ACIDS

 

ACIDS

An acid is a substance that gives hydrogen ion H+or a hydronium ion H3O+when dissolved in water. A substance, which has an acidic nature, contains one or more hydrogen and an anionic group in its formula.

Example: H2SO4 (sulfuric acid) contains two hydrogens or hydrogen ion 2H+and sulfate anion 

When sulfuric acid dissolves in water it produces Hydrogen ion or hydronium ion which is shown below

The process by which acids dissolve in water to produce hydrogen ions, H+, with the corresponding negative ions is known as ionization.

The process by which acids dissolve in water to produce hydrogen ions, H+, with the

corresponding negative ions is known as ionization.

Sources of acid

Acidic plants ( lemon – oranges – tomatoes ) contain Citric acid and Ascorbic acid , Dairy products ( milk – cheese – yoghurt ) contain Lactic acid , Soft drinks contain Carbonic acid and Phosphoric acid .



Classification Of Acids According To Its Strength (Degree Of Ionization )

Strong acids

The acids which are completely ionized in the water , Their aqueous solution conducts the electric current to a large degree, They are considered as strong electrolytes .

Examples : Hydroiodic acid HI, Hydrochloric acid HCl, Hydrobromic acid HBr, Sulphuric acid H2SO4, Nitric acid HNO3 & Perchloric acid HClO4 .

Weak acids

The acids which are incompletely ionized in the water , Their aqueous solution conducts the electric current to a small degree, They are considered as weak electrolytes .

Examples : Carbonic acid H2CO3 , Phosphoric acid H3PO4 , Acetic acid ( vinegar ) CH3COOH , Formic acid , Citric acid , Oxalic acid & Lactic acid .

There is no relation between the strength of the acid and the number of the hydrogen atoms in its molecular structure , Phosphoric acid ( H3PO4 ) is weaker than nitric acid ( HNO3 ) , although it contains greater number of hydrogen atoms .

CH3COOH ( aq ) + H2 O ( l ) ↔ CH3COO− ( aq ) + H3 O+ (aq)

Hydrochloric acid HCl is stronger than acetic acid because HCl is completely ionized in the water , but acetic acid is partially ionized in the water .

HCl ( aq ) + H2 O ( l ) → Cl− ( aq ) + H3 O+ (aq)

 

Classification Of Acids According To Their Sources

Acids are classified according to their sources ( origin ) into Organic acids and Mineral acids

Organic acids

Acids that have an organic origin ( plant or animal ) , They are extracted from the organs of living organisms , All of them are weak acids .

Examples : Lactic acid ( milk products ) , Acetic acid ( vinegar ) , Citric acid ( from lemon ) , Oxalic acid and Formic acid .

Mineral acids

Acids that have no organic origin , They usually have non-metallic elements in their structure like chlorine , sulphur , nitrogen and phosphorus , Some of them are strong acids and others are weak .

Examples : Carbonic acid H2CO3 , Hydrochloric acid HCl , Phosphoric acid H3PO4 , Perchloric acid HClO4 , Nitric acid HNO3 & Sulphuric acid H2SO4 .

Classification of acids according to basicity

The basicity of acid

It is the number of hydrogen ions ( H+ ) , which is produced by one molecule of the acid when it dissolves in the water .

Acids are classified according to the basicity into Monobasic acids ( Monoprotic ) , Dibasic acids ( Diprotic ) and Tribasic acids ( Triprotic ) .

Monobasic acids ( Monoprotic )

Acids where each molecule gives one proton H+ , when it dissolves in the water .

Examples : Organic monobasic acids ( Formic acid HCOOH , Acetic acid CH3COOH ) , Mineral monobasic acids ( Hydrochloric acid HCl , Nitric acid HNO3 ) .

Dibasic acids ( Diprotic )

Acids where each molecule gives one or two protons H+, when it dissolves in the water .

Examples : Organic diabasic acids ( Oxalic acid ) , Mineral dibasic acids ( Sulphuric acid H2SO4 , Carbonic acid H2CO3 ) .

Tribasic acids ( Triprotic )

Acids where each molecule gives one, two or three protons H+, when it dissolves in the water .

Examples : Organic tribasic acids ( Citric acid ) , Mineral tribasic acids ( Phosphoric acid H3PO4 ) .

Both of citric acid and phosphoric acid have the same degree of basicity, while they differ in their source ( origin), Because both of them are tribasic acids , but citric acid is an organic acid, while phosphoric acid is a mineral acid.

Properties Of Acid

1.Acids have a sour taste.

2.They change the colour of litmus dye or litmus paper into red .

3.Acids react with bases producing salt and water.   Na OH (aq ) + HNO3 ( l ) → NaNO3 (aq ) + H2 O ( l )

4.Acids react with active metals producing a salt of the acid and hydrogen gas H2 evolves.

Zn ( s ) + 2 HCL (aq ) → ZnCl2 (aq ) + H2 ( g ) ↑

5.Acids react with Carbonate or bicarbonate salts causing effervescence and producing carbon dioxide gas CO2 which makes limewater turbid. 

Na2 CO3 ( s ) + H2 SO4 ( aq ) → Na2 SO4(aq ) + H2 O ( l ) + CO2 ( g ) ↑

Chemical Properties of Acids

1. With the exception of trioxonitrate (v) acid, acids react with some metals like zinc, magnesium and iron to liberate hydrogen gas.

Zn(s)  +  H2SO4(aq)           ZnSO4(aq)  +  H2(g)                       ; H2SO4(aq)  +  Mg(s)         MgSO4(aq)  +  H2(g)

2HCl(aq)  +  Zn(s)             ZnCl2(aq)  +  H2(g)                          ; 2HCl(aq)  +  Fe(s)             FeCl2(aq)   +  H2(g)

2HCl(aq) + Mg(s)              MgCl2(aq) +  H2(g)

2. Acids react with bases to form salt and water only. This reaction is known as a neutralization reaction.

HCl(aq)  +  NaOH(aq)          NaCl(aq) + H2O(l)

H2SO4(aq) + CaO              CaSO4(aq) + H2O(l)

H2SO4(aq) + 2KOH(aq)           K2SO4(aq)  +  2H2O(l)

3. Acids reacts with trioxocarbonate(iv) salts to liberate carbon(iv)oxide

i. CaCO3(s)  +  2HCl(aq)          CaCl2(aq)  +   H2O(l) + CO2

CaCO3(s)  +  H2SO4(aq)           CaSO4(aq)  +  H2O(l)  +  CO2(g)

K2CO3(s)  +  H2SO4(aq)           K2SO4(aq)  +   H2O(l)  +  CO2(g)

 Preparation of Acids:

Acid is prepared by direct combination of the constituent elements in the presence of a catalyst.

The gas formed is then dissolved in water.

i. H2(g)  +   Cl2(g)                2HCl(g)                          ii. H2(g)  +  Br2(g)               2HBr(g)

2. Acid is prepared by the dissolution of an acid anhydride in water. An acid anhydride is a nonmetallic

oxide which dissolves in water to form acid. Some examples of acid anhydride are:

Carbon(iv)oxide, Sulphur(iv)oxide, Sulphur(vi)oxide etc.

i. CO2(g)  +   H2O(l)         H2CO3(aq)         ii. SO2(g)  +  H2O(l)          H2SO3(aq)                    iii. SO3 + H2O               H2SO4(aq)

3. Acid is prepared by the displacement of a weak acid or a volatile acid (from its salt) by a

strong acid.

i. 2KNO3(s)  +  H2SO4(aq)         K2SO4(aq) + HNO3(aq)

ii. 2NaCl(s) + H2SO4(aq)               Na2SO4(aq)  +  HCl(g)

Iii. HCl(g) + H2O                 HCl(aq)

Uses of Acids

1. They are used in the manufacture of drugs, fertilizers, soaps and detergents.

2. Industrially, some acids are used as: i. Drying agents ii. Oxidizing agents

iii. Catalysts

other uses are:

1. Steel used in construction is acid treated before painting. Dil. H2SO4 or HCl will remove any surface rust which would otherwise spread under the painted surface. Rust removal used to repair cars is dilute phosphoric acid, H3PO4

2. Baking powder contains tartaric acid.

3. ‘lime scale’ removers contain dil. acids. Lime scale is CaCO3 (also called furring).

4. A swasp sting is alkali, and may be neutralized with a weak acid (lemon juice or vinegar).

5. H2SO4 is used in the production of fertilizers, e.g., superphosphate of lime and ammonium sulphate. 

6. H2SO4 is used in the manufacture of chemicals, example, HCl, HNO3, sulphate salts, synthetic detergents, dyes and pigments, explosives, and drugs.

6. Citric acid is used in the food/beverage industry for pH adjustment, acidity, preservation, stabilization of colours and flavours.

7. Citric acid is used In the pharmaceutical industry for effervescence, stabilization of ingredients and taste of pharmaceutical products.

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