Stoichiometry: Definition and Laws | Chemistry Grade XI

Stoichiometry

Stoichiometry: Definition and Laws | Chemistry Grade XI

Stoichiometry: Definition and Laws

STOICHIOMETRY:
The branch of chemistry which deals with the calculation of mass and volume relationship among reactants and products of a chemical reaction is called stoichiometry.
 
LAWS OF STOICHIOMETRY:
The chemical combination is governed by basic 5 laws which is known as law of chemical combination.
 
1. Laws of conservation of mass:
This law was given by Antoine Lavoisier in 1780. Law of conservation of mass states that "the total sum of mass of reactant is equal to the total sum of mass of products".
 
Let's take a general chemical reaction
A+B  =     C+D
Let W1, W2, W3 and W4 are the mass of A, B, C and D respectively.
 
According to the law of conservation of mass,
W1+ W2= W3 + W4

Experimental Verification of Laws of Conservation of mass: To verify the law of conservation of mass experimentally, Landolt took a certain mass of silver nitrate solution in one limb and certain mass of sodium chloride solution in the next limb in Landolt tube. The tube was inverted and the reaction took place with the formation of white precipitate. Then, the weight of the reactants system was measured. There was no change in weight. This proved the law of conservation of mass.

AgNO3 + NaCL ------> AgCL + NaNO3 

 2.Law of definite proportion or law of constant composition:
Law of definite proportion states that a particular compound contains same elements which combine with each other in a fixed proportion by mass regardless of origin or mode of formation.
 
Illustration
Lets take example of water. Water can be obtained from different sources like tap, underground, river, lake, ocean, etc. Water can also be synthesized in the laboratory. Whatever be the source of pure water, the water molecule is composed of hydrogen and oxygen. In water, two parts of oxygen by mass is combined with sixteen part of oxygen by mass, that means, the ratio of hydrogen and oxygen by mass in water is 2:16 =1:8

The carbondioxide gas can be prepared by;

i. Oxidation of carbon:
C + O2 -------> CO2
 
ii. Decomposition of CaCO3 ( Calcium Carbonate):
Ca CO3 --------> CO2 + CaO

The carbondioxide gas obtained from the above reactions contain the same element i.e. C and O [ Carbon and Oxygen] and the ratio of C and O in carbondioxide by mass is 12:32= 3:8.

3. Law of multiple proportion:
This law  was developed by John Dalton in 1803. It states that, "when one element combines with another element to give two or more than two different compounds. Then the masses of one element which combines with constant mass of another element bears the simple whole-number ratio to one another.

ILLUSTRATION

i. Lets take  example of hydrogen and oxygen, hydrogen reacts with oxygen in two different ways to give different compounds.

law of multiple proportionsIn H2O,  2 parts by mass of H combines with 16 parts mass of O

In H2O2, 2 parts by mass of H combines with 32 parts by mass of O

Here, the ratio of masses of oxygen which combines with constant mass of hydrogen is 16:32 = 1:2

ii. Nitrogen combines with oxygen in different ways and gives different oxides of nitrogen.

  1. N2O: In N2O, 28 parts by mass of N combines with 16 parts by mass of oxygen. 14 parts by mass of N combines with 8 parts by mass of oxygen.
  2. NO: In NO, 14 parts by mass of nitrogen combine with 16 parts by mass of oxygen.
  3. NO2: In NO2,  14 parts by mass of N combines with 32 parts by mass of oxygen.
  4. N2O3: In N2O3, 28 parts by mass of N combines with 48 parts by mass of oxygen. In N2O3, 14 parts by mass of N combines with 24 parts by mass of oxygen.
  5. N2O5:  In N2O5, 28 parts by mass of N combines with 80 parts by mass of oxygen. In N2O5, 14 parts by mass of N combines with 40 parts by mass of oxygen.
The ratio of masses of oxygen which combines with a constant mass of nitrogen = 8:16:32:24:40 = 1:2:4:3:5
 
Hence, this way, the law of multiple proportion is verified.
 
4. Law of reciprocal proportion or law of equivalent proportion:
This law was developed by Ritcher in 1972 AD. This law states that when two different elements combine separately with the same mass of a third element, the ratio in which they do so will be same or simple multiple of the ratio in which they combine with each other.

ILLUSTRATION

i. Lets take three elements O, S and H. Sulphur and oxygen combine with hydrogen separately to give H2S and H2O respectively. Similarly, sulphur combines with oxygen to give SO2.Law of equivalent proportion

In H2S, 2 parts by mass of H combines with 32 parts by mass of S.
In H2O, 2 parts by mass of H combines with 16 parts by mass of O.

The ratio of masses of sulphur and oxygen which combines with same mass of hydrogen is 32:16 i.e. 2:1

Again,

In SO2, 32 parts by mass of sulphur combines with 32 parts by mass of oxygen. The ratio of masses of sulphur and oxygen is 32:32 i.e. 1:1

Here, 1/1 * 2 = 2/1

Here, ratio 2:1 is a simple multiple of the ratio 1:1. So, this example illustrates law of reciprocal proportion.

 2.        

In CH4, 4 parts by mass of hydrogen combines with 12 parts by mass of carbon.
In CH4, 2 parts by mass of hydrogen combines with 6 parts by mass of carbon.
In H2O, 2 Parts by mass of hydrogen combines with 16 parts by mass of oxygen.
 
The ratio of masses of carbon and oxygen which combines with same mass of hydrogen is 6:16 i.e. 3:8  ---- (i)
 
Again, in CO2, 12 parts by mass of carbon combines with 32 parts by mass of oxygen.
The ratio of masses of carbon and oxygen is 12:38 = 3:8  ------- (ii)
Here, the ratio (i) and (ii) are same. hence this result illustrates the law of equivalent proportion.
 
GAY-LUSSAC’S LAW OF GASEOUS VOLUME
This law was developed by Gay-Lussac in 1808 AD. This law states that when gases combine with each other, they do so in volume which bears the simple whole number ratio to one another and to the volume of the product if these are gaseous, provided all the measurement being made under the same condition of temperature and pressure.
 
ILLUSTRATION
When hydrogen gas combines with chlorine gas, it gives hydrogen chloride gas( HCl).
H2     +    Cl2 --------> 2HCl
(g)          (g)                 (g)
1 vol  +  1 vol              2 vol
 
ratio= 1:1:2
The ratio of reactants and product by volume in above reaction bears simple whole number i.e. 1:1:2.

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