Nuclear Fission for std 11 & 12 GSEB course Chemistry

Uranium-235 is highly unstable. When a neutron collides with a uranium-235 nucleus, the nucleus breaks up or gets fissioned into two smaller nuclei of comparable masses along with the liberation of a large amount of energy (Fig. 12.1). The product nuclei of fission reaction are called fission fragments.
The reaction in which a heavy nucleus splits up into two medium nuclei with the liberation of enormous amount of energy, is called nuclear fission.

Note  
The German scientists Otto Hahn and Fritz Strassmann discovered unclear fission in 1938.
Dr. H. J. Bhabha (1909-1966) an Indian physicist started the atomic energy era in India.
He is known as the architect of India’s atomic energy programme. His work was followed up by Dr. Vikram A Sarabhai (1919-1971) and other eminent Indian scientists. 

Nuclear reaction

Example : 

When uranium-235 atoms are bombarded with slow moving neutrons, fission takes place. It is represented as follows. 

Fission fragments need not always be barium and krypton. Sometimes, uranium splits upto into xenon and strontium and sometimes into lanthanum and bromine. 

The energy released by fission of 1 g of U-235 is equal to that due to combustion of 50 millions tons of coal; it is about 8.5 x 1010 J.
During fission, 2 or 3 neutrons, g -rays and enormous amount of heat energy and light energy are released. The fission fragments are radioactive and they decay with the emission of
  alpha-rays, beta-rays and gama- rays. 

CHAIN REACTION
During each fission of U-235, 2 or 3 neutrons are released. These neutrons may cause further fission. The reactions continue further and further in the form of a chain with a multiplying effect. Such a self sustained reaction is called a chain reaction (Fig. 12.2).
Chain reaction is a continuous unclear fission reaction.
In a chain reaction if fission neutrons are built upto a certain level and thereafter the number of fission producing neutrons, is kept constant, then it is called controlled chain reaction. In this case, energy is generated at a definite rate. Controlled chain reaction is the principle of a nuclear reactor.

Chain Reaction of nuclear energy

Critical size : 
It the size of the fissionable material is smaller than a certain minimum, fission neutrons escape through its surface and the chain reaction is not possible. At a particular size called critical size, fission neutrons are retained to continue the chain reaction. The minimum size of a fissionable material required to sustain fission chain reaction, is called critical size.
Atom bomb (Nuclear bomb)
An atom bomb consists of two separate pieces of fissionable material, Each of these is smaller than the critical size. When they are brought together, the resulting lump is much larger than the critical size. Therefore chain reaction builds up rapidly resulting in an explosion.
Atom bomb was dropped on the Japanese city of Hiroshima, on 6th Aug 1945. Nearly 50% of the city was destroyed killing about 80,000 people. Three days later another bomb was dropped on Nagasaki destroying half the city and killing thousands of people. One bomb hand done more damage than 20,000 tons of TNT. Unbelievable destruction and horror was unleashed.

Nuclear Energy for std 11 & 12 GSEB board

Review:
a) Energy is the capacity to do work.
b) Energy exists in different forms.
c) SI unit of work is joule.
d) Energy can be transformed from one form to another.
e) An isotope of an element X is represented by the symbol ZXA or AZX where Z is the atomic number and A is the mass number.

INTRODUCTION
Some isotopes are radioactive while others are not . Those which are not radioactive are stable while those which are radioactive are unstable.

Note:
The question arises as to why some isotopes are unstable.
The nucleons in a nucleus are bound together by forces called nuclear forces. Energy is required to separate the nucleons. The minimum energy required for the purpose is called binding energy. Stability is usually measured by binding energy per nucleon
[be per nucleon = (binding energy / mass numebr)]. Higher the binding energy per nucleon. greater the stability of the nucleus. 

MEANING OF NUCLEAR ENERGY

Any reaction that involves a change in the nucleus of an atom is called a nuclear reaction. The energy released during a nuclear reaction is called nuclear energy. This is sometimes called atomic energy.

Nuclear reaction
Transmutation of a nucleus and reaction between two nuclei are examples for nuclear reaction. It is totally different from a chemical reaction which involves only orbital electrons. Electrons have no role in nuclear reactions. A nuclear reaction is exothermic where as chemical reactions may be exothermic or endothermic.

Note:
Accurate measurements of nuclear masses using precision instruments such as mass spectrographs have shown that the mass of a nucleus is lesser than the sum of the masses of the constituent particles. The mass difference called Mass defect corresponds to binding energy.

Where does the nuclear energy come from ? 

During a nuclear reaction, the sum of the masses of the product (or resultant) particles is lesser than the sum of the masses reactant particles. This difference in mass is converted into energy. The conversion that takes place can be explained by Einstein’s mass-energy relation.

According to Einstein, mass and energy are interconvertible. The total of mass and energy is conserved. This is known as Einstein’s mass-energy relation. If a mass DM (The symbol D is used to indicate small quantity) is completely converted into energy DE, than 

DE = Mc2.

where c = velocity of light.

For example, if DM = 1 mg = 10-6 kg then DE =10-6(3x108)2

  = 9 *1010 J.

This is the basis on which energy released in a nuclear reaction may be explained.