The combination of elements to form compounds is governed by the following five basic laws.
Thus, irrespective of the source, a given compound always contains same elements in
1) Law of Conservation of Mass
It states that matter can neither be created nor destroyed.This law was put forth by Antoine Lavoisier in 1789. He performed careful experimental studies for combustion reactions for reaching to the above conclusion. This law formed the basis for several later developments in. chemistry Infact, this was the result of exact measurement of masses of reactants and products, and carefully planned experiments performed by Lavoisier.
2)Law of Definite Proportions
This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight. Proust worked with two samples of cupric carbonate— one of which was of natural origin and the other was synthetic one. He found that the composition of elements present in it was same for both the samples as shown below
|
|
% of copper
|
% of oxygen
|
% of carbon
|
|
Natural sample
|
51.35
|
9.74
|
38.91
|
|
Synthetic sample
|
51.35
|
9.74
|
38.91
|
the same proportion. The validity of this law has been confirmed by various experiments.
It is sometimes also referred to as Law of definite Composition.
3) Law of Multiple Proportions
This law was proposed by Dalton in 1803. According to this law, if two elements can
combine to form more than one compound, the masses of one element that combine with a
Hydrogen + Oxygen → Water
2g 16g 18g
Hydrogen + Oxygen → Hydrogen Peroxide
2g 32g 34g
Here, the masses of oxygen (i.e. 16 g and 32 g) which combine with a fixed mass of hydrogen
(2g) bear a simple ratio, i.e. 16:32 or 1: 2
4) Lussac’s Law of Gaseous Volumes
This law was given by Gay Lussac's in 1808. He observed that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volumep provided all gases are at
same temperature and pressure
Thus, 100 mL of hydrogen combine with 50 mL of oxygen to give 100 mL of water vapour.
Hydrogen + Oxygen → Water
100 mL 50 mL 100mL
Thus, the volumes of hydrogen and oxygen which combine together (i.e. 100 mL and
50 mL) bear a simple ratio of 2:1.Gay-Lussac’s discovery of integer ratio in volume relationship is actually the law of definite proportions by volume. The law of definite proportions, stated earlier, was with respect to mass. The Gay-Lussac’s law was explained properly by the work of Avogadro in 1811.
could explain the above result by considering the molecules to be polyatomic. If hydrogen
and oxygen were considered as diatomic as recognized now, then the above results are
easily understandable. However, Dalton and others believed at that time that atoms of the
same kind cannot combine and molecules of oxygen or hydrogen containing two atoms did
not exist. Avogadro’s proposal was published in the French Journal de Physidue. In spite
of being correct, it did not gain much support. After about 50 years, in 1860, first
international conference on chemistry was held in Karlsruhe, Germany to resolve various ideas.
At the meeting, Stanislao Cannizaro presented a sketch of a course of chemical philosophy
which emphasised the importance of Avogadro’s work.
It is sometimes also referred to as Law of definite Composition.
3) Law of Multiple Proportions
This law was proposed by Dalton in 1803. According to this law, if two elements can
combine to form more than one compound, the masses of one element that combine with a
fixed mass of the other element, are in the ratio of small whole numbers.
For example, hydrogen combines with oxygen to form two compounds, namely, water
and hydrogen peroxide.Hydrogen + Oxygen → Water
2g 16g 18g
Hydrogen + Oxygen → Hydrogen Peroxide
2g 32g 34g
Here, the masses of oxygen (i.e. 16 g and 32 g) which combine with a fixed mass of hydrogen
(2g) bear a simple ratio, i.e. 16:32 or 1: 2
4) Lussac’s Law of Gaseous Volumes
This law was given by Gay Lussac's in 1808. He observed that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volumep provided all gases are at
same temperature and pressure
Thus, 100 mL of hydrogen combine with 50 mL of oxygen to give 100 mL of water vapour.
Hydrogen + Oxygen → Water
100 mL 50 mL 100mL
Thus, the volumes of hydrogen and oxygen which combine together (i.e. 100 mL and
50 mL) bear a simple ratio of 2:1.Gay-Lussac’s discovery of integer ratio in volume relationship is actually the law of definite proportions by volume. The law of definite proportions, stated earlier, was with respect to mass. The Gay-Lussac’s law was explained properly by the work of Avogadro in 1811.
5) Avogadro law
In lumes1811, Avogadro proposed that equal volume of gases at the same temperature and
pressure should contain equal number of molecules. Avogadro made a distinction
between atoms and molecules which is quite understandable in the present times. If we consider again the reaction of hydrogen and oxygen to produce water, we see that two volumes of hydrogen combine with one volume of oxygen to give two volumes of water without leaving any unreacted oxygen. Note that in above the Fig. each box contains equal number of molecules. In fact, Avogadro
and oxygen were considered as diatomic as recognized now, then the above results are
easily understandable. However, Dalton and others believed at that time that atoms of the
same kind cannot combine and molecules of oxygen or hydrogen containing two atoms did
not exist. Avogadro’s proposal was published in the French Journal de Physidue. In spite
of being correct, it did not gain much support. After about 50 years, in 1860, first
international conference on chemistry was held in Karlsruhe, Germany to resolve various ideas.
At the meeting, Stanislao Cannizaro presented a sketch of a course of chemical philosophy
which emphasised the importance of Avogadro’s work.
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