8.Chemical Reaction

Chemical reaction is the change in both physical and chemical properties brought about by the addition, decomposition or displacement of an atoms or group of atoms.

Chemical equation is the symbolic and short representation of chemical reaction.

Nitrogen + Hydrogen  Ammonia

N2  + H2-  NH3 ↑

Here, N2   and   H2   are reactants and NH3   isthe product.

If the total number of atoms in the reactants and products are same then the  chemical reaction is said to be balanced chemical equation.

For example:

4Al + 3O2      →         2Al2O3

H2 + O2 → H2O
Here, the number of oxygen in the reactant is not equal to the oxygen in product side. So, to balance the equation we have to balance the numbers on the reactant as well as product side.
2H2 + 02 → 2H20
Here, all the elements in the product side are equal to the elements in the reactant side.

Factors influencing the chemical reaction

a. Simple Contact: When two elements are brought in contact they react. This is the simple condition under which chemical reaction takes place.

2Na + Cl2   → 2NaCl

b. Light: Light simulates the reactants for chemical reaction. When hydrogen react violently with chlorine in presence of diffused sunlight hydrogen chloride is produced.

H2 + Cl2   →Sunlight 2HCl

c. Temperature: Increase in temperature increases the rate of almost all chemical reactions while decrease in temperature decreases the reaction rate. The increase in temperature increases the kinetic energy of molecules. It causes the breaking of bonds n the reactant molecules and reaction takes place at a high speed.

d. Concentration: The rate of reaction increases with the increase in the concentration of reactants. This means that the rate of reaction is directly proportional to the concentration of the reactants. For e.g. when dilute HCl is poured into a piece of calcium carbonate there is effervescence with the evolution of CO2.The effervescence is brisk when concentrated HCl is added. So it increases the rate of chemical reaction between CaCO3 and HCl.

e. Pressure: Pressure is another important factor influencing the chemical reaction. With the application of high pressure nitrogen and hydrogen combined to form ammonia.

N2 + H2-  NH3

 f. Catalyst: A catalyst is defined as a chemical substance that changes the rate of chemical reaction itself without undergoing any permanent chemical change during the course of chemical reaction. They are of two types; they are positive catalyst and negative catalyst.

Manganese dioxide acts as a positive catalyst in the decomposition of hydrogen peroxide without heating. This is because it increases the decomposition rate of hydrogen peroxide. Also glycerin acts as a negative catalyst as it decreases the rate of reaction during the decomposition of hydrogen peroxide into water and oxygen.

2H2O2    →MnO2     2H2O + O2

Types of chemical reaction

There are 4 types of chemical reaction mainly. They are as follows:

a. Addition or Combination reaction.
Examples:
Sodium combines with chlorine to form sodium chloride.
2Na + Cl2 → 2NaCl
When iron fillings are heated with sulphur, ferrous sulphide is produced.
Fe + S → Fes

 b. Decomposition or dissociation reaction.
Examples:
When lead oxide is heated, it decomposes into lead and oxygen.
2PbO → 2Pb + O2
When sodium chloride is electrolyzed at a high temperature,it forms sodium and chlorine.
2NaCl → 2Na + Cl2

c. Displacement Reaction
Examples:
Single Displacement
Zn + H2SO4 → ZnSO4 + H2
In this reaction, H2 is replaced by Zn.
Double Displacement
•AgNO3 + NaCl → AgCl + NaNO3
In this reaction, Na is replaced by Ag and NO3 is replaced by Cl.

 Neutralization or Acid Base Reaction
Examples:
When NaOH is reacted with HCl, then there is formation of NaCl and H2O
NaOH + HCl → NaCl + H2O
When copper oxide reacts with hydrochloric acid then copper chloride is formed.
CuO + 2HCl → CuCl2 + H2O

The sum of atomic weights of all the atoms of the same or different elements present in the molecule is called molecular weight.

The molecular weight of a substance (elements or compounds) is expressed in term of gram, which is called its gram molecular weight. It is also called gram mole or only mole.

One mole of a substance is defined as a collection of 6.023*1023 particles of that substance.

6.023*1023 = 1 Avogadro’s number