Study of Reactions

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The study of reactions is one of the important areas of Chemistry. There are many aspects of a reaction like the rate of reaction, energy absorbed or evolved, mechanism of a reaction that can be evaluated only with weight and volume relationships. A study of reactions is possible by understanding the concept of atoms and molecules and the definite laws and patterns regulating them.

Law of Conservation of Mass

This law was put forth by the French chemist A. Lavoisier (1774). It states that in 'any chemical reaction, the total mass of the system before and after the reaction is the same although its matter can undergo a physical change'.

If 'a' grams of A and 'b' grams of B react to give 'c' grams of C and 'd' grams of D, then,

Total mass of the reactants = Total mass of the products

(a + b) grams = (c + d) grams

Therefore, matter is neither created nor destroyed as a result of any chemical or physical change.

This law can be verified by a simple experiment. A solution of sodium chloride and silver nitrate are taken separately in the two limbs of an 'H' shaped tube. The tube is sealed and weighed precisely. The two reactants are made to react by inverting the tube. The following reaction takes place.

The whole tube is kept undisturbed for sometime so that the reaction is complete. When the tube is weighed again it is observed that:

Weight before the reaction = Weight after the reaction

This verifies the law of conservation of mass.

Law of Definite Proportions or Law of Constant Composition

This law was proposed by Louis Proust in 1799, which states that:

'A chemical compound always consists of the same elements combined together in the same ratio, irrespective of the method of preparation or the source from where it is taken'.

For example, water is always found to contain only two elements hydrogen and oxygen combined in a fixed ratio by mass (1:8) irrespective of its source. Similarly carbon dioxide is always found to contain carbon and oxygen in the mass ratio of 3:8. This law can be verified by studying the electrolysis of different samples of water into hydrogen and oxygen when it is found that the ratio of hydrogen and oxygen is 2:1 (by volume) and 1:8 (by mass).

However, this law does not hold good when the compound is obtained by using different isotopes of the combining elements and when the compounds are non stoichiometric.

Law of Multiple Proportions

Proposed by Dalton in 1803, this law states that:

'When two elements combine to form two or more compounds, then the different masses of one element, which combine with a fixed mass of the other, bear a simple ratio to one another'.

This law is evident when one element combines with another to give more than one compound of different stoichiometry e.g., nitrogen reacts with oxygen to form nitrous oxide (N₂O) and nitric oxide (NO). This law does not hold good when different isotopes are used to obtain different compounds.

This law can be verified by heating 1.00 g each of the oxide of copper CuO and Cu₂O in a current of hydrogen. Both the oxides react with hydrogen producing metallic copper. From the weight of the copper obtained, the respective weights of oxygen in the two compounds are determined. From this, the different weights of oxygen, which combine with the same weight of copper in the two compounds are calculated. Since these weights are found to bear a simple whole number ratio, the law is verified.

Law of Reciprocal Proportions or Law of Equivalent Proportions

This law was proposed by Ritcher in 1792. It states that 'when two different elements combine separately with the same weight of a third element, the ratio of the masses in which they do so will be the same or some simple multiple of the mass ratio in which they combine with each other'.

For example, oxygen and sulphur react with copper to give copper oxide and copper sulphide respectively. Suphur and oxygen also react with each other to give SO₂. Then,

In CuS, Cu:S = 63.5:32

In CuO, Cu:O = 63.5:16

S:O = 32:16

S:O = 2:1

Now in SO₂

S:O = 32:32

S:O = 1:1

Thus the ratio between the two ratios is

Gay Lussac's Law of Gaseous Volumes

This law put forward by Gay Lussac states that 'whenever gases react together under constant conditions of temperature and pressure, the volumes of the reacting gases as well as products (gases) bear a simple whole number ratio between themselves'.

For example, in the reaction,

The ratio of volumes of hydrogen, chlorine, and hydrogen chloride is 1:1:2 (a simple ratio).