Chemical Reactions and Equations (Class 10 Science) Part 3

 

Types of chemical reaction

The chemical reaction are classified into different classes depending upon the type of chemocal changes taking place. These reactions are as follows :

 

1.  Combination Reaction :-

          A reaction in which two or more reactants combine to form a single product, is called combination reaction, e.g.

 

CaO_(s) + H₂O_(l) → Ca(OH)_2(aq) + Heat

 

C_(s) + O_2(g)   →   CO_2(g)

 

2.  Decomposition Reaction :-

          A reaction in which a single reactants breaks down to form two or more products, is known as decomposition reaction. This reaction is opposite to combination reaction. e.g.

CaCO_3(s)  →  CaO_(s)  +  CO_2(g)

 

2AgCl_(s)  →  2Ag_(s) +  Cl_2(g)

 

2NaCl_(l)  →  2Na_(s)  +  Cl_2(g)

 

 

3.  Exothermic and Endothermic reaction :-

          Depending upon whether heat is evolved or absorbed during a reaction, the reaction can be exothermic or endothermic.

(i)Exothermic reactions :-  The reaction which are accompanied by the evolution of heat, are called exothermic reactions or the reaction in which heat is released along with the formation of products are called exothermic reaction. e.g.

a)    Burning of natural gas

CH_4(g)  +  2O_2(g)  →  CO_2(g)

b)   Burning of Magnesium ribbon

2Mg_(s) + O_2(g)  →  2MgO_(s)  +  Heat

(ii)     Endothermic reaction :-  The reaction which occur by the absorption of heat/energy are called endothermic reactions. e.g.

6CO_2(aq)  +  12H₂O_(l)  →  C₆H₁₂O₆ + 6O₂ + 6H₂O

NH₄Cl_(s) + Heat → NH_3(g)  +  HCl_(g)

 

4.  Displacement reaction :-

          When a more reactive element displaces less reactive elwmwnt from its compound, it is called displacement reaction. this reaction is of two types :

(i)    Single displacement reaction :  It is a type of chemical reaction where an element reacts with a compound and takes the place of another element in that compound is called single displacement.

        Zn_(s)   +  CuSO_4(aq)  →  ZnSO_4(aq)  +  Cu_(s)

(ii)    Double displacement reaction :  The reaction in which two different ions or group of atoms in the reactant molecule are displaced by each other is called double displacement reaction.

Na₂SO_4(aq) + BaCl_2(aq)  →  BaSO_4(s)  +  2NaCl_(aq)

 

5.  Oxidation and Reduction reaction :-

OXIDATION :

          It can be defined as

The process in which oxygen is added to a substance.

Or

The process in which hydrogen is removed from a substance.

Or

The process in which a substance loss electrons.

e.g.

2H₂S  +  O₂  →  2S  +  2H₂O

2Cu  +  O₂  →  2CuO

 

REDUCTION :

          It can be defined as

The process in which oxygen is removed from a substance.

Or

The process in which hydrogen is added to a substance.

Or

The process in which a substance gains electrons.

e.g.

2Na  +  H₂  →  2NaH

Zn₂+  +  2^e−  →  Zn

 

Oxidising agents :  The substance which can bring about oxidation of other substances is called an oxidising agent.

Reduction agent :  The substance which can bring about reduction of other substance is called a reducing agent.

 

Redox Reaction :-

          Those reaction in which oxidation and reduction take place simultaneously, are called redox reactions. e.g.

CuO  +  H₂  →  Cu  +  H₂O

          In this reaction the copper oxide is losing oxygen and is being reduced. Whereas oxygen is added to hydrogen and is being oxidised.

 

 

 Corrosion 

          The phenomenon due to which open surface of the metals are slowly eaten away by the reaction of air, water, and chemicals present in the atmosphere is called corrosion.

e.g. Iron articles are shiny when new, but get coated with a reddish brown powder when left for some time.

The process of corrosion of iron is Calles rusting. The rusting of iron is a redox reaction. the black coating on silver and the green coating on copper are other examples of corrosion.

Painting, Galvanising, electroplating are some of the methods to prevent corrosion.

 

Effects of Corrosion

(i)              Corrosion causes damages to car bodies, bridges, iron railings, ships, and all objects made up of metals, specially those which are made up of iron.

(ii)           Corrosion is a wasteful process in most of cases. Every year tons of various metals especially iron get wasted in the country.

 

Rancidity

 

It is the process of slow oxidation of oil and fat present in the food material resulting in the change of smell and taste in them. The method to prevent rancidity are :

(i)              Keeping food material in air tight containers.

(ii)           Refrigeration of cooked food at low temperature.

(iii)        Packing of food items like potato wafers etc in packets containing nitrogen gas instead of air.

(iv)         Avoid keeping cooked food and food materials in  direct sunlight.

 

Part 1        Part 2