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Past
paper questions 3
Francine
Taylor-Campbell, Contributor
We
will attempt the following questions
as part of your revision programme
for the upcoming examination.
Before
we look at some questions please note
that:
- Scoring
maximum marks from the questions
on the exam paper will depend on
how well you follow the instructions
asked.
- If
you are asked to explain your answer
or to describe something, more information
is required than just listing.
- The
marks assigned to each part of a
question indicate how much answer
is required. The length of your
answer for a one-mark question should
be significantly less than the answer
you give for a question worth five
marks.
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.
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.
1.
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 a 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 oxidized while another is reduced.
Based on the above equation, chlorine
is acting as an oxidizing 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 oxidized (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 pools, and in many
organic compounds to make antiseptics
and insecticides.
To
ensure that you are fully preparing
for your exams, try practising past-paper
questions under exam conditions. That
is, try answering the questions in
the time normally set for the exams.
In this way you get used to working
fast and within a time limit.
Francine
Taylor-Campbell is an independent
contributor. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
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