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Ch - Chemical Reaction and Equation

Chemical Reaction : – Whenever a chemical change occurs we can say that a chemical reaction has taken place.

 eg – Food gets digested in our body
      – Rusting of iron.


⇒Identification of Chemical Reaction

A chemical reaction  can be identified by either of the following observation:-

* change in state 
* evolution of a gas
* change on colour
* change in temperature 
* formation of a precipitate

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

 eg magnesium is burnt into air to form magnesium oxide can be represented as
Mg + O2 → MgO
 :–    We can observe or recognise a chemical reaction by observing change in state, colour, by evolution of gas or by change in temperature.

Physical state of the reactant and products are mentioned to make chemical reaction more informative. eg 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.
eg:- Fe + H2O → Fe2O3 + H2 
 can be written as
3 Fe(s) + 4H2O(g) → Fe2O3 (s) +4H2 (g)

⇒Types of Chemical Reaction

The chemical reaction classified into different classes depending upon the types of chemical changes taking place.

These reaction are as follows:-

Combination Reaction :– The reaction in which two or more substances combine to form a new single substance. eg :-

CaO(s)     +   H2O(l)    →    Ca(OH)2 (aq)
calcium oxide          water                 calcium hydroxide 


-----   Ca(OH)2 slaked lime is used for white washing walls.
          It reacts will CO2 to form CaCO3 and gives a shiny finish to the walls.


Ca(OH)2(aq)          +    CO2 (g)   →   CaCO3(s)  +   H2O (l)
calcium hydroxide                               calcium carbonate 


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

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) 

        – Respiration is also exothermic reaction.
        – Decomposition of vegetable matter into compost.

Decomposition reaction :-  A reaction in which a single reactant break down into two or more products , is known as decomposition reaction . this reaction is opposite to  combination reaction.

Decomposition reactions can be of three types:-

1* Thermal Decomposition :- These reaction use the energy in the form of heat for decomposition of the reactant .

2* Electrolysis :- These reaction involves the use of electrical energy for decomposition of the reactant molecules.

3* Photolysis :-  These reaction involves the use of light energy for the purpose of decomposition.




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

2Ba(OH)2 + NH4Cl → 2BaCl2 + NH4OH


Displacement reaction :- The chemical Reaction in which an element displaces another element from its solution

Fe(s)     +      CuSO4(aq)       →      FeSO4      +     Cu(s)
Copper (aq) Sulphate                   Iron Sulphate




--------- The nail becomes brownish in colour and the blue colour of Copper Sulphate solution fade . eg:- 

Zn(s)     +     CuSO4     →            ZnSO4    +    Cu(s) 
 Copper Sulphate                          Zinc Sulphate

Zinc  are more reactive elements than copper. They displace copper from its compound .

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


Na2 SO4(aq) + BaCl2(aq)    →    BaSO4(s) + 2NaCl (aq) 

A white substance is formed due to above reaction. The insoluble substance is called precipitate.
                                                              
Precipitation Reaction – Any reaction that produces a precipitate is called a precipitate reaction . eg :-

Pb(NO3 ) 2 + 2KI→ PbI2  +2KNO3 

– Oxidation : Oxidation is the gain of oxygen or loss of hydrogen. eg :- 
2Cu + O2   → 2CuO 

When Copper is heated a black colour 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 oxidised while other get reduced.


eg.-
ZnO + C → Zn + CO


MnO2 + 4HCl → MnCl2 + 2H2O + Cl2

Corrosion : When a metal is attacked by substances around it such as moisture, acids etc.

 eg. Reddish brown coating on iron. 
(ii) Black coating on Silver.

Rancidity : When fats and oils are oxidised they become rancid and their smell and taste change.
 
 – Antioxidants are added to foods containing fats and oil.

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