Chemistry Reference
In-Depth Information
the iron acts as a catalyst fi rstly removing carbon
from the methane. When the surface of the iron
becomes saturated, nanotubes begin to be formed.
If He or Ar is added to the vacuum chamber then
'metallic' (electrical conducting) nanotubes are made
in the majority; this accounts for up to 91% of the
nanotubes produced (Figure 14.8).
CFC-12. CFCs escape into the atmosphere and,
because of their inertness, remain without further
reaction until they reach the stratosphere and the
ozone layer. In the stratosphere the high-energy
ultraviolet radiation causes a chlorine atom to split
off from the CFC molecule. This chlorine atom, or
free radical, then reacts with the ozone.
Cl(
g
) + O
3
(
g
) → OCl(
g
) + O
2
(
g
)
This is not the only problem with CFCs. They are
also signifi cant 'greenhouse gases' (Chapter 13,
p. 212). The ozone depletion and greenhouse
effects have become such serious problems that an
international agreement known as the
Montreal
Protocol on Substances that Deplete the Ozone Layer
was agreed in 1987.
Research is now going ahead, with some success,
to produce safer alternatives to CFCs. At present,
better alternatives called hydrochlorofl uorocarbons
(HCFCs) have been developed - these have lower
ozone-depletion effects and are not effective
greenhouse gases. It is believed that if the intended
agreements are adhered to then the ozone layer will
recover by 2050.
Figure 14.8
These nanotubes are being used in electrical circuitry.
Other uses of alkanes
Besides their major use as fuels (p. 90), some of
the heavier alkanes are used as waxes in candles, as
lubricating oils and in the manufacture of another
family of hydrocarbons - the alkenes.
Methane - another greenhouse gas!
Methane, the fi rst member of the alkanes, occurs
naturally. Cows produce it in huge quantities when
digesting their food. It is also formed by growing
rice. Like carbon dioxide, it is a greenhouse gas
(Chapter 13, p. 212) because it acts like the glass in a
greenhouse - it will let in heat from the Sun but will
not let all of the heat back out again. It is thought
that the greenhouse effect may contribute to climate
change, which could have disastrous effects for life on
this planet.
Questions
1
Write a balanced chemical equation to represent the
combustion of propane.
2
In what mole proportions should chlorine and methane be
mixed to produce:
a
mainly chloromethane?
b
mainly tetrachloromethane?
3
Describe a method you would use to separate
chloromethane from the other possible reaction products
when methane reacts with chlorine.
4
Explain why it is the C
—
Cl bond and not the C
—
F bond in
CFCs that is disrupted in the stratosphere.
Question
1
Use your research skills to fi nd out:
a
any other sources of methane found in nature
b
how climate change might affect your particular
environment.
●
Alkenes
Alkenes form another homologous series of
hydrocarbons of the general formula C
n
H
2
n
where
n
is the number of carbon atoms. The alkenes are
more reactive than the alkanes because they each
contain a double covalent bond between the carbon
atoms (Figure 14.9). Molecules that possess a double
covalent bond of this kind are said to be
Methane - the 21st century chemical
Methane is fi nding another use, which sets it apart
from the rest of the alkanes, in the production
of nanotubes for power generation and futuristic
circuitry. A common way to grow nanotubes is from
a mixture of methane gas and iron nanoparticles.
When heated to around 700-800°C in a vacuum
