CHEMICAL REACTIONS AND EQUATIONS AND THEIR IMPORTANCE

Chemical Reactions

Whenever a chemical change occurs, we can say that a chemical reaction has taken place

e.g.  –  Food gets digested in our body

        –   Rusting of iron.

Chemical Equation:- A chemical reaction can be expressed symbolically by using a chemical equation

e.g.  magnesium is burnt into the air to form magnesium oxide can be represented as

Mg+O₂  → MgO

We can observe or recognize a chemical reaction by observing a change in state, or color, by the evolution of gas, or by the change in temperature.

• The physical state of the reactant and products are mentioned to makes chemical reaction more informative.
• e.g.  we use (g) for gas, (l) for liquid, (s) for solid, and (aq) for aqueous.

• Balancing Equation :- We balance the chemical equation so that no. of atoms of each element involved in the reaction remain same at the reactant and product side.

     e.g.  Fe+ H₂O → Fe₂O₃ + H₂ can be written as

             3Fe_(s) + 4H₂O(g) → Fe₃O₄ + 4H₂(g)

Read More about Balancing Equation

• Combination Reaction:  The reaction in which two or more substances combine to form a new single substance

e.g. CaO_(s)            +    H₂O_(l)              →               Ca(OH)_2(aq)

       Calcium Oxide                          Water                           Calcium hydroxide

       ( Quick Lime)                                                           (Slaked lime)

• Ca(OH), slaked lime is used for white washing walls. It reacts with CO₂ to form CaCO₂ and gives a shiny finish to the walls.

          Ca(OH)₂(aq)         +    CO₂(g)              →       CaCO₂(s)                     +           H₂O(l)

    Calcium hydroxide                                               Calcium Carbonate

Exothermic Reactions : – Reaction in which heat is released along with the formation of products.

 e.g.        CH_4(g)         +           2O_2(g)                  →            CO_2(g)           +          2H₂O_(g)

    Respiration is also exothermic reaction.

    De composition of vegetable matter into compost.

Decomposition Reactions : – The reaction in which a single substance decomposes to give two or more substances.

Decomposition reactions can be of three types

• Types of Decomposition Reactions

Thermal Decomposition : –  When a decomposition reaction is carried out by heating

e.g.               2FeSO_4(S)                                      Fe₂O_3(g)     +    SO₂      +   SO₃

               Ferrous Sulphate                                              Ferric Oxide

                  Green Color                                                         Reddish brown color

                    CaCo_3(S)                                              Cao_(g)    +      CO_2(g)

                       PB(NO₃)_2(s)                                                 PbO_(S)      +        4NO_2(g)      +    O_2(g)

                  Lead Nitrate                                         Lead Oxide       Nitrogen            Oxygen

                White colour                                    Brown colour      Dioxide

Electrolytic Decomposition : –  When a decomposition reaction is carried out by electric current,

e.g.  H₂O_(l)

Photolytic Decomposition : – When a decomposition reaction is carried out by light

e.g.   2AgCl_(S)       2Ag_(S)    +    Cl_2(g)

        White color                    grey color

         Silver bromide behaves similarly

         2AgBr     2Ag_(S)    +  Br_{2 (g)}

           The above two reactions are used in black and white photography.

Endothermic Reactions : –  The reactions which require energy in the form of heat, light or electricity are called Endothermic Reactions.

2Ba(OH)₂        +        NH₄Cl     →   2BaCl₂      +  NH₄OH

Displacement Reaction : – The chemical Reaction in which an element displaces another element from its solution. This

depends on reactivity series.

 Fe_(S)       +     CuSO_4(aq)     →     FeSO₄       +   Cu_(s)

                               Copper                              (aq)

                             Sulphate                       Iron Sulphate

The nail becomes brownish in color and the blue color of Copper Sulphate solution fade.

Other examples :-

           Zn_(s)      +     CuSO_4(aq)         →     ZnSO₄       +     Cu_(S)

                                          Copper                      Zinc

                               Sulphate                  Sulphate

           Pb_(S)    +    Cucl_2(aq)               →   PbCl_{2 (aq)}    +   Cu_(S)

                                    Copper                           Lead

                             Chloride                            Chloride       

Zinc and lead are more reactive elements than copper.  They displace copper from its compounds.

Double Displacement Reaction : –    The reaction in which two different atoms or group of atoms are mutually exchanged

e.g.                         Na₂ SO_4(aq)        +      BaCl_2(aq)       →     BaSO_4(S)              +      2Nacl_(aq)

                               Sodium Sulphate       Barium Chloride         Barium Sulphate             Sodium Chloride

A white substance  is formed due to above reaction. The insoluble substance is called precipitate.

Precipitate Reaction : – Any reaction that produces a precipitate is called a precipitation reaction.

e.g.                       Pb(NO₃)_2(aq)      +      2KI               →      PbI_2(aq)¯     +2KNO₃

                            Lead Nitrate           Potassium                     Lead              Ptassium

                                                           Iodide                           Iodide            Nitrate

Oxidation :  Oxidation is the gain of oxygen or loss of hydrogen or loss of electron.

e.g.                      2Cu    +     O₂     2CuO

When  Copper is heated a black color appears. If this CuO is reacted with hydrogen gas then again, Cu becomes brown as reverse reaction takes place CuO + H₂  Cu +H₂O

Reduction : Reduction is the loss of oxygen or gain of hydrogen.

Redox Reaction : The reaction in which one reactant gets oxidized while other gets reduced

e.g.           ZnO    +    C         Zn   +  CO

                 MnO₂  +  4HCl     MnCl₂ +  2H₂O   +   Cl₂

Corrosion : 

• When a  metal is exposed to moisture, air, acid, etc. for some time, a layer of hydrated oxide is formed which weakens the metal and hence metal is said to be corroded.
• Rusting of iron, black coating on silver and green coating on copper are examples of corrosion.
• Corrosion can be prevented by painting, oiling, galvanization (Zn metal- layer), electroplating.

Rancidity : 

When fats and oils are oxidized, they become rancid and their smell and taste change.

Methods to prevent rancidity

• By adding antioxidants
• Keeping food in air tight containers
• Replacing air by nitrogen
• Refrigeration.

Read More at – MonoMath