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Chemical Reactions and Equations


HD 12:11 Animated video Lecture for Chemical Reactions and Equations
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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

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 "(→)".


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   (Un-balanced equation)
Step 1: 2-Nitrogen atoms, 2-Hydrogen atoms → 1 –Nitrogen atom, 3-Hydrogen atoms

Step 2: 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.

Step 3Hydrogen 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


HD 12:11 Animated video Lecture for Chemical Reactions and Equations
LearnNext Video
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Activities & Simulations

Physics-chemistry-interactive-flash-animation has created a web based simulation which tests the knowledge of  student about chemical formulae of different molecules and the chemical reaction between those molecules. It also tests method of balancing the chemical equations.
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Colorado.edu has developed an interactive simulation regarding balancing of chemical equations staring from basic equations upto different levels in difficulty by using helping tools.
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1 . what is rancidity ? write different methods of preventing rancidity .

Ans: Rancidity: It is defined as chemical decomposition of oils and fats which in another words is spoiling food materials that difficult fo...

2 . Formula of Sulphuric acid


3 . A substance X is pale green in colour. It changes to reddish brown if kept in air for long time.X decomposes to brown coloured metal oxide Y along with two oxides of a yellow coloured non-metal.Identi

Ans: The substance in pale green colour is FeSO?. When it is kept in air for longer time it decompose to form Ferric oxide,...

4 . "Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate". (1).Translate the above statement into a chemical equation. (2). state two types in

Aluminum sulfate  + Barium chloride ? Aluminum chloride + Barium sulfate.
Al 2(SO 4) 3 + 3BaCl...

5 . what change in colour is observed when white silver chloride is left exposed to sunlight ? state the type of chemical reaction in this change .also write the equation.

Ans: When white Silver chloride exposed to sunlight it produces to black metallic colured Silver along with liberation of Chlorine gas....