1. Chemical Equations

CHAPTER → 1

CHEMICAL REACTIONS AND EQUATIONS

*  CHEMICAL EQUATIONS

CHEMICAL REACTIONS:- The process in which two or more substance combine with each other to form new substances with new properties is called chemical reaction.

There are two parts of a chemical reactions :-

(i) Reactants:-   The substances which take part in a chemical reaction are known as reactants.

(ii) Products:-  The new substances formed during a chemical reaction are known as products.

There are 5 ways to tell if a chemical reaction has occurred.

• Change in state.
• Change  in colour.
• Change in temperature.
• Evolution of a gas.
• Formation of precipitate.

  Chemical reaction in everyday life:-

• Digestion of food.
• Respiration.
• Rusting of iron.
• Formation of curd.
• Burning of magnesium ribbon.

Chemical Equations:-  A chemical equation is a written representation of a chemical reaction.

The representation of chemical reaction using symbols and formulae of the substances is called chemical equation.

A   +   B       →      C   +     D

Reactants                   Products   

n this equation, A and B are called reactants and C and D are called the products. The arrow shows the direction of the chemical reaction. The necessary condition such as temperature, pressure or any catalyst should be written on arrow between reactants and products.

E.g. Magnesium is burnt in air to form magnesium oxide.

(i) Word equation for above reaction would be -

 Magnesium + oxygen     →           Magnesium oxide

   ( Reactants )                                      ( Product )

Skeletal equation for above reaction would be - 

Mg +    O₂        →           MgO

BALANCING CHEMICAL EQUATIONS:-

• LAW OF CONSERVATION OF MASS :-  Mass can neither be created nor be destroyed in a chemical reaction.
• So number of elements involved in chemical reaction should remain same at reactant and products side.

For Example ,

Zn   +   H₂SO₄             →     ZnSO₄  +         H₂

(Zinc)    ( Sulphuric Acid)        (Zinc Sulphate)   ( hydrogen) 

Let us check the number of atoms of different elements on both sides of the arrow .

 

As the number of atoms of each element is same on both sides of arrow. This is   a balanced chemical equation.

 Let us take another example :-   

Fe     +    H₂O   →  Fe₃O₄   +   H₂

STEP 1 :-   Write a chemical equation.

Fe   +    H₂O  →   Fe₃O₄  +  H₂

 STEP 2:-  List the number of atoms of different elements present in the unbalanced equation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

STEP 3 :_ Select the element which has the maximum number of atoms . now equalize the number of atoms by putting coefficient in front of it.

Fe   +      4  H₂O      →     Fe₃O₄   +     H₂

STEP 4 :-  Fe and H atoms are still not balanced choose any elements now to balance. To equalize the number of H atoms,

Fe   +     4 H₂O   →      Fe₃O₄   +     4 H₂

STEP 5 :- Now, take Fe and equalize the number of Fe atoms.

3 Fe   + 4 H₂O     →Fe₃O ₄     +    4 H₂

Now all the atoms of elements are equal on both sides.

STEP 6 :-  To make the chemical equation more information ,write the physical states of reactants and products.

Solid state = (s)

Liquid state = (l)

Gaseous State = (g)

Aqueous state = (aq)

3 Fe (S)    +   4H₂O(g)       →   Fe₃O₄S+  4H₂(g) 

STEP 7:-  Write necessary conditions of temperature pressure or catalyst on above or below arrow.

For Example:-