Chapter 28
The following information is referred to in questions 16, 17 and 18.
The Down’s cell is used for the industrial preparation of sodium and chlorine. The reaction occurring
in the
Down’s cell is
2NaCl(l) 2Na(l) + Cl2(g)
Question 16 (02 Chem2)
An iron mesh screen is a necessary part of the Down’s cell.
The primary role of the screen is to
A. allow electric current to flow from the anode compartment to the cathode compartment.
B. provide a way for the liquid sodium to be run into a collection vessel.
C. prevent the chlorine and sodium from coming into contact and thereby exploding.
D. prevent chloride ions from moving from the anode into the cathode compartment.
Question 17 (02 Chem2)
Pure NaCl is not used in a Down’s cell. The liquid that is in a Down’s cell is a mixture of CaCl2 and
NaCl.
The CaCl2/NaCl mixture is used instead of pure NaCl because it
A. has a higher melting temperature.
B. improves the yield of chlorine produced.
C. improves the purity of the sodium produced.
D. enables the process to be carried out at a lower temperature.
Question 18 (02 Chem2)
A particular Down’s cell operates for 1.00 × 104 s at a current of 96.5 A.
The amount of chlorine produced, in mole, is
A. 0.050
B. 5.00
C. 10.0
D. 20.0
Question 20 (02 Chem2)
5 L volumes of 1.0 M KNO3(aq), 1.0 M AgNO3(aq) and 1.0 M Cu(NO3)2(aq) are placed in three
separate
electrolytic cells using platinum electrodes. Over a period of 30 minutes, 96 500 coulombs of electric
charge
are passed through each solution.
Which of the following alternatives correctly indicates the amount, in mole, of metal deposited at the
cathode
in each cell?
n(K) n(Ag) n(Cu)
A. 0 1 2
B. 0 1 0.5
C. 1 1 0.5
D. 0.5 0.5 1
Questions 5 and 6 refer to the following information.
A student electrolyses a 0.1 M aqueous solution of sodium chloride. A colourless, odourless gas is
evolved at one electrode. A second colourless, odourless gas is evolved at the other electrode.
Question 5 (03 Chem2)
The gases evolved at the positive and negative electrodes are
Positive electrode
Negative electrode
oxygen
hydrogen
A.
hydrogen
oxygen
B.
C.
D.
chlorine
hydrogen
hydrogen
chlorine
Question 6 (03 Chem2)
During this electrolysis, the pH of the solution will
A. decrease around the cathode and increase around the anode.
B. increase around the cathode and decrease around the anode.
C. decrease at both electrodes.
D. increase at both electrodes.
Question 4 (03 Chem2)
A piece of silver jewellery is coated with gold (Au) in an electrolytic cell that contains gold ions in an
aqueous solution. Use the information below to determine the oxidation number of the gold ions in the
aqueous solution.
volume of gold deposited 0.150 cm3
density of gold 19.3 g cm–3
cell current 4.00 A
time taken to plate jewellery 17.75 minutes
a. Calculate the amount of electrons, in mole, passed through the cell.
2 marks
b. Calculate the amount of gold, in mole, deposited on the jewellery.
2 marks
c. i. Determine the ration(e–)n(Au).
ii. Hence give the oxidation state of gold in the gold ions in this solution.
2 marks
Total 6 marks
Question 5 (03 Chem2)
Give a concise answer to each of the following questions.
a. The Downs Cell uses a molten electrolyte containing NaC1 to produce sodium at an iron cathode.
Explain why an aqueous solution of NaC1 cannot be used to produce sodium at an iron cathode.
1 mark
b. Alumina dissolved in molten cryolite is used in preference to molten alumina in the electrolytic
production of aluminium. Explain why.
1 mark
Question 3 (04 Chem2)
In the electrolytic production of aluminium metal from alumina (Al2O3), the alumina is dissolved in
cryolite (Na3A1F6). The anodes and cathodes are made of carbon. The electrode reactions may be
represented by
C + 2O2– → CO2 + 4e–
Al3+ + 3e– → Al
The net reaction for this electrolysis process is
A. 2Al2O3 → 4Al + 3O2
B. Al3+ + C + 2O2– → Al + CO2
C. 2Al2O3 + 3C → 4Al + 3CO2
D. 4Al3+ + 6O2– + O2 + C → 2Al2O3 + CO2
Question 9 (04 Chem2)
96.5 C of electricity is used to completely deposit silver metal (Ag) from an aqueous solution in
which the silver ion is present as Ag+(aq). Another 96.5 C is used to completely deposit copper (Cu)
from an aqueous solution in which the copper ion is present as Cu2+(aq).
The silver metal deposited would
A. have half the mass of the copper deposited.
B. have twice the mass of the copper deposited.
C. be half the mole of the copper deposited.
D. be twice the mole of the copper deposited.
Question 9 (05 Chem2)
The disc to be plated is connected to the
A. positive terminal of a battery so that oxidation occurs at the disc.
B. positive terminal of a battery so that reduction occurs at the disc.
C. negative terminal of a battery so that oxidation occurs at the disc.
D. negative terminal of a battery so that reduction occurs at the disc.
Question 10 (05 Chem2)
The mass of silver to be deposited is 0.150 g.
If the current is held steady at 1.50 amps, the time, in seconds, that it takes to complete the
plating is closest
to
A. 90
B. 180
C. 200
D. 360
Question 11 (05 Chem2)
An identical disc is to be gold-plated with a solution containing Au3+(aq) as the electrolyte
using a current of
1.50 amps.
The ratio of the time that is needed to plate the disc with 0.150 g of gold to the time needed
to plate the disc
with 0.150 g of silver is closest to
A. 1 to 3
B. 1 to 1.6
C. 1.6 to 1
D. 3 to 1
Question 12 (05 Chem2)
An electrolytic cell is used commercially to extract aluminium from its ore. The anode and
cathode of this
electrolytic cell are composed of
anode cathode
A. carbon carbon
B. carbon iron
C. iron carbon
D. iron iron
Question 7 (05 Chem2)
A mineral ore contains a mixture of compounds of lead and calcium, in approximately equal
proportions.
A chemist extracts the metal ions by roasting the ore in air and treating the product with acid.
The solution that
contains the Pb2+(aq) and Ca2+(aq) is then placed in an electrolytic cell as shown in the
diagram below.
a.
Label the anode and cathode of the cell.
1 mark
b. When the current begins to ß ow in the cell, write equations for the half reaction that is
likely to occur at
the
. positive electrode
. negative electrode
2 marks
c. After some time has elapsed, a new half reaction occurs at one of the electrodes. Write
the equation for
this half reaction.
1 mark
d. If the chemist had used copper electrodes instead of platinum electrodes, how would
this have affected
the half reaction at the anode?
1 mark
Total 5 marks
Question 8 (05 Chem2)
One type of .breathalyser. instrument used by police for the measurement of the
concentration of alcohol in a
driver.s breath is a fuel cell. An acidic electrolyte is used. Ethanol is oxidised to ethanoic acid
at one electrode
and oxygen from the air is converted to water at the other.
The overall equation for this reaction is
C2H5OH(aq) + O2(g) CH3COOH(aq) + H2O(l)
a. Write the equation for the half reaction at the anode.
2 marks
b. A motorist who has consumed alcohol blows into the fuel cell. If the breath entering the
cell provides
alcohol at the rate of 3.0 × 10.5 g per second, calculate the maximum current, in amps, that
the cell would
produce.
3 marks
c. The nature of the electrodes in the cell is essential to the effective operation of the
breathalyser. State two
important functions that the electrodes must perform.
Function 1
Function 2
2 marks
Total 7 marks
Question 7 (06 Chem2)
Tin metal is electroplated onto sheets of iron using a tin anode and a well-stirred solution
containing
Sn2+ ions.
During this process
A. the anode increases in mass.
B. Sn2+ ions move towards the cathode.
C. the concentration of Sn2+ in the solution decreases.
D. the concentration of Sn2+ in the solution increases.
Question 8 (06 Chem2)
The passage of 0.019 faradays of electricity through a molten chromium compound yields
0.50 g of chromium
metal.
The oxidation number of chromium in the compound is likely to be
A. +2
B. +3
C. +4
D. +6
Question 6 (06 Chem2)
The simpliÞ ed diagram below shows the Hall Cell that is used for the industrial production of
aluminium.
a. In circles X and Y in the diagram, show the polarity of the electrodes.
1 mark
b. In the Hall Cell, the electrolyte consists of alumina mixed with another compound. In box
Z, write the
name or the formula of this compound.
1 mark
c. Write a half equation for the reaction that occurs at the
i. anode
ii. cathode.
1 + 1 = 2 marks
d. Suppose the electrolyte was replaced with an aqueous solution of Al(NO3)3 at 25°C. Write
an equation for
the half reaction that would occur at the cathode.
1 mark
Total 5 marks
Question 7 (06 Chem2)
Give concise explanations in answer to each of the following.
a. Calcium chloride is added to the molten sodium chloride electrolyte in the Downs Cell for
the production
of sodium. Explain why calcium metal is not produced in the cell.
1 mark
Questions 12, 13 and 14 refer to the following information.
The diagram below represents a diaphragm cell used for the commercial production of
chlorine gas.
Question 12 (07
Chem2)
The gases labelled X and Y are
XY
A. chlorine oxygen
B. oxygen chlorine
C. chlorine hydrogen
D. hydrogen chlorine
Question 13 (07 Chem2)
One function of the porous diaphragm in the cell is to
A. act as a catalyst to increase the rate of the reaction.
B. allow movement of ions between the cell compartments.
C. prevent sodium ions from entering the solution near the anode.
D. prevent the electrolyte from making contact with the gases produced.
Question 14 (07 Chem2)
A highly concentrated salt solution, called brine, is used as the electrolyte in this cell.
The main reason that a highly concentrated, rather than a dilute, solution is used is in order
to
A. allow an electric current to pass through the cell.
B. produce chlorine gas, in preference to oxygen gas.
C. allow sodium hydroxide to be separated from the salt by crystallisation.
D. create non-standard conditions that ensure hydrogen gas production.