity acids were classified into different types. They are
- Mono-basic acids
- Di-basic acids
- Tri-basic acids
Mono-basic acids:
Acids which on ionisation produces one hydronium ion in water are termed as mono-basic acids.
Example: HCl, HBr, HNO3
Di-basic acids:
Acids which on ionisation produces two hydronium ions are called as di-basic acids.
Example: H2SO4, H2CO3 etc
Tri-basic acids:
Acids which on ionisation produces three hydronium ions are called astri-basic acids.
Example: H3PO4
Properties of acids:
- Acids have corrosive action on skin.
- They are good conductors of electricity.
- Acids neutralizes bases to form salt and water.
- They are sour in taste. (But do not try to taste acids in Lab)
Chemical properties of acids
1. Reaction of acids with active metals:
Acids reacts with active metals to form metal salts. In this reaction, hydrogen gas is liberated. Active metals are more reactive metals like Sodium, Potassium, Zinc, Calcium, Magnesium etc.
Example: In the reaction of hydrochloric acid reacts with zinc produces hydrogen gas and zinc chloride.
2HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g)
2. Reaction of acids with metal carbonates:
Acids reacts with metal carbonates to form corresponding salts, carbon dioxide and water.
Example: Hydrochloric acid on reaction with sodium carbonate forms sodium chloride, carbon dioxide and water.
2HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + CO2 (g) + H2O (l)
3. Reaction of acids with metal hydrogen carbonates:
Acids reacts with metal hydrogen carbonates and form corresponding salts, carbon dioxide and water.
Example: Hydrochloric acid on reaction with sodium bicarbonate forms sodium chloride, carbon dioxide and water.
HCl (aq) + NaHCO3 (aq) → NaCl (aq) + CO2 (g)+ H2O (l)
4. Reaction of acids with metal oxides:
Acids reacts with metal oxide to form salt and water.
Example:
Sulphuric acid on reaction with cupric oxide forms copper sulphate and water.
CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)
Indicators:
An acid base indicator is a substance which exhibits different colour in acids and bases.
Red cabbage is a visual indicator used to detect acids.
Onions are called olfactory indicators. They change their odour with change in the nature of solution.
Litmus is a natural indicator and is extracted from lichens.
Apart from natural indicators there are a few synthetic indicators, such as methyl orange and phenolphthalein.
Following table gives colours of the indicators in presence of acids and bases.
| Indicator | Acid | Base |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
| Blue litmus paper | Red colour | No Change |
| Red litmus paper | No change | Blue colour |
Universal indicator is a mixture of different number of indicators which shows different colours in different solutions.
Uses of acids:
| Sulphuric acid (H2SO4) |
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| Nitric acid (HNO3) |
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| Hydrochloric acid (HCl) |
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| Ethanoic acid (CH3COOH) |
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| Boric acid (H3BO3) |
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| Carbonic acid (H2CO3) |
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