April 07, 2012

Nuclear Chemistry Five Marks

1. Differentiate between chemical reaction and nuclear reaction. Or Distinguish chemical reactions from nuclear reactions.
Chemical reactions
Nuclear reactions
Involve some loss, gain or overlap of outer orbital electrons of the reactant atoms.
Involve emission of alpha, beta and gamma particles from the nucleus.
Balanced in terms of mass only
Balanced in terms of both mass and energy.
Energy changes are very less when compared with nuclear reactions.
Energy changes are far exceeding
when compared with chemical reactions.
Energy is expressed in kJ / mol.
Energy is expressed in MeV / individual nucleus.
No new element is produced since nucleus is unaffected.
New element / isotope may be
2. Distinguish between nuclear fusion reactions and nuclear fission reactions.
Both, nuclear fusion and nuclear fission reactions lead to the production of new nuclei, there are some basic differences between the two.
Nuclear Fission Reaction
Nuclear Fusion Reaction
Splitting of a heavy nucleus into two or more light nuclei
Combination of two light nuclei to form a heavy nucleus
Fission reactions can be spontaneous.
Fusion reaction can never be spontaneous.
Fission reaction can be started by bombarding one nuclei with high energy on the other nuclei.
Fusion reaction can be started by increasing the temperature of the nuclei to be fused.
Takes place at room temperature
Requires a very high temperature of 108 K
 It is a chain reaction
It is not a chain reaction
It is accompanied by emission of neutrons
It is not accompanied by emission of neutrons
Nuclear fission can be sustained and controlled in practical situations.
Nuclear fusion has not been sustained in the laboratory conditions.
Comparatively less amount of energy is released
Enormous amount of energy is released
Fission reaction can be controlled and the energy released can be used to generate electricity
Fusion reaction cannot be controlled and hence the energy released cannot be used to generate electricity
Nuclear fission has been used for making bombs as well as power generation
Nuclear fusion as yet has been used for making hydrogen bombs only.
 It leaves behind radioactive wastes
It does not leave behind any radioactive wastes
3. Explain nuclear fission reaction with an example.
In the fission process, the heavy nucleus absorbs a neutron and forms an unstable compound nucleus. The compound nucleus then breaks up more or less in the middle to give fission product.
A typical example of the fission process in the fission of uranium by neutrons is explained by the following equation.
92U235 + 0n1  56Ba141 + 3 0n1 + 200 MeV
Further, the neutrons released (say three) from the fission of first uranium atoms can hit three other uranium atoms. In this way a chain reaction is set up resulting into the liberation of an enormous amount of energy. In the case of nuclear fission, 92U236 formed breaks up in several ways.                                             56Ba140 + 36Kr93+ 3 0n1
92U235 + 0n1  92U 236    54Xe144 + 38Sr90+ 2 0n1
                                                       55Cs144 + 37Rb90+ 2 0n1
4. Explain nuclear reactions that take place in sun.
It has been estimated that the sun is giving out energy equally in all possible directions at the rate of 3.7 × 1033 ergs/sec. The energy of the sun is supposed to arise from the fusion of hydrogen nuclei into helium nuclei which in going on inside it all the time.
The various fusion reactions taking place in the sun are as follows:
Proton - proton chain reaction:
1H1 + 1H1 1H2 + +1e0 + energy
1H2 + 1H1 2He3 + energy
2He3 +1H1 2He4 + +1e0 + energy
The overall reaction, therefore, may be written as:
4 1H1 2He4 + 2 +1e0 + energy
5. Explain Radio carbon dating. Or Write a note on / briefly about radio carbon dating.
Radio carbon dating is based on the fact that 6C14, radioactive isotope of carbon is formed in the upper atmosphere by reaction with neutrons (from cosmic rays).
7N14 + 0n1  6C14 + 1H1
The C14 atoms thus produced are rapidly oxidised to 14CO2
which in turn is incorporated in plants as result of photosynthesis. Animals too consume C14 by eating plants.
On death, organisms cease to take in fresh carbonations. Carbon-14 begins to decay.
6C14 7N14 + -1e0
5700 years a fossil (plant or animal) will lose half the amount of Carbon-14 present in its living state.
Therefore by knowing either the amount of C14 or the number of β-particles emitted per minute per gram of carbon at the initial and final stages, the age of carbon material can be determined by the following equation.

Uses of Radio carbon dating
1. For correlating facts of historical importance.
2. In understanding the evolution of life and
3. In understanding the rise and fall of civilizations or used to determine the age or period.
Explain the principle in the function of hydrogen bomb. Or Explain the principle underlying the function of hydrogen bomb.
The highly destructive hydrogen bomb is also based on the fusion reactions of hydrogen to form helium producing large amount of energy. Hydrogen bomb consists of an arrangement for nuclear fission in the centre surrounded by a mixture of deuterium (1H2) and lithium isotope (3Li6). Fission reaction provides the high temperature necessary to start the fusion.
i) Fission Heat + Neutrons
ii) 3Li6 + 0n1  1H3 + 2He4 + 4.78 MeV
     1H2 + 1H3  2He4 + 0n1 + 17.6 MeV
7. Explain the uses of radioactive isotopes with examples.
Answer: Q.No. 8 & Q.No. 9
8. How are radioactive isotopes useful in medicine? Or List the medicinal uses of radioactive isotopes.
Radioactive isotopes which are useful in medicine
Isotope                        Use
1H3                 Tritium                         Measure water content of the body
6C11               Carbon - 11                 Brain scan
6C14               Carbon - 14                 Radio immunology
53I131             Iodine - 131                Diagnosis of damaged heart muscles and hyper thyroidism
80Hg197         Mercury - 197             Kidney scan
15P32             Phosphorous - 32        Detection of eye tumours
26Fe59           Iron - 59                      Diagnosis of anemia
27Co60           Cobalt - 60                  Treatment of cancer
11Na24           Sodium - 24                Location of blood clots and circularity disorders
79Au198         Gold - 198                   Curing of cancers
9. Mention the use of radio isotopes in the field of a) study of hydrolysis of ester, b) mechanism of photosynthesis in plants.
a) Study of hydrolysis of ester
By labelling oxygen, the mechanism of ester hydrolysis can be studied by using water labelled with O18. The hydrolysis of an ester by water enriched with radioactive oxygen is indicated as:

Therefore it is the acid and not alcohol produced which is radioactive confirming the above mechanism.
b) Mechanism of photosynthesis in plants
A small quantity of Radioactive C*O2 containing radioactive oxygen O18 is mixed with ordinary carbondioxide and the process is carried out. It has been found that oxygen gas evolved along with sugar formation is non-radioactive. Therefore O2 produced comes from water and not from carbondioxide. So the correct mechanism is as follows.

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