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Revision
programme for examination
Francine
Taylor-Campbell, Contributor
We
will attempt the following questions
as part of your revision programme
for your upcoming examinations.
Question
1
(a)
How many electrons, protons and neutrons
are present in one atom of the chlorine
isotope 37Cl?
b)
When concentrated aqueous sodium chloride
is electrolysed using graphite electrodes,
hydrogen is collected at the cathode
and chlorine at the anode.
i)
Give the equations for the electrode
reactions by which hydrogen and chlorine
are formed.
ii)
Name the product other than hydrogen
and chlorine which is manufactured
by the electrolysis of concentrated
aqueous sodium chloride. Give a major
use of this product.
iii)
Why is concentrated hydrochloric acid
not electrolysed for the manufacture
of chlorine?
c)
Household bleaches containing sodium
chlorate (I), NaOCl, can be made by
reacting chlorine with sodium hydroxide.
2NaOH
+ Cl2 ===== NaCl + NaOCl + H2O
Explain
why this is a redox reaction.
ii)
Give three uses of chlorine.
ANSWERS
1.(a)
The isotope 37Cl contains 17 protons
and 20 neutrons.
Comments:
The mass number 37 is made up of the
number of protons and neutrons and
since this is an isotope of chlorine
then it contains the same proton number
as 35Cl.
b
(i) At the cathode: 2H+ + 2e == H2
(g) hydrogen gas is liberated at the
cathode.
At
the anode: 2Cl- == Cl2 (g) + 2e chlorine
is produced at the anode.
(ii)
Comment: As hydrogen and chlorine
are removed from the concentrated
aqueous sodium chloride solution,
sodium Na+ and OH- ions remain, resulting
in an alkaline solution of sodium
hydroxide (NaOH). Sodium hydroxide
is an important industrial chemical
which is used in the manufacture of
soaps.
(iii)
Concentrated hydrochloric acid is
a very dangerous chemical to work
with especially as an electrolyte.
It is much easier, safer and less
expensive to use sodium chloride which
is abundant in the oceans as the electrolyte
from which chlorine can be obtained.
In most cases the solution used in
this electrolysis is concentrated
brine (salt water) taken from the
seas.
2NaOH
+ Cl2 ===== NaCl + NaOCl + H2O
(c)
In a redox reaction, one substance
is oxidised while another is reduced.
Based on the above equation, chlorine
is acting as an oxidising agent and
has been reduced in the same reaction.
If one should look at oxidation numbers,
chlorine is changing from an oxidation
number of 0 in Cl2 gas to -1 in NaCl
and +1 in NaOCl. This is an example
of a reaction where a substance can
be both oxidised (increase in oxidation
number) and reduced (decrease in oxidation
number) at the same time.
(ii)
Chlorine is used as a domestic bleach,
a disinfectant in pool, and in many
organic compounds to make antiseptics
and insecticides.
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Members
of Jamaica's successful Under-15
cricket team on arrival at the
Norman Manley International
Airport last week Monday. The
team successfully defended the
Clico West Indies Under-15 Trophy
over the Easter weekend.
- Contributed
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Francine
Taylor-Campbell is an independent
contributor.
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