Text 17
Free Radicals
An atom or group of atoms with one or more unshared electrons, which
may enter into chcmical-bond formation is callcd a
free radical. (The same
group in a molcculc is callcd a radical; for example the methyl radical in
methyl cyanide or other molcculcs.) Free radicals arc usually highly rcactivc
and difficult to prepare in any except low concentration.
One way of making the methyl radical as a dilute gas is by heating
mercury dimethyl, Hg(CHj)., which decomposes to give metallic mcrcury
and methyl radical. Methyl radical can also be made conveniently by the
decomposition of diacctyl, (CHjCO*)», by cither heat or ultraviolet light.
The diacctyl molcculc liberates two molcculcs of carbon dioxide and two
methyl radicals.
The American chcmist Moses Gombcrg (1866-1947) discovered in
1900 that some hydrocarbon free radicals are stable. He attempted to
synthesize the substancc hcxaphcnylcthanc,
( Q H
j
)
j
C
—C(C
6
Hj)j, which
he cxpcctcd to be a stable, white crystalline substancc. Instead, he obtained
a strongly coloured solution, with the property of combining readily with
oxygen. He concluded correctly that the solution did not contain the
hcxaphcnyl derivative of ethane, but instead, the free radical triphcny-
Imcthyl, with the formula (QH
3
)jC\ Many similar hydrocarbon free radicals
have been made, and it has been shown that they arc paramagnetic, and
accordingly contain unpaired electrons (the paramagnetism is due to the
magnetic moment o f the clcctron spin o f the unpaired clcctron). The
stability of the triphcnylmcthyl radical, which is responsible for
the low
bond energy of the carbon-carbon bond in the substituted ethane, is
attributed to the resonance energy of the unpaired clcctron among the
various carbon atoms of the molcculc.
Text 18
The Manufacture of Sulphuric Acid
It is a matter of common knowledge among chcmists that sulphuric
acid is made by two processes, the
contact process and the
lead-chamber
process, which arc now about equally important.
In the contact process sulphur trioxidc is made by the catalytic oxidation
of sulphur dioxide (the name of the proccss refers to the fact that reaction
occurs on contact of the gases with the solid catalyst). The catalyst formerly
used was finely divided platinum; it has now been largely replaced by
vanadium pcntoxidc, V:0 }. The gas containing sulphur trioxidc is then
bubbled through sulphuric acid, which absorbs the sulphur trioxidc. Water
is added at the
proper rate, and 98% acid is drawn off.
In the Icad-chambcr proccss oxygen, sulphur dioxide, nitric oxide, and
a small amount of water vapour arc introduced into a large lead-lined
246
chambcr. White crystals of nitrosulphuric acid, NOHSO
4
(to put it in
another way, sulphuric acid in which one hydrogen ion is replaced by the
nitronium ion, : N a O*:), arc formed. When steam is introduced the crystals
react to form drops of sulphuric acid, liberating oxides o f nitrogen. In
effect, the oxides of nitrogen serve to catalyze the oxidation of sulphur
dioxide by oxygen. The reactions that occur may be summarized as
2SO:+NO + NOj+ 0 :+ H :0 -> 2N0HS0,;
2
NOHSO
4
+ H :0 -> 2H:S0*+ NO + NO:.
The oxides of nitrogen, NO and N 0 2, that take part in the first reaction,
arc released by the second reaction, and can serve over and over again.
It should be pointed out in conclusion that however widespread these
processes may be, they arc by no means tlic only ways o f the manufacture
of sulphuric acid.
Text 19
W hat Is Light? W hat Is an Electron?
During recent years many people have asked the following questions:
“Docs light
really consist of waves or of particles? Is the clcctron
really a
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