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Chemical Reactions and Equations. class 10th Chemistry




CHEMICAL REACTIONS AND EQUATIONS


Chemical Reaction :- “Chemical reactions are the process in which new substances with new properties are formed.”

A chemical reaction represents the change of the species ( atoms or molecules ) taking

part in the reaction into new species.
A substances which take part in the chemical reaction are called reactants.

A new substances produced as a result of chemical reaction is called products.

Some examples of chemical reaction
Souring of milk, Cooking of food, Rusting of Iron, Ripening of food, Burning of foods etc.

Characteristic Feature Of Chemical Changes


  • There is generally a change in colour of reacting species taking part in the change.
  • The physical state of substance normally change.
  • In some chemical reaction, gas may evolved.
  • In some chemical reaction precipitate may formed.
  • In most of chemical reaction temperature is changed.


Before we go further , we should know the meaning of term ‘exothermic’ and ‘endothermic’
When enegry is released or given out, the reaction is exothermic in nature.
When energy is absorbed or taken up, the reaction is endothermic in nature.

Chemical Equation

The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it known as a chemical equation.
We can represents a chemical equation in two forms:

  • Word Equations.
  • Symbol Equation.

For e.g.
 Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas
The statement can be written as Word Equation
   Zinc + Sulphuric acid ---> Zinc sulphate + Hydrogen

The above symbol equation can be written in Symbol Equation
Zn + H2SO4 ----> ZnSO4 + H2

Balanced And Unbalanced Chemical Equations

A balanced chemical equation has an equal number of atoms of different elements in the reactants.
An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.

MAKING A CHEMICAL EQUATION MORE INFORMATIVE:

We can make our chemical equation more informative in the follwing ways :
1) To mentioned the physical state of matter by following symbol:

  • (s)  for solid state
  • (l) for liquid state
  • (g) for gaseous state
  • (aq) for aqueous state ( water solution.)

2) To Indicate the gas evolved in the reaction by an arrow pointing upward (↑)

3) To represent the precipitate if formed in the reaction by an arrow pointing downward (↓) or by using the abbreviation  ‘ppt’

4) To mention the actual heat evolved in the exothermic reaction by plus ( + ) sign and heat absorbed in the endothermic reaction bu minus (-) sign on the product side.

6) In some chemical reactions conditions of temperature, pressure, catalyst etc. are mentioned either above or below the arrow head.



Types of Chemical Equation

Some of important types of chemical equations are :
a) Combination Reactions,
b) Decomposition Reactions,
c) Displacement Reactions,
d) Double Displacement Reactions,
e) Oxidation And Reduction Reactions.

Now let us know about them in some more detail

Combination Reaction : Those reaction in which two or more substances combine to form a single substance, are called combination reaction.

i.e. A + B ------> C
here A and B both combine and form a single substance C

e.g.1  2Mg (s)      +      O2   ------>      2MgO (s)
 Magnesium      Oxyge     Magnesium Oxide


2)   2H2(g)   +       O2    ------>    2H2O
 Hydrogen Oxygen       water

Decomposition Reaction : Those reaction in which a compound splits up into two or more simpler substances under suitable conditions are known as decomposition reactions.

It is just opposite of combination reaction

i.e.               C    --------> A  +  B
here  C  is substance which decompose ( splits ) into two new substances A and B.



                               heat
e.g.    2FeSO4(s)   ----------->   Fe2O3(s)     + SO2(g)   +    SO3(g)
 Ferrous Sulphate ---->   Ferric oxide +    sulphur dioxide        +       sulphur trioxide


                 electrolysis        
2H2O (l)  ---------------> 2H2        +        O2
 

Displacement Reaction :   Those reaction in which one element takes the position or place of another element present in a compound is known as displacement reaction.

 i.e.        AB    +    C ----------->  AC    +  B
In this reaction, C displaces B from compound AB.

e.g.  Zn (s)     +     FeSO4(aq)  ------------> ZnSO4(aq)      +     Fe(s)
zinc + ferrous sulphate ----->  Zinc Sulphate          + Iron


Double Displacement reaction : Those chemical reaction in which two compounds react by an exchange of ions to form two new compounds are called double displacement reaction.

i.e.  AB     +    CD  ---------->   AC     +      BD
in this reaction, an exchange of ions takes place.
e.g.
 AgNO3 (aq)   +   NaCl(aq) ----------> AgCl (s)      +     NaNO3(aq)
   silver nitrate  +  Sodium  Chloride ------>          Silver Chloride    +  sodium nitrate



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