Chemical Reactions and Equations


Chemical reaction:
The process of combination of different atoms to form different products is known as chemical reaction.

A chemical reaction involves a change in the physical and chemical properties, the composition and the physical state of a substance.

Example: Carbon burns in oxygen to form carbon dioxide.

Chemical equation:
A chemical equation is a symbolic representation of the reactants and the products using their chemical formulae.

A chemical equation contains:

  • Reactants
  • Products
  • An arrow separates  the reactants and products  

+ O2            →  CO2
Reactants               Product

Representation of physical states of reactants and products:

  • For  solids it is "(s)".
  • For  liquids it is "(l)".
  • For  gases it is "(g)".
  • For aqueous solutions it is  "(aq)".
  • For gas produced in the reaction it is "(↑)".
  • For precipitate formed in the reaction it is  "(↓)".
  • Direction of reaction is indicated by "(→)".

Zn (s) + dil.H2SO4 (aq) → ZnSO4 (aq) + H2 (g) ()
  (Reactants)                               (Products)

A chemical equation is helpful to understand a chemical reaction in a easy way.

In a chemical equation the masses of reactants and products may or may not be equal. But according to law of conservation of mass "the total mass of the reactants and the products should be equal". So in order maintain the law true it is necessary to balance a chemical equation.

Steps involved in balancing of chemical equation:

  • Determining the reactants and products in a reaction.
  • Counting the number of atoms of each element on both sides of the equation.
  • Selecting the elements that occur for the least number of times in the equation.
  • Balance atoms of each element on both sides of the reaction.
  • Always leave hydrogen and oxygen for last to balance.
  • Balance the hydrogen atoms lastly followed by balancing of oxygen atoms.

Formation of Ammonia

N2 + H2 → NH3

Step1: 2-Nitrogen atoms, 2-Hydrogen atoms → 1 –Nitrogen atom, 3-Hydrogen atoms

Step2: In the above equation number of Nitrogen atoms on both sides are not equal, multiply with suitable integer to balance the Nitrogen atoms on both sides.

So, multiply with "2" on the product side.

N2 + H2 → 2NH3

In the above equation the number of Nitrogen atoms were balanced.

Step3: Hydrogen atoms on both sides were not balanced. So, multiply with suitable integer.

Multiply with "3" to Hydrogen on reactant side.

N2 + 3H2 → 2NH3

Now the above equation is balanced.

In general complicated rections like redox reactions can be balanced using two methods. They are:

  • Hit and trial method
  • Half equation method


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