Chemistry Reference
In-Depth Information
Tests for aqueous cations
Aqueous sodium hydroxide can be used to identify
salts of Al
3+
, Ca
2+
, Cr
3+
, Cu
2+
, Fe
2+
, Fe
3+
and Zn
2+
.
The colour of the precipitate and its behaviour in
excess sodium hydroxide solution will help identify
the metal present. In the case of ammonium salts,
ammonia gas is produced on warming.
Questions
1
Suggest a reason for only 'roughly' twice as much hydrogen
gas being produced at the cathode as oxygen gas at the
anode in the electrolysis of water.
2
Account for the following observations which were made
when concentrated sodium chloride solution, to which a
little universal indicator had been added, was electrolysed
in the laboratory in a Hofmann voltameter.
a
The universal indicator initially turns red in the region of the
anode, but as the electrolysis proceeds it loses its colour.
b
The universal indicator turns blue in the region of the
cathode.
3
Why is it important to remove compounds of calcium,
strontium and magnesium before brine is electrolysed?
4
The uses of sodium hydroxide can be separated on a
percentage basis as follows:
Neutralisation
Table 5.1
Cation solution
Effect of aqueous sodium hydroxide
Aluminium (Al
3
+
)
White precipitate, soluble in excess giving a
colourless solution
Ammonium (NH
4
+
)
Ammonia produced on warming
Calcium (Ca
2
+
)
White precipitate, insoluble in excess
5%
Chromium (Cr
3
+
)
Paper manufacture
5%
Green precipitate, soluble in excess
Oil refi ning
5%
Copper (Cu
2
+
)
Light blue precipitate, insoluble in excess
Soap/detergents
5%
Manufacture of rayon and acetate fi bres
16%
Iron(
ii
) (Fe
2
+
)
Green precipitate, insoluble in excess
Manufacture of chemicals
30%
Iron(
iii
) (Fe
3
+
)
Red-brown precipitate, insoluble in excess
Miscellaneous uses
34%
Use a graph-plotting program to create a 'pie'
chart of these data.
Zinc (Zn
2
+
)
White precipitate, soluble in excess, giving a
colourless solution
If the nitrate ion is present in solution, then upon
addition of aqueous sodium hydroxide and a little
aluminium foil, ammonia gas is produced on warming.
●
Electrolysis of copper(ii)
II
)
sulfate solution
Copper
(
ii
) sulfate solution (CuSO
4
(
aq
)) may be
electrolysed using inert graphite electrodes in a
cell similar to that shown in Figure 5.14. When
the solution is electrolysed, oxygen gas and
copper metal are formed at the anode and cathode
respectively. Four ions are present in solution:
●
Electrolysis of
concentrated
hydrochloric acid
When concentrated hydrochloric acid is electrolysed,
two gases are produced. Both chloride ions and
hydroxide ions would be attracted to the anode. The
chloride ions are at a much higher concentration than
the hydroxide ions and go on to produce chlorine gas
by losing electrons (oxidation).
2Cl
-
(
aq
) → Cl
2
(
g
) + 2e
-
The chlorine gas can be tested by placing a piece of
damp blue litmus paper near the anode. The paper will
fi rst turn red and then it will be bleached (turned white).
The only positively charged ion is the hydrogen ion.
These are attracted to the cathode where they produce
hydrogen gas by the gain of electrons (reduction).
2H
+
(
aq
) + 2e
-
→ H
2
(
g
)
Hydrogen gas, if collected from the cathode, will give
a 'squeaky pop' when mixed with air and ignited.
H
+
(
aq
)
OH
−
(
aq
)
from the water:
Cu
2+
(
aq
) SO
4
2−
(
aq
)
from the copper(ii)
ii
) sulfate:
oxygen
copper (II)
sulfate
solution
copper forms
on surface of
carbon electrode
bung
Figure 5.14
The electrolysis of copper(ii)
ii
) sulfate solution using inert
electrodes.
