Chemical reactions and equation

Chemical reactions and equations (I)




Introduction

 Chemical changes are undergoing in nature. Due to chemical changes we have seen change in our surroundings. These changes are irreversible. For a chemical change some conditions are required. When these conditions are fulfilled, then chemical change may take place.


In a chemical change following things are possible to happen:

1. In a chemical change, atoms or molecules take part. Because of reaction between atoms or molecules of matter change takes place. In other words, in a chemical change atoms take place.
2.  In a chemical change, heat plays a vital role. For some changes, heats is given externally and in the changes which need heat for reaction are called endothermic reaction and those which liberate heat due to reaction are called exothermic reaction.
3.  Due to chemical change, the substances lose their properties and new substances are formed which have different properties than that of reactant.
Calcium carbonate (marble or lime stone) CaCO3
When limestone is heated at high temperature in a furnace, limestone undergoes chemical change. As a result, lime (calcium oxide) and carbon dioxide are produced. This reaction needs heat externally (endothermic reaction). The properties of reactant and product are different. Thus, a chemical reaction has taken place.
        CaCO3 = CaO + CO2

Chemical Equation

A chemical equation represents a chemical reaction with the help of symbols and formulae. A chemical equation is a symbolic represent of an actual change.
A chemical equation representing in words is called word equation which is time, space and effort consuming. Therefore, symbolic equations are used in chemistry and is called the language of chemistry. In a chemical equation, the chemicals which take part in the reaction are called reactant and the substances which are formed due to a certain condition present is called products. In the representation of chemical equation, the reactants are written on the left hand-side of the arrow-head and the products on the right hand-side.

 


A chemical equation has three essential characteristics:

1.  It should represent an actual chemical change.
2.  It should be balanced
3.  It should be molecular.

There are four types of chemical reactions. They are:

a)      Combination reaction
b)     Decomposition reaction
c)      Displacement reaction
d)     Acid-base reaction or Neutralization reaction

Combination Reaction

Combination reaction

Combination reaction

By the term it is referred that two or more elements or compounds combine to give a single product. This is also called a synthesis or addition reaction. Generally two or more elements combine to give a single product. Such a reaction is termed as combination reaction.
Example: Iron + Sulphur  =  Iron sulphide.

                 Fe + s = FeS


Decomposition Reaction

When a substance goes decomposition, two or more substances or elements are produced. Similarly, in decomposition reaction, a chemical compound under certain condition decomposes to give two or more compound. Thus, decomposition reaction appears to be opposite to combination reaction.

Example: Calcium carbon = Calcium oxide + Carbon dioxide
             CaCO3 = CaO + CO2

Displacement Reaction

 

displacement reaction
When two or more elements and compounds are in a state of reaction, one element of a compound is displaced by another. Thus, new substances are produced. Such a reaction is known as displacement reaction. An element which is less reactive is displaced by a more reactive metallic or electropositive radical. For example, in the reaction of sulphuric acid and zinc, zinc displaces hydrogen from acid.
Example: Zinc + Sulphuric acid = Zinc sulphate + Hydrogen
            Zn + H2SO4 = ZnSO4 + H2

Acid-Base Reaction (Neutralization Reaction):

In this type of reaction two chemical substances react in which one is acid and the other base. They react in such a way that both lose their properties and new substances are formed with different properties than that of reactants. The new products are always salt and water. The properties of acid and base are lost, i.e, they neutralize each other.
Example:Sodium hydroxide + hydrochloric acid = Sodium chloride + Water
                 NaOH + HCl = NaCl + H2O

Balancing a chemical equation

 Before we go on balancing chemical equations we should have sound knowledge about the chemical formulae of substances. The chemical equation may be word equation form or skeleton. Primarily there are two methods for balancing chemical equations. They are
a)  Hit and trial method
b)  Partial equation method

The method of balancing the chemical equation by Hit and trial method:

1)  Write word equation clearly.
2)  Write down the molecular formula of each chemical substance.
3)  Count the number of atoms or molecules on both sides.
4)  Increase the coefficients of the molecule which are not equal on both sides.
5)  Do not change the molecular formula.
6)  Write the balanced equation in molecular form.
Example:
Word equation:  Potassium chlorate = Potassium chloride + Oxygen.
Symbolic equation:    KClO3 = KCl + O (skeleton)
                                   KClO3 = KCl + 3O (atomic equation)
                                   2KClO3 = 2KCl + 3O2 (molecular equation)

Limitation of the Hit and Trial Method:

The hit and trial methods is very useful for balancing simple chemical equations but it has own drawbacks also. They are:
1)  It takes a very long time to balance complicated equations in which the same element occurs in a number of compounds. This will be evident if you try to balance the following skeleton equation.
                           Zn + HNO3 = Zn(NO3)2 + N2O + H2O
2)  In a number of cases the mechanism of the reaction is not clear. For example, if we write the action of chlorine or on sodium hydroxide in the cold, we have
                         NaOH + Cl2 = NaCl + NaClO + H2O
To equalize the hydrogen atoms, multiple NaOH by two. This gives us the balanced equation.                              2NaOH + Cl2 = NaCl + NaClO + H2O
It is cleared that balancing of the equations, is very easy but the mechanism of the reaction as to how chlorine gas, which is not an acid can react with NaOH- an alkali, to give NaCl and NaClO (Salt of hydrochloric acid and Hypochlorous acid respectively) is not clear.
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