electrolysis revision
DESCRIPTION
IGCSE questions on ElectrolysisTRANSCRIPT
12
0620/22/M/J/12© UCLES 2012
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7 A student placed some crystals of salt at the bottom of a beaker of distilled water. She left the contents of the beaker to stand for one hour.
The diagram below shows her observations.
distilled water
at start after 15 minutes after 1 hour
XX
salt crystals
After one hour, all the salt had disappeared but the solution at point X tasted salty.
(a) Use the kinetic particle theory to explain these observations.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [4]
(b) Salt is sodium chloride, NaCl.
(i) Which one of the following statements about bond formation in sodium chloride is true?
Tick one box.
A sodium atom shares one electron with a chlorine atom.
A sodium atom loses its outermost electron and a chlorine atom gains an electron.
A sodium atom shares two electrons with a chlorine atom.
A sodium atom gains an electron and a chlorine atom loses its outermost electrons.
[1]
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(ii) Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does conduct.
....................................................................................................................................
....................................................................................................................................
.............................................................................................................................. [2]
(iii) State the name of the product formed at each electrode when a concentrated aqueous solution of sodium chloride is electrolysed using graphite electrodes.
at the positive electrode .............................................................................................
at the negative electrode ..................................................................................... [2]
(iv) What is the name of the negative electrode? Put a ring around the correct answer.
anion anode cation cathode electrolyte[1]
(v) Suggest why graphite is a suitable material for an electrode.
.............................................................................................................................. [1]
[Total: 11]
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6 During electrolysis, ions move in the electrolyte and electrons move in the external circuit. Reactions occur at the electrodes.
(a) The diagram shows the electrolysis of molten lithium iodide.
molten lithium iodide
electrode electrode
+ –
external circuit
heat
(i) Draw an arrow on the diagram to show the direction of the electron fl ow in the external circuit. [1]
(ii) Electrons are supplied to the external circuit. How and where is this done?
....................................................................................................................................
.............................................................................................................................. [2]
(iii) Explain why solid lithium iodide does not conduct electricity but when molten it is a good conductor.
....................................................................................................................................
.............................................................................................................................. [1]
(b) The results of experiments on electrolysis are shown in the following table. Complete the table. The fi rst line has been done as an example.
electrolyte electrodes product atcathode
product atanode
change toelectrolyte
molten lithium iodide carbon lithium iodine used up
aqueous copper(II) sulfate platinum oxygen
concentrated aqueouspotassium chloride carbon chlorine
[4]
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(c) The diagram below shows the electrolysis of dilute sulfuric acid. Hydrogen is formed at the negative electrode (cathode) and oxygen at the positive electrode (anode) and the concentration of sulfuric acid increases.
carbon cathode
dilute sulfuric acid
bubbles ofhydrogen gas
bubbles ofoxygen gas
carbon anode
+ –
The ions present in the dilute acid are H+(aq), OH–(aq) and SO42–(aq).
(i) Write an equation for the reaction at the negative electrode (cathode).
.............................................................................................................................. [2]
(ii) Complete the equation for the reaction at the positive electrode (anode).
4OH–(aq) → O2(g) + .....H2O(l) + ....... [1]
(iii) Suggest an explanation of why the concentration of the sulfuric acid increases.
.............................................................................................................................. [1]
(d) In the apparatus used in (c), the power supply is removed and immediately replaced by a voltmeter.
carbon cathode
dilute sulfuric acid
bubbles ofhydrogen gas
bubbles ofoxygen gas
carbon anode
Vvoltmeter
A reading on the voltmeter shows that electrical energy is being produced. Suggest an explanation for how this energy is produced.
...........................................................................................................................................
...........................................................................................................................................
..................................................................................................................................... [3]
[Total: 15]
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(c) The diagram below shows an electrolysis cell used to manufacture sodium from molten sodium chloride.
– –
+
A
B
C
D
E
(i) Which letter in the diagram above represents
the anode? ..............
the electrolyte? .............. [2]
(ii) State the name of the product formed
at the positive electrode, ............................................................................................
at the negative electrode. .................................................................................... [2]
(iii) Which one of the following substances is most likely to be used for the anode? Put a ring around the correct answer.
graphite iodine magnesium sodium[1]
(d) Lithium, sodium and potassium are metals with a low density. State two other physical properties of these metals.
1. .......................................................................................................................................
2. ................................................................................................................................. [2]
[Total: 15]
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0620/33/O/N/10© UCLES 2010
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4 The electrolysis of concentrated aqueous sodium chloride, between inert electrodes, is used to make four important chemicals.
hydrogen chlorine sodium hydroxide sodium chlorate(I)
(a) The ions present in the electrolyte are Na+, H+, Cl – and OH –.
(i) Hydrogen ions are discharged at the negative electrode (cathode). Write an equation for this reaction.
.............................................................................................................................. [2]
(ii) The hydrogen ions are from the water.
H2O H+ + OH –
Suggest an explanation why the concentration of hydroxide ions increases.
....................................................................................................................................
.............................................................................................................................. [2]
(iii) When a dilute solution of sodium chloride is used, chlorine is not formed at the positive electrode (anode), a different gas is produced. Name this gas.
.............................................................................................................................. [1]
(iv) State an example of an inert electrode.
.............................................................................................................................. [1]
(b) (i) State a use of hydrogen.
.............................................................................................................................. [1]
(ii) Why is chlorine used to treat the water supply?
.............................................................................................................................. [1]
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2 The results of experiments on electrolysis using inert electrodes are given in the table. Complete the table; the first line has been completed as an example.
electrolyte change at negative electrode
change at positive electrode
change to electrolyte
molten lead(II) bromide
lead formed
bromine formed
used up
potassium formed
iodine formed
used up
dilute aqueous sodium chloride
aqueous copper(II) sulfate
hydrogen formed
bromine formed
potassium hydroxide formed
[Total: 8]
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© UCLES 2007 0620/03/M/J/07 [Turn over
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6 Aluminium is extracted by the electrolysis of a molten mixture that contains alumina, which is aluminium oxide, Al2O3.
(a) The ore of aluminium is bauxite. This contains alumina, which is amphoteric, and
iron(III) oxide, which is basic. The ore is heated with aqueous sodium hydroxide. Complete the following sentences.
The dissolves to give a solution of
The does not dissolve and can be removed by [4]
(b) Complete the labelling of the diagram.
waste gases
carbon anode (+)
mixture of aluminium
oxide and .........................
................................
temperature is .........................
.......................
....................... (–)
[4] (c) The ions that are involved in the electrolysis are Al3+ and O2-. (i) Write an equation for the reaction at the cathode.
[2]
(ii) Explain how carbon dioxide is formed at the anode.
[2]
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0620/3 M/J/02
1 In 1886, the modern electrolytic process for the extraction of aluminium was discovered inthe USA by C. Hall.
(a) Before this discovery, the only method of extracting the metal was by displacement.
(i) Name a metal that can displace aluminium from aluminium chloride.
..................................................................................................................................[1]
(ii) Write a word equation for this displacement reaction.
..................................................................................................................................[1]
(iii) Complete the equation for the reaction.
AlCl3 + ....................... → ...................................... + ........................................[2]
(b) Aluminium is produced by the electrolysis of an electrolyte that contains aluminiumoxide.
(i) Write an ionic equation for the reduction of the aluminium ion at the cathode.
..................................................................................................................................[2]
(ii) Name the main ore of aluminium.
..................................................................................................................................[1]
(iii) Complete the following description of the electrolyte by filling the spaces.
The electrolyte is a ................................................ mixture of aluminium oxide
and .................................................. which is maintained at 900 °C. [2]
(iv) Explain why the gas given off at the anode is a mixture of oxygen and carbondioxide.
...................................................................................................................................
...............................................................................................................................[2]
(c) One property of aluminium is that it resists corrosion because it is covered with a layerof its oxide.
(i) Give one use of the metal that depends on this property.
..................................................................................................................................[1]
(ii) Give another use of the metal that depends on a different property.
use .............................................................................................................................
property..................................................................................................................[2]
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© UCLES 2005 0620/03/O/N/05
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(c) The major ore of strontium is its carbonate, SrCO3. Strontium is extracted by the electrolysis of its molten chloride.
(i) Name the reagent that will react with the carbonate to form the chloride.
[1]
(ii) The electrolysis of molten strontium chloride produces strontium metal and
chlorine. Write ionic equations for the reactions at the electrodes.
negative electrode (cathode)
positive electrode (anode) [2]
(iii) One of the products of the electrolysis of concentrated aqueous strontium chloride
is chlorine. Name the other two.
[2]
(d) Both metals react with water. (i) Write a word equation for the reaction of zinc and water and state the reaction
conditions.
word equation [1]
conditions [2]
(ii) Write an equation for the reaction of strontium with water and give the reaction
condition.
equation [2]
condition [1]
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UCLES 2004 0620/03/M/J/04 [Turn over
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(iii) If aluminium chloride had been used instead of iron(III) chloride, the shape of the
graph would be different. How are the shapes of these two graphs different and
why?
difference in shape
reason for difference
[2]
5 (a) Copper has the structure of a typical metal. It has a lattice of positive ions and a “sea”
of mobile electrons. The lattice can accommodate ions of a different metal.
Give a different use of copper that depends on each of the following.
(i) the ability of the ions in the lattice to move past each other
[1]
(ii) the presence of mobile electrons
[1]
(iii) the ability to accommodate ions of a different metal in the lattice
[1]
(b) Aqueous copper(II) sulphate solution can be electrolysed using carbon electrodes. The
ions present in the solution are as follows.
Cu2+
(aq), SO4
2–
(aq), H+
(aq), OH–
(aq)
(i) Write an ionic equation for the reaction at the negative electrode (cathode).
[1]
(ii) A colourless gas was given off at the positive electrode (anode) and the solution
changes from blue to colourless.
Explain these observations.
[2]
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UCLES 2004 0620/03/M/J/04
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(c) Aqueous copper(II) sulphate can be electrolysed using copper electrodes. The
reaction at the negative electrode is the same but the positive electrode becomes
smaller and the solution remains blue.
(i) Write a word equation for the reaction at the positive electrode.
[1]
(ii) Explain why the colour of the solution does not change.
[2]
(iii) What is the large scale use of this electrolysis?
[1]
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© UCLES 2006 0620/02/O/N/06
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6 The diagram shows the structure of lead bromide.
Pb2+ Pb2+
Pb2+ Pb2+
Pb2+ Pb2+
Br– Br– Br– Br–
Br– Br– Br– Br–
Br– Br– Br– Br–
(a) What is the simplest formula for lead bromide?
[1]
(b) What type of structure and bonding is present in lead bromide? Choose two words from the following:
atomic covalent giant ionic metallic molecular
[2]
(c) Lead bromide is electrolysed using the apparatus shown below.
+ –
heat
molten lead bromide
electrodes
BA
C
(i) Which letter, A, B or C represents the cathode?
[1]
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(ii) State the name of a metal which can be used for the electrodes.
[1]
(iii) Why does lead bromide have to be molten for electrolysis to occur?
[1]
(iv) State the name of the products formed during this electrolysis;
at the anode,
at the cathode. [2]
(d) A student bubbled chlorine gas through an aqueous solution of sodium bromide. (i) Complete the equation for this reaction.
Cl2 + 2NaBr ............ + 2NaCl
chlorine sodiumbromide
bromine sodiumchloride
[1] (ii) What colour is the solution at the end of the reaction?
[1]
(iii) An aqueous solution of iodine does not react with a solution of sodium bromide.
Explain why there is no reaction.
[1]
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© UCLES 2008 0620/02/M/J/08
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(d) Copper can be purified by electrolysis.
impure copper foilpure copper foil
copper(II) sulphate
solution
+–
(i) Choose a word from the list below which describes the pure copper foil. Put a ring around the correct answer.
anion anode cathode cation electrolyte
[1]
(ii) Describe what happens during this electrolysis to
the pure copper foil,
the impure copper foil. [2]
[Total: 9]
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