Wednesday, May 20, 2020

Laws, Dalton's & Modern Atomic Theory


LAWS OF CHEMICAL COMBINATION:
There are five laws of chemical combination. Out of the five four deals with mass relationships while the fifth one deals with the volume of reacting gases
1. LAW OF CONSERVATION OF MASS:
 This law was given by Antoine Lavoisier in 1789. It states that matter can neither be created nor be destroyed. In other words, the total mass,i.e. the sum of mass of reacting mixture and products formed remains constant.
For example: If we consider a chemical reaction
Ca(OH)2 + CO2---- CaCO3 + H2O
Given the masses of Ca,O, H and C are 40u, 16u, 1u and 12u respectively.
we will first find the molecular masses of each of the terms in the given reaction:
Ca(OH)2: 40+32+2= 74
CO2: 12+32= 44
CaCO3= 40+12+48 =100
H2O= 2+16= 18
Substituting these values into the equation,
74+44= 100 + 18
118= 118
Therefore, this shows that law of conservation of mass is followed by the given equation. experimental verification of law of Conservation of mass:
This law was verified by Landolt. He took the solutions of sodium chloride(NaCl) and silver nitrate ( AgNO3) in two limbs of a 'H' shaped tube. This tube was sealed and weighed. After this the two solutions were mixed thoroughly by shaking the tube. As a result, the reaction occurred between AgNO3 and NaCl and a white ppt. of AgCl is formed as:
AgNO3 + NaCl----- AgCl + NaNO3
                       (white ppt)
fig.1. Landolt experimetal setup for verification law of conservation of mass.

After the reaction took place the tube was again weighed. The weight remained practically unchanged. This verifies the law of conservation of mass.
2. LAW OF CONSTANT PROPORTIONS:
This law is given by Joseph Proust in 1799. It states that when a compound is broken ,the masses of the constituent elements remain in the same proportion. or in the chemical compound, the elements are always present in definite proportions by mass.
This means each compound has the same elements in the same proportions, regardless of where the compound was obtained.
For example: A person living in Australia sent 100 ml sample of CaCO3 (calcium carbonate)to a person living in India. The person living in India made his own sample of 200 ml and compared it to his friend's sample.
now the question is which of the two compounds has a greater ratio of Ca:C
The answer to this is both contain equal ratios of Ca and C. This is explained in the law of constant proportions.
3. LAW OF MULTIPLE PROPORTIONS:
This law was proposed by John Dalton. It states that when two elements form two or more compounds between them, the ratio of the masses of the second element in each compound can be expressed in the form of small whole numbers.
For example:
Carbon combines with oxygen to form two different compounds . One is the most common gas CO2 and the other is CO.
The question is Do they obey the law of multiple proportions?
The answer to this question is yes they do obey this law let us see further:
We know that the mass of carbon is 12 u and the mass of oxygen of 16u . so, we can say that 12 g of carbon combines with 32 g of oxygen to form CO2. Similarly, 12 g of carbon combines with 16g of oxygen to form CO.
So, the ratio of oxygen on the first and the second compound is 2:1 =32/16 =2 which is a whole number.
4. LAWS OF RECIPROCAL PROPORTIONS:
This law was proposed by Jeremias Ritcher in 1792. The law of reciprocal proportions states that when two different elements combine with same quantity of the third element, the ratio in which they will do so will be the same or a multiple of the proportion in which they combine with each other.
for example:
Oxygen and Sulphur react with copper to create copper oxide and copper sulfide respectively. Sulphur and oxygen also react with each other to form S02. Therefore,
In CuS , Cu: S = 63.5: 32
In CuO , Cu:0= 63.5: 16
 i.e.                       S:O = 32: 16
                                    = 2:1
now in SO2, 32:32= 1:1
Thus the ratio between the two ratios is the following:
2/1:1/1= 2:1
which is a simple multiple ratio.
5. GAY LUSSAC'S LAW OF GASEOUS VOLUMES:
This law was given by Gay Lussac in 1808. This law states that when gases are produced or combine in a chemical reaction, they do so in simple ratio by volume given that all gases are at same temperature and pressure. This law can be considered as another form of law of definite proportions. The only difference between these two laws of chemical combination is that GAY LUSSAC 's LAW is stated with respect to volume while law of definite proportions is stated with respect to mass.
Experimental verfication of Gay Lussac's law of combining volumes:
It has been experimentally found that 1 volume of hydrogen reacts with 1 volume of chlorine to give 2 volumes of hydrogen chloride as shown in fig(b) 
fig.2.

Thus, the volume ratio of hydrogen : chlorine : hydrogen chloride is 1:1:2. This is simple whole number ratio and is also in agreement with their molar ratios when they are involved in the reaction.
H2  + Cl2 ---- 2HCl .

DALTON'S ATOMIC THEORY:
The basic postulates of the theory are:
·         Matter is made up of extremely small indivisible and indestructible ultimate particles called Atoms.
·         Atoms of the same element are identical in all respects i.e. in shape, size, mass and chemical properties.
·          Atoms of different elements are different in all respects and have different masses and chemical properties.
·          Atom is the smallest unit that takes part in a chemical combinations.
·         Atoms of two or more elements combine in a fixed ratio to form compound atoms ( molecules)
·         Atoms can neither be created nor be destroyed during any physical or chemical change.
·         Chemical reactions involve only combination, separation or rearrangement of atoms.
Limitations of Dalton's atomic theory:
·         It could not explain the laws of chemical combination by mass but failed to explain the law in gaseous volumes.
·          It could not explain why atoms of different elements have different masses, sizes, valencies etc.
·         It could not explain why and how atoms of different elements combine with each other to form compounds or molecules.
·         It failed to explain the nature of forces that bind together atoms in a molecule.
·         It could not explain the difference between particle of an element and that of a compound.

MODERN ATOMIC THEORY:
The main modifications made in the Dalton’s atomic theory:
·                    ·     Atom is no longer considered to be indivisible:
It is found that atom is complex structure and is made up of subatomic particles such ad electrons, protons and neutrons.
·                   ·      Atoms of same elements may not be similar in all respects:
Atoms of same element may possess different relative masses. For example: we have two different types of atoms of Chlorinr with atomic masses 35 a.m.u and 37 a.m.u resp. Such atoms of same element having different atomic masses are called Isotopes.
·                   ·      Atoms of different elements may have similar one or more properties:
There are certain atoms of different elements which possess same relative mass. For example: atomic mass of Calcium and Argon is same (40 a.m.u) but their chemical properties are entirely different. Such atoms of different elements having same mass are called Isobars.
·                  ·      Atom is the smallest unit which takes part in chemical reactions:
An atom is composed of subatomic particles yet it is the smallest particle which takes place in chemical reactions.
·         The ratio in which the different atoms combine may be fixed and integral but may not always be simple:
For example: in sugar molecule C12 H22 O11) the ratio of C,H and O atoms is 12: 22: 11, which is not simple.
·                 ·       Atom of one element may be changed into atoms of other element:
For example: atoms of nitrogen can be changed into oxygen atoms by interaction with alpha rays. This process is called Transmutation.
·                ·       The mass of atom can be changed into energy:
According to Einstein’s equation
                  E=mc2
 E = energy
 m = mass
 c= velocity of light.                                                                     Mass and energy are interconvertible.


to be conti...