10 Class- Chemical Reactions and Equations

Chemical Reactions and Equations

• ·       Every chemical change is a chemical reaction.

• ·       General observation to identify whether a chemical reaction take place or not-

1.    Change in state.

2.    Change in color.

3.    Evolution of gas.

4.    Change in temperature.

Chemical Equations:

Any equation which describes the change of reactants to products is called chemical reaction.

Reactant1+Reactant2+…………--->   Product1+Product2+………

In chemical equations reactants are placed on left hand side and products are placed on right hand side and arrow sign is placed between them. Direction of arrow shows the direction of reaction.

Example: chemical equations in words.

Magnesium + Oxygen {Reactants} .------->  Magnesium-Oxide {Product}

Writing a Chemical Equation by using Chemical Formula:

In it we use symbols instead of words to write a chemical equation.

Example: Mg + O₂ .-----> MgO

     1.    Unbalanced Chemical Equation:

Equation in which number of atoms on reactant side is not equal to number of atoms on product side, means mass is not same on both sides.

         Example: Mg + O₂ -----> MgO

                          Fe +  H₂O -----> Fe₃O₄ + H₂

     2.    Balanced Chemical Equation:

Equation in which number of atoms on reactant side is equal to number of atoms on product side, means mass is same on both sides.

         Example: 2Mg + O₂ ----> 2MgO

                          3Fe + 4H₂O -----> Fe₃O₄ + 4H₂

Writing Symbols of Physical State:

                                                          Reactants and products are written with their physical state to make equation more informative.

Physical State	Notation
Gaseous	(g)
Liquid	(l)
Aqueous	(aq)
Solid	(s)

Example: 3Fe (s) + 4H₂O (g) -----> Fe₃O₄ (s) + 4H₂ (g)

·       Example of reactions with reaction condition like pressure, temperature, catalyst etc.

Types of Chemical Reactions:

                                                Chemical reactions are reactions which involve making and breaking of bonds between atom to yield new substances.

    1.    Combination Reaction:

These are the reactions in which one single product is formed from two or more reactants.

Example: i. Formation of Calcium Hydroxide.

CaO (s)(quick lime) + H₂O (l) -------> Ca(OH)₂ (aq)(slaked lime)

ii. Burning of Coal.

C (s) + O₂ (g) ------> CO₂ (g)

iii. Formation of Water.

2H₂ (g) + O₂ (g) ------> 2H₂O (l)

    2.    Displacement Reaction:

These are the reactions in which highly reactive metal displaces less reactive metal from their solution.

Example: i. Fe (s) + CuSO₄ (aq)(copper sulphate) -------> FeSO₄ (aq)(iron sulphate) + Cu (s)

 ii. Zn (s) + CuSO₄ (aq)(copper sulphate) -------> ZnSO₄ (aq)(zinc sulphate) + Cu (s)

iii. Pb (s) + CuCl₂ (aq)(copper chloride) -------> PbCl₂ (aq)(lead chloride) + Cu (s)

    3.    Double Displacement Reaction:

These are the reactions in which exchange of ions between reactants occur.

Example: Na­₂SO₄ (aq)(sodium sulphate) + BaCl₂ (aq)(barium chloride) -------> BaSO₄ (s)(barium sulphate) + 2NaCl (aq)(sodium chloride)

    4.    Oxidation and Reduction Reaction:

1.1.        Oxidation:

These are the reactions in which a substance loses hydrogen or gain oxygen.

          Example: 2Cu + O₂ + Heat ----------> 2CuO

1.2.        Reduction:

These are the reactions in which a substance gain hydrogen or loses oxygen.

          Example: CuO + H₂ + Heat ----------> Cu + H₂O

1.3.        Redox Reaction:

These are the reactions in which one reactant gets reduced while other reactant gets oxidized. This type of reaction is also known as Oxidation-Reduction Reaction or Redox Reaction.

Example:

i.                   CuO + H₂ + Heat ---------> Cu + H₂O

In above reaction-

                   H₂ ---------> H₂O (oxidation)

                   CuO --------> Cu (reduction)

ii.                ZnO + C --------->  Zn + CO

iii.             MnO₂ + 4HCl --------> MnCl₂ + 2H₂O + Cl₂

Observable Effect of Oxidation Reduction in Everyday Life:

1.    Corrosion:

   Attack on metals by moisture, acids, oxygen etc., which corrode metals is called corrosion.

Example: Rusting of Iron, Black coating on Silver, Green coating on Copper.

2.    Rancidity:

   Change in taste and smell of food material prepared by using fat or oil, because fat or oil get oxidized and become rancid.

Endothermic Reaction:

                                      These are the reactions in which heat is absorbed during reaction.

Example: NH₄Cl (s) + H₂O (l) + Heat --------> NH₄Cl (aq)

Exothermic Reaction:

                                      These are the reactions in which heat is evolved during reaction.

Example: CH₄ (g) + 2O₂ (g) --------> CO₂ (g) + 2H₂O (g) + Heat

Precipitation Reaction:

                                      These are the reactions in which insoluble substance called precipitate is obtained when reaction completes, is known as precipitation reaction.

Example: Na₂SO₄ (aq) + BaCl₂ (aq) -------> BaSO₄ (s) + 2NaCl (aq)