LAWS OF CHEMICAL COMBINATION

      LAWS OF CHEMICAL COMBINATION

In this topic we will learn about the various laws that are involved during the formation of chemical reaction. The observation during the chemical changes, are made of certain generalisation These generalisations are known as laws of chemical combination. These are:

1. Law of conservation of mass 

2. Law of constant composition or definite proportions

3. Law of multiple proportions

4. Law of reciprocal proportions

5. Law of combining volumes

1. Law of Conservation of Mass

This law deals with the relation between the mass  of the reactants and the products during the chemical changes. It was postulated by a French the Antoine Lavoisier (Father of chemistry) in 1789. He performed chemical experimental studies for combustion reactions before concluding the law. 

Law of conservation of mass states that “ during any physical or chemical change, total mass of the products is equal to total mass of the reactants”.

In other words, matter can neither be created no destroyed during any physical or chemical change. Therefore, this law is also known as law of indestructibility of matter. Experimental verification of the law of conservation of mass.

The law can be verified with the help of Landolt’s experiment. Landolt’s took the solutions of sodium chloride (Na Cl) and silver nitrate (A g NO3) separately in two limbs of a H' shaped tube (known as Landolt's tube). The tube was sealed and weighed After weighing, the two solutions were mixed thoroughly by shaking the tube. As a result, the reaction occurred between silver nitrate and sodium chloride and a white ppt. of silver chloride is formed as:

          AgNO3 + Na Cl → Ag Cl + Na NO3 ↓

After the reaction, the tube was again weighed was observed that the weight remained practically unchanged. This verified the law of conservation of law of conservation of mass. 

2. Law of Constant Composition or Definite Proportions

The law of constant composition deals with the composition of various elements present in a compound. It was stated by a French chemist, Joseph Proust, in 1799.

 Law of constant composition states that a pure chemical compound always contains same elements combined together in the same definite proportion by weight, Proust worked with two samples of cupric carbonate; one of which was naturally occurring cupric carbonate and other was prepared in the laboratory. 

Limitations of Law of Constant Composition 

1. The law of constant composition does not hold good when a compound is obtained by using different isotopes of the combining elements. For example, when CO is formed from C-12 the ratio between C and O is 12:32. But when CO is formed from C-14 isotope. 

3. Law of Multiple Proportions 

This law was given by Dalton in 1803. Law of multiple proportions states that when two elements combine to form more than one compound, then the masses of one of the elements which combine with a fixed mass of the other element are in a simple whole number ratio.

Experimental Verification of Law of Multiple Proportions

Copper forms two oxides cuprous oxide (Cu,0) and cupric oxide (Cu O). 1 gram of each oxide of copper (Cu O and Cu, respectively) is heated in a current of hydrogen. Both the oxides react with hydrogen producing metallic copper. From the weight of copper obtained, the respective weights of oxygen in the two compounds are obtained. Then, the different weights of oxygen which combine with the same weight of copper in the two compounds are calculated. These weights are found to bear a simple whole number ratio. Thus, the law has been verified.

4. Law of Reciprocal Proportions or Law of Equivalent Proportions

This law was given by Richter in 1792. This law states that when two different elements combine separately with a fixed mass of a third element, the r in which they do so will be the same or some simple multiple of the ratio in which combine with each other. 

In other words, the mass ratio of two elements and B which combine with the fixed mass of separately, is either the same or some simple whole number multiple of the mass ratio in which A and B combine together. This law may be illustrated by the following examples :porous oxide (Cu) and f each oxide of copper heated in a current of react with hydrogen the weight of copper of oxygen in the two the different weights the same weight of are calculated. These and O which combine with the same weight of H e whole number ratio.

Consider three elements sulphur, oxygen and hydrogen. Both sulphur and oxygen separately combine to form hydrogen sulphide (H,S) and water (H,O) respectively. They also combine with each other to form sulphur dioxide (SO,) as shown: According to the law, the ratio of weights will either be same or a simple multiple of the ratio in which S and O combine with each other.

5. Gay Lussac’s Law of Combining Volumes

Gay Lussac’s performed a number of experiments on reactions involving gases and found that some regular exists between the volumes of the gaseous reactants and products. In 1808, he put forward a generalisation known as Gay Lussac's law of combining volumes. It states that when gases react together or produced in a chemical reaction, they do so in a simple ratio volume to one another and to the volumes of the products (if these are also gases) provided gases are at the same temperature and pressure.