Online Notes on Chapter=1 ( Chemical Reactions and Equations) Part 1

This is Online Notes on Chapter=1 ( Chemical Reactions and Equations))  Part 1 for preparation of CBSE BOARD Examination, NTSE etc.





=> The process in which new substances with new properties are formed from
one or more substances is called Chemical Reaction.

* The substances which take part in chemical reaction are called Reactants.


* The substances which are formed in a chemical reaction are called Products.

Examples :
(i) Digestion of food
(ii) Respiration
(iii) Rusting of iron
(iv) Burning of Magnesium ribbon
(v) Formation of curd

=> Chemical reaction involves :
• Change in state
• Change in colour
• Change in temperature

• Evolution of gas

=> Chemical Equation:

* A chemical reaction can be represented by chemical equation. It involves uses
of symbol of elements or chemical formula of reactant and product with
mention of physical state.

* The necessary conditions such as temperature, pressure or any catalyst should
be written on arrow between reactant and products.

e.g., Magnesium is burnt in air to form magnesium oxide.

Mg + O2 → MgO

=> Balancing Chemical Equation:

* Law of conservation of Mass. Matter can neither be created nor destroyed in a
chemical reaction.

* So number of elements involved in chemical reaction should remain same at
reactant and product side.

=>  STEPWISE BALANCING (Hit and Trial):

Step 1. Write a chemical equation and draw boxes around each formula.

Fe + H2O → Fe3O4 + H2

* Do not change anything inside the box.
Step 2. Count the number of atoms of each element on both the sides of chemical

equation.

Step 3. Equalise the number of atoms of element which has maximum number by putting in front of it.

Fe + 4H2O → Fe3O4 + H2

Step 4. Try to equalize all the atoms of elements on reactant and product side by
adding coefficient in front of it.

3Fe + 4H2O → Fe3O4 + 4H2

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

Step 5. Write the physical states of reactants and products.

3Fe (s) + 4H2O (g) → Fe3O4 ( s) + 4H2 (g)
Solid state = (s)
Liquid state = (l)
Gaseous state = (g)
Aqueous state = (aq)

Step 6. Write necessary conditions of temperature, pressure or catalyst on arrow

above or below.