4 April 2014

Chemical Reactions and Equations

CBSE NCERT Class X (10th) | Science

Chapter 1 Chemical Reactions and Equations

A chemical reaction involves a change in the physical and chemical properties, the composition and the physical state of a substance.
Class 10th Science: Chapter 1 Chemical Reactions and Equations, NCERT CBSE Solved Question Answers, KEY NOTES, NCERT Revision Notes, Free NCERT Solutions Online

 A chemical equation is a symbolic representation of the reactants and the products using their chemical formulae.
A chemical equation contains:
-An arrow separates  the reactants and products  
Symbols for physical state of reactants and products:
Gas (g)
Aqueous solution (aq)
Gas produced  ((↑))
Precipitate formed  ((↓))
Direction of reaction ((→))
The law of conservation of mass states that the total mass of the reactants and the products should be equal.
The process of balancing chemical equation involves four steps:
1. Determining the reactants and products in a reaction.
2. Counting the number of atoms of each on both sides.
3. Selecting the elements that occur for the least number of times in the equation.
4. Changing the coefficient of the molecules of reactants or products as required.
Represents an unbalanced equation for the formation of water
Represents balanced equation for the formation of water
Represents an unbalanced chemical equation for the combustion of ethyl alcohol.
Represents balanced chemical equation for the combustion of ethyl alcohol.
Represents an unbalanced chemical equation for the decomposition of potassium chlorate.
Represents balanced chemical equation for the decomposition of potassium chlorate.
Represents an unbalanced chemical equation for the reaction of sulphuric acid with copper to form copper sulphate, water and sulphur dioxide.
Represents a balanced chemical equation for the reaction of sulphuric aicd with copper to form copper sulphate, water and sulphur dioxide.
Represents an unbalanced chemical equation for the decomposition of sodium thiosulphate with hydrochloric acid to give sodium chloride, sulphur dioxide, sulphur and water.

Types of Chemical Reactions

In general chemical reactions are classified into:
  •    Combination reactions
  •    Decomposition reactions
  •    Single displacement reactions
  •    Double displacement reactions

Combination reactions:
A combination reaction is one in which two or more reactants combine to form a single product. Combination reactions are again of three types.

They are:
Combination reactions between elements.
Combination reactions between compounds.
Combination reactions between elements and compounds.

Combination reactions between elements:
In these reactions elements were combined to form a product.

Examples :
Formation of calcium oxide by the combination elements calcium and oxygen.
2Ca + O 2CaO
Formation of ammonia by the combination of elements nitrogen and hydrogen.
N2 + 3H2 → 2NH3

Combination reactions between compounds:
In these reactions compounds were combined to form product.

Reaction of calcium oxide in water to form calcium hydroxide
CaO + H2O  → Ca(OH)2

Combination reactions between elements and compounds:
In these reactions elements and compounds combined to form product.

Formation of sulphur trioxide by the combination of sulphur dioxide and oxygen.
2SO2 + O  2SO3

Decomposition reaction:
Decomposition reactions are those in which a substance splits into two or more simpler substances. Decomposition reactions are the opposite of combination reactions.  Decomposition reactions are of three types:

  •    Thermal decomposition
  •    Electrolysis
  •    Photolysis

Decomposition by the appication of heat on a substance is called thermal decomposition. 

Thermal decomposition of calcium carbonate.
CaCO3 → CaO + CO2

Decomposition of a substance by passing current through it is called electrolysis. 
Electrolysis of water 

2H2O → 2H2 + O2

The decomposition reaction resulting from action of light energy is called photolysis. 
Photolysis of silver chloride
2AgCl  → 2Ag + Cl2

Single displacement reaction: 
Single displacement reaction is the one in which one element substitutes or displaces another element in a compound to give new compound. Generally in a single displacement reaction,  less active metal is displaced by a more active metal.

It is important to note that in a single displacement reaction, one of the reactants is always an element. 

The reaction of magnesium with copper chloride

Mg + CuCl  MgCl2 + Cu

Double displacement reaction:
The reactions which involves exchange of ions (cations and anions) between the reactants are called double displacement reactions.
AB + CD → AC + BD

Double displacement reactions classified into different types. They are: 

  • Neutralization reactions
  • Precipitation reactions
  • Gas forming reactions

Neutralization reaction:
Reaction in which hydrogen ions of an acid react with hydroxide ions of a base to form water is an neutralization reaction. 
Example :

The reaction of hydrochloric acid with sodium hydroxide.
H+Cl  + Na+OH → Na+Cl + H2O

Precipitation reaction:
Reactions which proceeds through the formation of precipitation are called precipitation reactions.
In the reaction of barium chloride with sodium sulphate produces precipitate of barium sulphate along with formation of sodium chloride.
BaCl2 (aq) + Na2SO4 (aq) → BaSO(s) + NaCl (aq)

Gas forming reaction:
In these reactions gas is produces as one of the product during reaction.
Na2CO3 (s) + 2HCl (aq) → 2NaCl (aq) + H2(l) + CO2 (g)

Combustion reaction:
Burning of a substance in the presence of oxygen which resulting release of energy is called combustion.
Combustion of ethylene gas.
C2H4 + 3O2 → 2CO2 + 2H2O

Oxidation reaction: 
Oxidation reaction involves the addition of oxygen or the removal of hydrogen from a substance.
Rusting of iron

Rusting of iron:
Iron when reacted with both water and oxygen are present (moist air), corrodes. Its silvery colour changes to a reddish-brown, because hydrated oxides are formed which is commonly called as rust.
Reaction showing the formation of rust when reacted with water in presence of oxygen (atmosphere).                        
4Fe + 3O₂+ XH₂O → 2Fe₂O₃.XH₂O

Reduction reaction:
The reaction which involves addition of hydrogen or removal of oxygen from a substance is called reduction reaction.
Photosynthesis is a reduction reaction.

Redox reaction:
The reactions in which both oxidation and reduction occurring together are known as redox reactions.
CuO + H→ Cu + H2O

The above is a redox reaction as copper is reduced and hydrogen is oxidised.

Based on energy differeneces between reactants and products the chemical reactions are classified into two types. They are exothermic reaction and endothermic reaction.

Exothermic reaction:
Chemical reactions in which heat energy released are known as exothermic reactions.
Burning of methane gas in air produces heat energy.
CH(g) + 2O(g) → CO(g) +  2H2O (l)+ heat

Endothermic reaction:
Reactions which involves absorption of energy are known as endothermic reactions.
Formation of nitric oxide from nitrogen and oxygen.
N2 (g)+ O(g) + Heat  2N

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