Chemistry Reference
In-Depth Information
A. Give the charge and mass characteristics of the (a) neutron, and
(b) positron.
B. Complete the following equation:
235
U
n £
140
Ba
2 n
?
21.4
Nuclear Fusion
In contrast to nuclear fission reactions, in which large nuclei are broken into
smaller nuclei,
nuclear fusion
involves the combination of small nuclei into
larger ones. In nuclear fusion reactions, tremendous quantities of energy are
released. The atomic bomb uses a fission reaction as its source of energy; the
so-called
hydrogen bomb
uses a fusion reaction as its source of energy. In this
fusion reaction,
deuterium
nuclei, also called
deuterons,
with symbol d,
are combined into nuclei of
tritium
and then into nuclei of an isotope of
helium. This reaction is less “dirty” than a fission reaction in that the products
of fusion are not radioactive. However, fusion reactions require an extremely
high temperature to get them started, so a small atomic bomb is used as the
energy source to trigger a hydrogen bomb, and that bomb is a source of
radioactivity.
(
1
H)
(
1
H)
● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
In 1989, the scientific community was startled by the announcement of
two chemists that they had succeeded in causing a fusion reaction to occur
near room temperature. This
cold fusion
would have enabled the popula-
tion of the Earth to be supplied with almost limitless energy without the
radioactivity associated with the operation of ordinary nuclear power
plants. The effect on the scientific and economic communities was pro-
found. Unfortunately, so far, the results reported by the scientists have not
been repeated or confirmed, and cold fusion is still a dream.
ITEM OF INTEREST
The energy of the Sun and stars comes from nuclear fusion reactions, which
have the overall effect of transforming hydrogen nuclei to alpha particles
(helium nuclei). The temperature of the particular star determines the mecha-
nism by which this transformation takes place. The Sun, a moderately small star,
is thought to be powered by the following sequence of reactions:
1
H
1
1
H £
1
H
0
b
1
H
2
1
H £
2
H
0
0
g
2
He
3
2
He £
2
He
2
1
H
Each of these reactions emits energy, as well as the products listed. The
positron released in the first reaction can react with an electron to yield even
more energy. The net reaction is the conversion of four protons into an alpha
particle plus a great deal of energy. Hotter stars convert protons to alpha
particles via another series of reactions (see Problems 21.41 and 21.42 at the
end of the chapter).
