CHEMISTRY ADDA: Laws of chemical combination and Landolt Experiment ( IX Atoms and Molecules)

How and why elements combine and what happens when they combine. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

(a) The Law of conservation of mass was stated by Antoine L. Lavoisier in 1785 as” Mass can neither be created nor destroyed in a chemical reaction” [The Law of conservation of mass is the 2^nd postulate of Dalton's atomic theory. It states that Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.]

Example:
ð Water forms by the union of hydrogen and oxygen. If we weigh the reactants (hydrogen and oxygen) before the chemical reaction, we find the weight of the product (water) equal to the combined weight of reactants.
ð The weight of iron increases on rusting. The increase in weight is equal to the weight of oxygen added to iron.

ð Carbon combines with Sulphur to form Carbon disulphide. The mass of reactants i.e. carbon and sulphur is same mass of products (carbon disulphide).

Carbon + Sulphur -----➜ Carbon DiSulphide

C + S -----➜ C₂S

1g + 5. 34g = 6.34 g

LHS = RHS

The verification of the Law of conservation of mass by Landolt Experiment

H. Landolt was German Chemist. He proved the law of conservation of mass by using an H-shaped glass tube. He filled silver nitrate in limb A and hydrochloric acid in limb B. The tube was sealed and weighed before the chemical reaction. The reactants were mixed by inverting and shaking the tube. A white precipitate of silver chloride was formed along with Sodium nitrate . The tube was weighed again. He found that there was no change in weight during the following chemical reaction.

AgNO₃ + NaCl ® AgCl₂ (white precipitate) + NaNO₃

Limitations [Is there any exception to law of conservation of mass?]

In all the chemical reactions, energy is evolved or absorbed which would be at the expense of change in mass. In ordinary chemical reactions, this change in mass is so small that it cannot be registered on the most sensitive balance. This suggests that some matter of the reaction mixture gets converted into energy such as light, heat etc. Thus mass and energy are interconvertible. The mass is converted to energy by Einstein’s relation E = mc².

(b) The law of constant proportions which is also known as the law of definite proportions was stated by Proust in 1799 as “In a chemical substance the elements are always present in definite proportions by mass”.

[The Law of constant proportions is the 6^th postulate of Dalton's atomic theory. The relative number and kinds of atoms are constant in a given compound.]

E.g. In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

if the element ‘A’ and ‘B’ combine chemically to form the compound AB, then in whatever manner AB is formed, it is always composed of same two elements ‘A’ and ‘B’ combined together in the same fixed ratio or proportion by weight.
For example: Sulphur dioxide can be obtained b following sources:

         (i).     Sulphur is burnt in air,
                   S               +    O₂---------®   SO₂
         (ii).    Copper is heated with conc. sulphuric acid
                   Cu           +   2H₂SO₄       ---------®  CuSO₄   +   2H₂O   +   SO₂
         (iii). Dilute hydrochloric acid is added to sodium bisulphate
                   NaHSO₃   +   HCl            ----------®   NaCl   +   H₂O   +   SO₂
In each case, The ratio of sulphur and oxygen in the sulphur dioxide obtained is of 32 : 32 or 1 : 1 by mass.

The Law of Multiple Proportions
when two elements A and B combine to form more than one compounds, then the weight of one is constant and the other has a simple ratio. [The Law of Multiple Proportions is the third postulate of Dalton's atomic theory. It states that the masses of one element which combine with a fixed mass of the second element are in a ratio of whole numbers.]
E.g. Two different compounds are formed by the elements carbon and oxygen.

C (12gm) + ½ O₂ (16gm) ---------® CO (28gm)

C (12gm) + O₂ (32gm) ---------® CO₂ (44gm)

Here, 12 gm of carbon combine with 16g and 32gm of Oxygen to form Carbon monoxide and Carbon dioxide respectively. The ratio of oxygen combining with 12 gm of Carbon is 16: 32 or, 1:2 which is in a simple ratio.

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