Chemical Reactions and Equations

Summary

Chemical reaction:
The process of combination of different atoms to form different products is known as chemical reaction. Chemical reaction involves a change in the physical and chemical properties like:


Change in the colour of the substance
Change in the state of the substance
Change in heat energy - Absorption of energy or release of energy
Release of gas
Evolution of light and sound

Example:
Burning of coal: During the burning coal, the solid state coal burns with release of heat and gas.


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

Reactants are the substances which take parts in the chemical reaction.
Products are the substances which produced during chemical reaction.

+ 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 "(→)".

Example:



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.

Example:
Formation of Ammonia:
         N2 + H2 → NH3   (Un-balanced equation)
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.

Reaction of potassium hydroxide with sulphuric acid:
Write down the equation
        KOH + H2SO4 → K2SO4 + H2O (This is not balanced equation)

Check the number of atoms of each element on both sides

Potassium (K) Atoms on both sides are not equal, balance the Potassium atoms by multiplying with suitable integer.
           2KOH + H2SO4 → K2SO4 + H2O

Now Potassium  atoms on both sides are balanced.

Number of Hydrogen atoms on both sides is not equal, balance hydrogen atoms by multiplying with suitable integer.
              2KOH + H2SO4 → K2SO4 + 2H2O

Now Hydrogen atoms on both sides are balanced. And Oxygen atoms on both sides get balanced.
This is balanced equation as atoms of all elements on both sides are balanced
               2KOH + H2SO4 → K2SO4 + 2H2O

Videos

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Activities & Simulations

http://www.physics-chemistry-interactive-flash-animation.com/chemistry_interactive/basic_chemical_reaction_balance_learning.htm

http://phet.colorado.edu/sims/html/balancing-chemical-equations/latest/balancing-chemical-equations_en.html