Page 191 - Essentials of physical chemistry

Basic Chemical Kinetics 153

C. Deuteron bombardment

7 Li þ D ! Be þ n:
2
1
8
3 1 4 0
D. Neutron bombardment
28 1 28 1
0
1
14 Si þ n ! 13 Al þ H:
E. Electron bombardment

9 0 8 1 8 8 0 e:
3
4
0
3
4 B þ 1 e ! Li þ n followed by Li ! Be þ 1
We can see that an a-particle is a nucleus of He as the dication He . When electrons are given off
2þ
from a nuclear reaction they are called ‘‘b-particles’’ and electrons can be absorbed or emitted. In
some heavy elements, 1s electrons can sometimes be captured into a nucleus and combine with a
proton to form an additional neutron in the nucleus which reduces the number of protons and
1
1
changes the elements atomic number (Z)to(Z 1) as 1 0 e þ H ! n. Thus, for the purpose of
0
1
chemistry we can regard a neutron as the combination of an electron and a proton. While this has
been controversial in the past it is known that neutrons are unstable outside a nucleus and they decay
1 0 1
1
0
into an electron and a proton as n ! 1 e þ H with t 1=2 ¼ 10.3 min.
Of course we have not balanced the energy by this sort of analysis and in actual nuclear reactions
gamma rays are often emitted (particularly in neutron bombardment) to balance the energy and the
energy of fast emitted particles is part of the energy balance. Finally, as deduced by Lise Meitner,
the sum of the masses may not add up and missing mass has been converted into tremendous
amounts of energy often measured in ‘‘millions of electron volts’’ as MeV. The missing amounts of
mass are usually small enough that we can count the particles as integers in our simple ‘‘particle
balance.’’ Today nuclear physics is a maturing science but there are still some subatomic particles to
be detected experimentally. Even so, Lise Meitner is the ‘‘Mother of Fission’’ and the research
discoveries by the Curies (Marie, Pierre, and Irene) led to modern nuclear chemistry. An astute
student should note the connections between physics and chemistry in these discoveries.

PROBLEMS
90
7.1 An unstable isotope that can occur in fallout from a nuclear blast is Sr, which has a half-life of
38
29.1 years and has similar chemistry to Ca, so that when it falls on vegetation (grass),
herbivores (cows) can ingest it and it can enter the human food chain through milk. Since it
is radioactive and children need the nutrition and Ca in milk, 90 Sr is considered a long duration
38
health hazard from nuclear fallout and was one of the main reasons for an international ban on
above-ground testing of nuclear weapons. Calculate how much of a 1 g sample of 90 Sr will
38
remain after 50 years.
7.2 If a 500 gal tank has a valve which allows it to drain from 500 to 250 gal in 2 h, how much of
300 gal in the same tank will drain in 1 h?
7.3 Using the data in Example 2, determine the time when the concentration of propionaldehyde
will be exactly 0.020 M.
7.4 Using the data of Example 3, determine the time when the volume of the aliquot titration will be
35.00 mL using the same pipette and the same NaOH titration concentration.
7.5 Using the data of Example 4, determine the time at which the thiosulfate ion concentration will
be 0.0035.