What are the law of chemical combination ? Illustrate with example
Answers
1) Law of Conservation of Mass
French chemist, Antoine Lavoisier in 1789, studied this law. This law states that “In all physical and chemical changes, the total mass of the reactants is equal to that of the products” or “Mass can neither be created nor destroyed.”
We also refer to this law as the law of indestructibility of matter. The mass and energy are interconvertible but the total sum of the mass and energy during any physical or chemical change remains constant.
2) Law of Constant Composition or Definite Proportions
French chemist, J.L. Proust in 1799, discovered this law. It states that “A chemical compound is always found to be made up of the same elements combined together in the same fixed proportion by mass”.
For example, a sample of pure water from various sources or any country is always made up of only hydrogen and oxygen. These elements are always in the same fixed ratio of 1:8 by mass. We can prepare a sample of carbon dioxide in the laboratory. We can do this in various ways like:
Heating limestone
Burning coal in air
The action of dilute hydrochloric acid on marble
Heating sodium carbonate
However, we will always find that it contains the same elements, carbon, and oxygen, in the same fixed ratio of 3:8 by mass.
Limitation of this Law
It is not applicable if an element exists in different isotopes which may be involved in the formation of the compound. The elements may combine in the same ratio but the compounds formed may be different.
3) Law of Multiple Proportions
When two elements combine to form two or more chemical compounds, then the masses of one of the elements which combined with a fixed mass of the other, bear a simple ratio to one another. For eg, Carbon combines with oxygen to form two compounds namely carbon dioxide and carbon monoxide.
In carbon dioxide, 12 parts by mass of carbon combine with 32 parts by mass of oxygen while in carbon monoxide, 12 parts by mass of carbon combine with 16 parts by mass of oxygen. The masses of oxygen which combined with a fixed mass of carbon in carbon monoxide and carbon dioxide are 16 and 32. These masses of oxygen bear a simple ratio of 16:32 or 1:2 to each other.
Taking another example of the Compounds of Sulphur and oxygen, we see something similar. The element sulphur also forms two oxides Sulphur dioxide and sulphur trioxide. In sulphur dioxide, 32 parts by mass of Sulphur combine with 32 parts by mass of oxygen. On the other hand, in sulphur trioxide, 32 parts by mass of Sulphur combines with 48 parts by mass of oxygen.
The masses of oxygen which combined with the fixed mass of sulphur in the two oxides are 32 and 48. These bear a simple ratio of 32:48 or 2:3 to each other.
4) Law of Reciprocal Proportion
This law was put forward by Richter in 1792. It states that “The ratio of masses of 2 elements, A and B which combines separately with a fixed mass of the third element C is either the same or some multiple of the ratio of the masses in which A and B combine directly with each other”.
For example, the elements carbon and oxygen combine separately with the third element hydrogen to form methane and water. However, they combine directly with each other to form carbon dioxide. In methane, 12 parts by mass of carbon combine with 4 parts by mass of hydrogen. In water, 2 parts by mass of hydrogen combine with 16 parts by mass of oxygen.
The masses of carbon and oxygen which combine with a fixed mass of hydrogen are 12 and 32 ie.they are in the ratio 12:32 or 3:8. In carbon dioxide, 12 parts by mass of carbon combine directly with 32 parts by mass of oxygen ie they combined directly in the ratio of 12:32 or 3:8. This is exactly the same as the first ratio.
5) Gay Lussac’s Law of Gaseous Volume
When gases react together they always do so in volumes which bear a simple ratio to one another and to the volume of the products, if these are also gases. This holds true provided all measurements of volumes are done under similar conditions of temperature and pressure.