Chemical reactions - Chemistry - page 109

agreements are adhered to then the ozone layer will
recover by 2050.
Photosynthesis is a very important reaction
(Chapter 11, p. 172) which occurs only when
sunlight falls on leaves containing the green pigment
chlorophyll. The chlorophyll acts as a catalyst
and the rate of photosynthesis depends on the
intensity of the light. Another chemical reaction
that takes place only in light is that which occurs in
photographic fi lm. This is a transparent plastic strip
coated with emulsion: a layer of gelatin throughout
which are spread many millions of tiny crystals of
silver halides, in particular, silver bromide (AgBr).
The emulsion used is similar for both black-and-
white and colour fi lm.
When light hits a silver bromide crystal, silver
cations (Ag + ) accept an electron (reduction) from
the bromide ions (Br − ), which are oxidised. Hence,
silver atoms and bromine atoms are produced in the
emulsion.
silver ion
Ag +

Figure 7.12 Diagram showing the hole in the ozone layer
over Antarctica

stratosphere and the ozone layer. In the stratosphere,
the high-energy ultraviolet radiation causes a
chemical bond in the CFC molecule to break and a
chlorine radical to split off from the CFC molecule.
For example:

+
+

electron
e −

silver atom
Ag

→
→

uv light

+

CF 2 Cl 2 CF 2 Cl Cl

chlorine radical

The more light that falls on the fi lm, the greater the
amount of silver deposited. Places where the most
silver is deposited are darkest when the negative is
developed and lightest on the photographic print.

This chlorine radical then reacts with the ozone,
destroying it:

Cl( g ) + O 3 ( g ) → OCl( g ) + O 2 ( g )
One of the worrying things about this process is
that, although the above equation indicates that
the chlorine radical has also been destroyed, it has
been found that it is then regenerated by a further
reaction. It has consequently been found that a
single chlorine radical is capable of destroying
hundreds of thousands of ozone molecules.
This is not the only problem with CFCs. They are
also signifi cant 'greenhouse gases' (see Chapter 13,
p. 212).
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

Question
1 Devise an experiment to show how sunlight affects
the rate of formation of silver from the silver salts silver
chloride and silver bromide.

Catalysts
Over 90% of industrial processes use catalysts . A
catalyst is a substance which can alter the rate of a
reaction without being chemically changed itself. In
the laboratory, the effect of a catalyst can be observed
using the decomposition of hydrogen peroxide as an
example.
hydrogen peroxide
2H 2 O 2 ( aq )

water
2H 2 O( l )

+
+

oxygen
O 2 ( g )

→
→

The rate of decomposition at room temperature is
very slow. There are substances, however, which will
speed up this reaction, one being manganese( iv )
oxide. When black manganese( iv ) oxide powder