Chemistry - Laws of Chemical Combination

Laws of Chemical Combination : 

The elements combine with each other and form compounds. This process is governed by five basic laws discovered before the knowledge of molecular formulae.

Law of conservation of mass : 

Antoine Lavoisier (1743-1794) a French scientist is often referred to as the father
of modern chemistry. many combustion experiments, namely, burning of phosphorus and mercury, both in the presence of air. Both resulted in an increase in weight. After several experiments he found that the weight gained by the phosphorus was exactly the same as the weight lost by the air. He observed that,

Total mass of reactants = Total mass of products

When hydrogen gas burns and combines
with oxygen to yield water, the mass of the
water formed is equal to the mass of the
hydrogen and oxygen consumed. Thus, the
law of conservation of mass states that 'mass
can neither be created nor destroyed.'

Law of Definite Proportions :

French chemist, Joseph Proust performed
experiments on two samples of cupric
carbonate. One of the samples was natural in
origin and the other was a synthetic one. He
found that the composition of elements present
in it was same for both the samples as shown
below :

This led Joseph Proust to state the law of
definite proportion as follows :
'A given compound always contains exactly
the same proportion of elements by weight.'
Irrespective of the source, a given compound
always contains same elements in the same
proportion. The validity of this law has been
confirmed by various experiments. This law
is sometimes referred to as Law of definite
composition.

Law of multiple proportions :

This law was proposed by John Dalton in
1803. It has been observed that two or more
elements may form more than one compound.
Law of multiple proportions summarizes
many experiments on such compounds. When
two elements A and B form more than one
compounds, the masses of element B that
combine with a given mass of A are always
in the ratio of small whole numbers.

For example, i. Hydrogen combines with oxygen to form two compounds, namely water and hydrogen peroxide.

Gay Lussac Law of Gaseous Volume :

This law was put forth by Gay Lussacin 1808. The law states that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume, provided all gases are atsame temperature and pressure.

Illustration :

i. Under the same conditions of temperature and pressure, 100 mL of hydrogen combines with 50 mL of oxygen to give 100 mL of water vapour.
Hydrogen (g) + Oxygen (g) ----> Water(g)
100 mL                  50 mL               100 mL
(2 vol)                   (1 vol)                  (2 vol)

Thus, the volumes of hydrogen gas and oxygen
gas which combine together i.e. 100 mL and
50 mL producing two volumes of water vapour
which amounts to 100 mL bear a simple ratio
of 2:1:2

ii. Under the same condition of temperature
and pressure.
1 L of nitrogen gas combines with 3 L of
hydrogen gas to produce 2 L of ammonia gas.

Nitrogen (g) + Hydrogen (g) ----> Ammonia(g)
1 L                            3 L                          2 L
(1 vol)                  (3 vol)                     (2 vol)

Thus, the volume of nitrogen gas and hydrogen gas which combine together  i.e.  1 L and 3 L and volume of ammonia gas produced i.e. 2 L bear a simple ratio of 1:3:2

Avogadro Law :

In 1811, Avogadro proposed that equal volumes of all gases at the same temperature and pressure contain equal number of molecules.

If we consider the reaction of hydrogen and oxygen to produce water vapour.

Hydrogen (g) + Oxygen (g) ----> Water (g)
100 mL                50 mL               100 mL
(2 vol)                  (1 vol)                 (2 vol)
(Gay Lussac Law)

2n molecules n molecules 2n molecules
(Avogadro law)

2 molecules 1 molecule 2 molecules
We see that 2 volumes of hydrogen
combine with 1 volume of oxygen to give 2
volumes of water vapour, without leaving any
unreacted oxygen. According to Avogadro
law, if 1 volume contains n molecules, then
2n molecules of hydrogen combine with n
molecules of oxygen to give 2n molecules of
water.

Two volume of hydrogen react with one volume of oxygen to give two volumes of water vapour

Therefore, 2 molecules of hydrogen
gas combine with 1 molecule of oxygen to
give 2 molecules of water vapour. Avogadro
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.

Dalton's Atomic Theory :

In 1808, Dalton published ''A New System of chemical philosophy'' in which he proposed the following features, which later became famous as Dalton's atomic theory.

1. Matter consists of tiny, indivisible particles
called atoms.

2. All the atoms of a given elements have
identical properties including mass. Atoms
of different elements differ in mass.

3. Compounds are formed when atoms of
different elements combine in a fixed ratio.

4. Chemical reactions involve only the
reorganization of atoms. Atoms are neither
created nor destroyed in a chemical reaction.
Dalton's theory could explain all the laws of
chemical combination.

Atomic and molecular masses :

You know about the terms atoms and molecules. Thus it is appropriate here to understand what we mean by atomic and molecular masses.

Atomic Mass :

Every element has a characteristic atomic mass. Atomic mass is the mass of an atom. It is actually very very small.
For example, the mass of one hydrogen atom
is 1.6736 × 10^-24 g. This is very small quantity
and not easy to measure.
In the present system, mass of an atom
is determined relative to the mass of a carbon
- 12 atom as the standard and this has been
agreed upon in 1961. In this system, an atom
of carbon-12 is assigned a mass of exactly
12.00000 atomic mass unit (amu) and all
other atoms of other elements are given a
relative atomic mass, to that of carbon - 12.
The atomic masses are expressed in amu.
One amu is defined as a mass exactly equal
to one twelth of the mass of one carbon-12
atom. Later on the exact value of atomic mass
unit in grams was experimentally established.

Recently, amu has been replaced by
unified mass unit called dalton (symbol 'u' or
'Da'), 'u' means unified mass.

Average Atomic Mass :

Many naturally occuring elements exist as mixture of more than one isotope. Isotopes have different atomic masses. The atomic mass of such an element is the weighted average of atomic masses of its isotopes (taking into account the atomic masses of isotopes and their relative abundance i.e. percent occurrance). This is called average atomic mass of an element.

For example, carbon has the following three
isotopes with relative abundances and atomic
masses as shown against each of them.