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 2nd 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 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
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 6th 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:
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.
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.
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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