Chemical Reaction

Chemical Reaction

In the previous class, you have read about physical an chemical changes. Chemical changes result from chemical reaction taking placed between substances. In this chapter we shall deal with the chemical reactions and their representation in the form of chemical equations.

The processes in which a substance or substances undergo change to produce new substances with new properties are known as chemical reactions. for example, when calcium carbonate is heated, calcium oxide (lime) and carbon dioxide are formed. The breaking up of calcium carbonate into calcium oxide and carbon dioxide is, thus, a chemical reaction because calcium carbonate changes into new substances, calcium oxide and carbon dioxide.

Reactant

The substance which takes part in a chemical reaction is called reactant. For example, in the breaking up of calcium carbonate into calcium oxide and carbon dioxide, calcium carbonate is the reactant. Similarly, sodium and water are the reactants when they react.

Product

A product is a new substance formed in a chemical reaction. For example, hydrogen and sodium hydroxide are the products of the reaction between sodium and water.

\underset{{sodium}}{\mathop{{Na}}}\,+\underset{{water}}{\mathop{{{{H}_{2}}O}}}\,\to \underset{{sodiumhy\,\,droxide}}{\mathop{{NaOH}}}\,+\underset{{hydrogen}}{\mathop{{{{H}_{2}}}}}\,$

Similarly, in the breaking up of calcium carbonate, calcium oxide and carbon dioxide are the products.

\underset{{calcium\,carbonate}}{\mathop{{CaC{{O}_{3}}}}}\,\to \underset{{calcium\,oxide\,}}{\mathop{{CaO}}}\,+\underset{{carbon\,dioxide}}{\mathop{{C{{O}_{2}}}}}\,$

You know, atoms in a molecule are held together by a force of attraction called bond. The molecules do not participate directly in a chemical reaction. First they break down into atoms and these atoms then take part in the reaction. New bonds are formed between the atoms to form the products. That is, there take place rearrangement or regroupings of atoms in various ways to give products. For example, when ammonium cyanate is heated, different bonds in ammonium cyanate molecules are broken and new bonds are formed to produce urea.

Chemical Reaction Here, we see that the molecular formulae of both ammonium cyanate and urea are the same, but their properties are quite different and they are two different compounds. Such compounds are known as isomers of each other and the reactions that produce such isomers are called isomerization reactions.

Valency

The number of electrons shared by an atom is called its valency. It is also called the combining capacity of an atom, e.g., Cl atom can share one valence electron, its valency is 1, Oxygen can share two valence electrons, its valency is 2. Nitrogen can share 3 valence electrons, its valency is 3, Carbon can share 4 valency electrons, therefore its valency is 4 and so on.

It means if carbon combines with Chlorine, Carbon will share four valence electrons with four chlorine atoms, therefore the molecular formula of the covalent compound will be

valency

Some more examples are :

Some Common Monoatomic Ions

+1 Charge Formula +2Charge Formula +3 Charge Formula
Name of ion Name of ion Name of ion
Copper ion

(Cuprous ion)

Cu+ Barium ion

Cobalt ion

Ba2+

Co2+

Aluminium ion

Auric ion

Al3+

Au3+

Potassium ion K+ Strontium ion Sr2+ Chromium (III) ion Cr3+
Silver ion Ag+ Iron (II) ion

(Ferrous ion)

Fe2+ Iron (III) ion

(Ferric ion)

Fe3+
Sodium ion Na+ *Copper (II) ion Cu2+ Scandium ion Sc3+
Lithium ion Li+ *Lead (II) ion Pb2+ Arsenic ion As3+
    Cadmium ion Cd2+ Bismuth ion Bi3+
    Magnesium ion Mg2+ Antimony ion Sb3+
Aurous Au+ Manganese (II) ion Mn2+    
    *Mercury (I) ion    
    Zinc ion Zn2+    

 

– 1 Charge Formula – 2Charge Formula – 3 Charge Formula
Name of ion Name of ion Name of ion
Bromide ion Br Oxide ion O2– Nitride ion N3–
Chloride ion Cl Sulphide ion S2– Phosphide ion P3–
Fluoride ion F     Boride ion B3–
Iodide ion I        

 

These elements show more than one valency. So a Roman numeral shows their valency in a bracket.

Some Common Poly atomic Ions

– 1 Charge Formula – 2Charge Formula – 3 Charge Formula
Name of ion Name of ion Name of ion
Hydrogen carbonate

or bicarbonate ion

HCO_{3}^{-}$ Carbonate ion

Manganate ion

CO_{3}^{{2-}}$

MnO_{4}^{{2-}}$

Phosphate ion

Arsenate ion

PO_{4}^{{3-}}$

AsO_{4}^{{3-}}$

Hydrogen sulphate

or (bisulphate ion)

HSO_{4}^{-}$ Thiosulphate ion

Silicate ion

{{S}_{2}}O_{3}^{{2-}}$

SiO_{3}^{{2-}}$

Arsenite ion AsO_{3}^{{3-}}$
Hydroxide ion OH Sulphate ion SO_{4}^{{2-}}$ Phosphite ion PO_{3}^{{3-}}$
Nitrate ion NO_{3}^{-}$ Sulphite ion SO_{3}^{{2-}}$    
Chlorate ion ClO_{3}^{-}$ Chromate ion CrO_{4}^{{2-}}$ Borate ion BO_{3}^{{3-}}$
Nitrite ion NO_{2}^{-}$ Dichromate ion C{{r}_{2}}O_{7}^{{2-}}$ Ferricyanide ion [Fe(CN)6]3–
Permanganate ion MnO_{4}^{-}$ Hydrogen phosphate ion HPO_{4}^{{2-}}$    
Acetate ion CH3COO Oxalate ion {{C}_{2}}O_{4}^{{2-}}$    
Cyanide ion CN        
Hypophosphite ion {{H}_{2}}PO_{2}^{-}$       – 4 Charge
Meta aluminate ion AlO_{2}^{-}$     Carbide ion C4–
  +1 Charge     Ferrocyanide ion [Fe(CN)6]4–
Ammonium ion NH_{4}^{+}$        

 

Tests of Chemical Reaction

A chemical reaction must satisfy the following

(i)   There must be either evolution or absorption of heat, i.e., a chemical reaction must be accompanied with change in temperature.

(ii)  The reaction must occur between fixed quantities of the reactants.

(iii) There must not be either gain or loss of matter, i.e., a chemical reaction should follow the law of conservation of mass.

(iv) The products obtained as a result of chemical reaction must have properties different from those of the reactants.