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Acids,
bases and salts II
Francine
Taylor-Campbell, Contributor
Let
us examine a few questions on the
topic.
Question
1
1.
(A) Sulphur (IV) oxide, a primary
pollutant, can dissolve in water droplets
in the atmosphere.
(i)
Write the formula for sulphur (IV)
oxide. (1 mark)
(ii)
Write an equation for the reaction
of sulphur (IV) oxide with water.
(1 mark)
(iii)
What happens to the pH of water when
sulphur (IV) oxide dissolves in it?
(2 marks)
(B)
It has been suggested that the amount
of polluting gases, like the oxides
of sulphur and nitrogen, could be
reduced by passing exhaust air from
factories, through beds of powdered
reagents.
(i)
Name ONE reagent that could be used
for this purpose. (1 mark)
(ii)
Explain why the reagent should be
used in a powdered form. (1 mark)
(iii)
Explain how the reagent could work
to reduce the quantity of polluting
gases in the atmosphere. (1 mark)
(C)
Zinc is one of the metals that forms
an amphoteric oxide.
(i)
What is meant by the term 'amphoteric
oxide'? (2 marks)
ANSWERS
Question
1
1.
(A) Sulphur (IV) oxide has the formula
SO2.
Comment:
The number IV represents the oxidation
state of sulphur. Since oxygen generally
has an oxidation number of -2, then
2 oxygen atoms contribute -2*2 = —4
and sulphur is + 4.
—4
+ 4 = 0.
(ii)
SO2 (g) + H2O (l) = H2SO3 (aq). SO2
is an acidic oxide and dissolves in
water to form acids.
(iii)
The pH of water decreases, that is,
becomes more acidic. On the pH scale,
acids are from 0-6 and bases are from
8-14; thus, an acidic substance would
lower the pH.
B.
(i) Calcium carbonate or calcium oxide
could be used.
Comment:
The acidic oxides of sulphur and nitrogen
can be neutralised by reacting with
a basic oxide or a carbonate.
(ii)
The reagent should be used in powdered
form to increase the surface area
in contact with the gas. A larger
surface area means an increase in
reaction rate.
(iii)
By reacting with the gases to form
a neutral product, the quantity of
polluting gases in the atmosphere
could be reduced. SO2 (g) + CaO (s)
= CaSO3 (s) CaCO3 (s) + SO2 (g) =
CO2 (g) + CaSO3 (s)
C.
(i) An amphoteric oxide is one that
shows both basic and acidic properties.
It can react with both acids and bases.
Question
2
1.
Describe, in detail, how you would
prepare a sample of copper (II) nitrate
(Cu(NO3) 2. 3H2O) crystals from copper
(II) carbonate and nitric acid. (5marks)
Answer
Copper
(II) nitrate (Cu(NO3)2. 3H2O) are
hydrated crystals of a soluble salt.
(All nitrates are soluble). To prepare
this salt, copper (II) carbonate and
nitric acid would be reacted together
in a reaction flask, where nitric
acid would be added gradually from
a funnel, on to the copper carbonate
crystals. The copper nitrate formed
is in solution as it is soluble.
The
solution would need to be filtered
to remove any excess solid, then evaporated
to get the salt. The copper nitrate
must contain water of crystallisation,
hence it will not be evaporated to
dryness but, instead, to a small bulk
and then left to crystallise out of
solution. The crystals can then be
collected.
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Even
with the scorching heat, these
fifth-form students are out
celebrating!.
- Contributed
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Francine
Taylor-Campbell is an independent
contributor.
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