Laws of Chemical
Combinations
Ø Law
of Conservation of Mass: Antoine Lavoisier established the Law of Conservation of Mass. It states
that matter can neither be created nor destroyed.
In other words, we can say
that during any physical or chemical change, the total mass of reactants is
equal to the total mass of products. For eg.
Consider the reaction 2H2
+ O2 → 2H2O
Here 4 g of H2 combines
with 32 g of O2 to form 36 g of water.
Total mass of reactants = 4 + 32 =
36g. Total mass of products = 36 g
Ø Law
of Definite Proportions: A given compound always contains
the same elements in the same proportion by mass.
For
Example: Carbon dioxide can be formed in the atmosphere by various methods like
respiration, burning of fuels, reaction of metal carbonates and bicarbonates
with acid etc. All these samples of CO2 contain only two elements Carbon and
Oxygen combined in a mass ratio 3:8.
Ø Law
of Multiple Proportions: 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.
Hydrogen
combines with oxygen to form two compounds – 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.
Ø Gay
Lussac’s Law: Under similar conditions of temperature
and pressure whenever gases combine together, they do so in terms of volume.
e.g., 2 H2 (g) + O2
(g) →
2 H2O (g)
2 vol
1 vol 2 Vol (at same T, P)
Ø Avogadro’s
Law:
Under similar conditions of temperature and pressure equal volume of all gases
contains equal number of molecules.
For example: If we take 10L each of NH3, N2, O2
and CO2 at the same temperature and pressure, all of them contain
the same number of moles and molecules.
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