5 Electrochemistry

7/27/2019 5 Electrochemistry

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Classified Chemistry IGCSE Paper_3_Topic 5

5 - Electrochemistry Mostafa Barakat (0100 165 44 62)

67

Topic 5

Oxidation and reduction

Electrolysis

Application of electrolysis

1.Electroplating

2.Refining of copper

3.Extraction of aluminium


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1) Nov 2001 Q (1_a, iii, b, ii)

(iii) The following reaction is used to detect carbon monoxide.

What type of chemical reaction is the change Pd2+ to Pd? Give a reason foryour answer.

........................................................................................................................[2]

b) (ii) The main impurity in the nickel is copper. What technique is used topurify copper after it has been separated from the nickel?

........................................................................................................................[1]

2) June 2002 Q (1_b)

(b) Aluminium is produced by the electrolysis of an electrolyte that containsaluminium oxide.

(i) Write an ionic equation for the reduction of the aluminium ion at thecathode...

.[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 oxideand ................................................. which is maintained at 900 C. [2]

(iv) Explain why the gas given off at the anode is a mixture of oxygen andcarbon dioxide...

.[2]

3) June 2002 Q (4_b)

(b) Bromine is obtained from the bromide ions in sea water. Sea water isconcentrated by evaporation. Chlorine gas is bubbled through the solution.Chlorine oxidises the bromide ion to bromine.(i) Complete the following equation.

2...... .......... .............Cl Br

[2]

(ii) Explain using the idea of electron transfer why the bromide ion is oxidisedby chlorine.

The bromide ion is oxidised because .................................................................

............................................................................................................................

reduction because it gain electrons

electrolysis

AlAl + 3e3+ -

Bauxite

moltencryolite

oxygen formed at the anode react with carbon to form carbon dioxide

it lose electrons

2 2 Cl- Br

2


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Chlorine is the oxidising agent because ............................................................

........................................................................................................................[2]

(iii) Name a reagent that can be oxidised by bromine molecules.

........................................................................................................................[1]4) Nov 2002 Q (4_b, c)

(b) Copper is refined by the electrolysis of aqueous copper(II) sulfate usingcopper electrodes. Describe the change that occurs at the electrodes

(i) cathode (pure copper)

[2]

(ii) anode (impure copper) ..

.[2]

(iii) write an ionic equation for the reaction at the cathode.

[1]

(iv) if carbon electrodes are used, a colourless gas is given off at the anodeand the electrolyte changes from a blue to a colourless solution.

The colourless gas is ................................................. .

The solution changes into ............................................ [2]

(c) Electrolysis and cells both involve chemical reactions and electricity.What is the essential difference between them?

[2]

5) June 2003 Q (1_e)

(e) One of the methods used to prevent iron or steel from rusting is to

electroplate it with another metal, such as tin. Complete the following.

The anode is made of ................................................

The cathode is made of ..............................................

The electrolyte is a solution of .................................... [3]

it gain electrons

iodine

mass increase

mass decrease

Cu + 2 e Cu2+ -

oxygen

sulfuric acid

cell device that change chemical energy to electric energy while electrolysis

change electric energy into chemical energy

tin

iron or steel

tin nitrate


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6) June 2003 Q (5_b)

(b)All three metals can be obtained by the electrolysis of a molten halide. Theelectrolysis of the aqueous halides does not produce the metal.

(i) Complete the equation for the reduction of lanthanum ions at the negativeelectrode (cathode).

.......... ...............La

(ii) Name the three products formed by the electrolysis of aqueous caesiumbromide...

.[4]

7) Nov 2003 Q (3_c)

(c)A solution of an impure zinc ore contained zinc, lead and silver(I) ions. Theaddition of zinc dust will displace both lead and silver.

(i) The ionic equation for the displacement of lead is as follows.

Which change is reduction? Explain your answer.

............................................................................................................................

........................................................................................................................[2]

(ii) Write an ionic equation for the reaction between zinc atoms and silver(I)ions.

........................................................................................................................[2]

8) June 2004 Q (1_c, i, ii)

(c) Silicon is made by the carbon reduction of the macromolecular compound,silicon(IV) oxide.

(i) Balance the equation for the reduction of silicon (IV) oxide.

2....... .......SiO C Si CO

(ii) Explain why the silicon(IV) oxide is said to be reduced.

............................................................................................................................

........................................................................................................................[1]

3 e La

Hydrogen, Bromine and caesium hydroxide

change 2 because if gain electron

Zn + 2 Ag Zn + 2 Ag2++

22

because it gain electron

-


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9) June 2004 Q (5_b, c)

(b) Aqueous copper (II) sulphate solution can be electrolysed using carbonelectrodes. The ions present in the solution are as follows

2 2

( ) 4( ) ( ) ( ), , ,aq aq aq aqCu SO H OH

(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 thesolution changes from blue to colourless.

Explain these observations.

..

.[2]

(c) Aqueous copper(II) sulphate can be electrolysed using copper electrodes.The reaction at the negative electrode is the same but the positive electrodebecomes 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]

10) June 2005 Q (6_a)

6 The position of aluminium in the reactivity series of metals is shown below.

magnesium

aluminium

zinc

copper

(a) Aluminium is extracted by the electrolysis of its molten oxide.

Cu + 2e Cu2+ -

oxygen gas formed at anode

copper (II) sulfate changed into sulfuric acid

copper atom - electron copper ion

because copper ions are removed then replaced from the anode

refining copper or electroplating


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(i) Name the main ore of aluminium..

.[1]

(ii) Why does the molten electrolyte contain cryolite?..

.[1]

(iii) Oxygen is produced at the positive electrode (anode). Name another gaswhich is given off at this electrode...

.[1]

11) Nov 2005 Q (5_c)(c) The major ore of strontium is its carbonate, SrCO3. Strontium is extractedby 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 andchlorine. 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 strontiumchloride is chlorine. Name the other two.

[2]

Bauxite

to reduce the melting point of the mixture

carbon dioxide

hydrochloric acid

Sr + 2e Sr2+

2Cl - 2e Cl-

hydrogen and strontium hydroxide

2


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12) June 2006 Q (3_b)

(b) The following apparatus was set up to investigate the electricalconductivity of dilute acids.

Dilute sulphuric acid is a strong acid. If it was replaced by a weak acid, whattwo differences in the observations would you expect to make?

..

..

.[2]

13) Nov 2006 Q (6_b)

(b) Impure copper is extracted from the ore. This copper is refined byelectrolysis.

(i) Name; the material used for the positive electrode (anode),

..

the material used for the negative electrode (cathode),

..

a suitable electrolyte.

.[3]

(ii) Write an ionic equation for the reaction at the negative electrode.

[1]

(iii) One use of this pure copper is electrical conductors, another is to makealloys. Name the metal that is alloyed with copper to make brass.

[1]

with weak acid

slower rate of bubbles

bulb less brighter

impure copper

pure copper

copper (II) sulfate

Cu + 2e Cu2+

zinc


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14) June 2007 Q (6_a, b, c)

6) Aluminium is extracted by the electrolysis of a molten mixture that containsalumina, 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 aqueoussodium 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.

[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]

(d) Give an explanation for each of the following.

(i) Aluminium is used extensively in the manufacture of aircraft...

.[1]

(ii) Aluminium is used to make food containers...

.[2]

aluminium oxide

iron (III) oxide

sodium aluminate

filteration

carbon

cathode

900 Caluminium

cryolite

Al + 3e Al-3+

oxygen is formed react with carbon

because it has low density

because it is unreactive


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(iii) Aluminium electricity cables have a steel core...

.[1]

15) Nov 2007 Q (3_b, iii)b) (iii) The reaction between magnesium and bromine is redox. Complete thesentences.

Magnesium is . the agent because it has electrons.

Bromine has been because it has electrons. [4]

16) Nov 2007 Q (4_c)

(c) The remaining zinc oxide reacts with sulfuric acid to give aqueous zincsulfate. This is electrolysed with inert electrodes (the electrolysis is the sameas that of copper(II) sulfate with inert electrodes). ions present:

2 2

( ) 4( ) ( ) ( ), , ,aq aq aq aqZn SO H OH

(i) Zinc forms at the negative electrode (cathode). Write the equation for thisreaction

[1]

(ii) Write the equation for the reaction at the positive electrode (anode).

[2](iii) The electrolyte changes from aqueous zinc sulfate to

[1]

17) June 2008 Q (3_a)

3 Copper is purified by electrolysis.(a) Complete the following.

The positive electrode (anode) is made from ..

The negative electrode (cathode) is made from ..

The electrolyte is aqueous .[3]

(b) Write an ionic equation for the reaction at the positive electrode (anode)...

.[2]

(c) (i) Give two reasons why copper is used,

in electric wiring, ...

.[2]

for strength

reducing

oxidized

lost

gained

Zn + 2e Zn2+

4OH - 4e O + H O-

sulfuric acid

2 2

impure copper

pure copper

copper (II) sulfate

Cu - 2e Cu2+

because it is good conductor for electricity and it is ductile


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in cooking utensils. ...

....[2]

(ii) Give another use of copper

[1]

18) Nov 2008 Q (5_a)

5) The electrolysis of concentrated aqueous sodium chloride produces threecommercially important chemicals hydrogen, chlorine and sodium hydroxide.

(a) The ions present are ( ) ( ) ( ) ( ), ,aq aq aq aqa H Cl and OH

(i) Complete the ionic equation for the reaction at the negative electrode(cathode).

2......... .......... H [1]

(ii) Complete the ionic equation for the reaction at the positive electrode(anode).

2......... .......... Cl [1]

(iii) Explain why the solution changes from sodium chloride to sodiumhydroxide.

[1]

19) Nov 2008 Q (6_a, iii)

(iii) The following is a redox reaction.

2 2n Sn Mn Sn

Indicate on the equation the change which is oxidation.Give a reason for your choice.

............................................................................................................................

........................................................................................................................[2]

because it is good conductor for heat and it is unreactive

making alloys as brass

2 H 2 e+

2 Cl 2 e-

because Na ion and OH ion left in the solution react with each other to form

sodium hydroxide

+ -

Mn is oxidized because its oxidation number changed from zero to +2

or because it lose electrons


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20) June 2009 Q (2)

2) The results of experiments on electrolysis using inert electrodes are givenin the table. Complete the table; the first line has been completed as anexample.

[8]

21) Nov 2009 Q (4_b)

(b) Ozone is an oxidant. It can oxidise an iodide to iodine.

3 2 2 22 2I O H I O H O

(i) What would you see when ozone is bubbled through aqueous acidifiedpotassium iodide?

............................................................................................................................

............................................................................................................................

............................................................................................................................

........................................................................................................................[2]

(ii) Explain in terms of electron transfer why the change from iodide ions toiodine molecules is oxidation.

............................................................................................................................

........................................................................................................................[1]

molten

potassium iodide

hydrogen formed oxygen formed solution becomes

more concentrated

sulfuric acid

formed

oxygen formedcopper formed

concentrated

potassium bromide

the colour of solution changed from colurless to brwon as iodine is formed

because iodide ion loses electrons to form iodine molecule


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(iii) Explain, using your answer to b(ii), why ozone is the oxidant in thisreaction.

............................................................................................................................

........................................................................................................................[1]

22) June 2010 Q [31] (7_c)

7 Titanium is a transition element. It is isolated by the following reactions.

(c) Explain why the change, titanium(IV) chloride to titanium, is reduction.

............................................................................................................................

....................................................................................................................... [1]

23) June 2010 Q [32] (3_a, iii)

3 The following are examples of redox reactions.(a) Bromine water was added to aqueous sodium sulfide.

2

2( ) ( ) ( ) ( )2

aq aq aq sr S Br S

(iii) Explain, in terms of electron transfer, why bromine is the oxidant(oxidising agent) in this reaction.

............................................................................................................................

....................................................................................................................... [2]24) June 2010 [32] Q (7)

7Aluminium was first isolated in 1827 using sodium

3 3 3C Na A NaC

Aluminium, obtained by this method, was more expensive than gold.

(a) Suggest an explanation why aluminium was so expensive.

.......................................................................................................................................

.................................................................................................................................. [1]

(b) The modern method for extracting aluminium is the electrolysis of a moltenelectrolyte, aluminium oxide dissolved in cryolite. The aluminium oxidedecomposes.

2 3 22 4 3Al O Al O

Both electrodes are made of carbon.

(i) Give two reasons why the oxide is dissolved in cryolite....................................................................................................................................

...................................................................................................................................

.............................................................................................................................. [2]

because it gained electrons

because oxidation number of Ti reduced from +4 to zero

because its oxidation number changed from zero to -1 or because it gain electrons

sodium is expensive

or high energy costs in extraction of sodium

it reduce the melting point of mixture

it improve the conductivity of the mixture


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(ii) Complete the ionic equation for the reaction at the anode.

2

2........ ........O O e

[2]

(iii) Why do the carbon anodes need to be replaced frequently?...................................................................................................................................

.............................................................................................................................. [1]

(c) The electrolysis of a molten electrolyte is one method of extracting a metalfrom its ore.

Other methods are the electrolysis of an aqueous solution and the reductionof the oxide by carbon. Explain why these last two methods cannot be used toextract aluminium.

electrolysis of an aqueous solution .........................................................................

.......................................................................................................................................

using carbon ......................................................................................................

....................................................................................................................... [2]

25) Nov 2010 [31] Q (2_c, iii)

2About 4000 years ago the Bronze Age started in Britain. Bronze is an alloyof copper and tin

(iii) Copper oxide is reduced to copper which is then refined by electrolysis.

Label the diagram of the apparatus which could be used to refine copper.

2 4

because they burn away (or react with oxygen gives carbon dioxide)

hydrogen is formed not aluminium because

aluminium is more active than hydrogen

because aluminium is more active than carbon so carbon does not

reduce aluminium oxide

impure copper

copper (II) sulfate

pure copper


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26) Nov 2010 [32] Q (4_b)

(b) Chromium is used to electroplate steel objects. The diagram shows howthis could be done.

(i) Give two reasons why steel objects are plated with chromium.

............................................................................................................................

....................................................................................................................... [2]

(ii) The formula of the chromium(III) ion is Cr3+ and of the sulfate ion is 24

SO

Give the formula of chromium(III) sulfate.

....................................................................................................................... [1]

(iii) Write the equation for the reaction at the negative electrode (cathode).

....................................................................................................................... [2]

(iv)A colourless gas, which relights a glowing splint, is formed at the positive

electrode (anode). Name this gas.

....................................................................................................................... [1]

(v) During electrolysis, it is necessary to add more chromium(III) sulfate butduring copper-plating using a copper anode, it is not necessary to add morecopper(II) sulfate. Explain.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

27) Nov 2010 [33] Q (4_a)

4 The electrolysis of concentrated aqueous sodium chloride, between inertelectrodes, 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).

to make it shinyto resist corrosion

4Cr (SO )32

Cr + 3e Cr3+

oxygen

add more chromium (III) sulfate to replace chromium ions used up to plate steel

but it is not necessary to add copper (II) sulfate because copper ions replaced from

the anode


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Write an equation for this reaction.

....................................................................................................................... [2]

(ii) The hydrogen ions are from the water.

2O 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 formedat the positive electrode (anode), a different gas is produced. Name this gas.

....................................................................................................................... [1]

(iv) State an example of an inert electrode.

....................................................................................................................... [1]

28) June 2011 [31] Q (2_c)

c) The selenide ion reacts with water

2

2Se H O HSe OH

What type of reagent is the selenide ion in this reaction? Give a reasonfor your answer

............................................................................................................................

....................................................................................................................... [2]

29) June 2011 [31] Q (5_b)

Hydriodic acid HI(aq) is a strong acid, its salts are iodides

a) it has the reactions of a typical strong acid. Complete the following equations

i) ........ ........ .............. ..............Li HI [1]

ii)..................... ..................... ..............

..................... .....................

Zinc hydroiodic

carbonate acid[2]

iii) ........ .............. ..............gO HI [1]

b) two of the reactions in (a) are acid/base and one is redox. Which one isredox? Explain your choice

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

2H + 2e H+

Hydrogen ion is removed from the solution in the form of hydrogen gas

so that the equilibrium shift right and the concentration of hydroxide ion increase

Oxygen

Chlorine

2

base because it accept a proton

2 2 LiOH H2

zinc

iodide

carbon

dioxide

water

MgI H O22 2

reaction (i) is redox reaction because oxidation number of Li changed from 0 to +1


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30) June 2011 [32] Q (2_b)

(b) Aqueous tin(II) sulfate is electrolysed using carbon electrodes. Thiselectrolysis is similar to that of aqueous copper(II) sulfate using carbonelectrodes.

(i) What is the product at the negative electrode (cathode)?

....................................................................................................................... [1]

(ii) Write the equation for the reaction at the positive electrode (anode).

....................................................................................................................... [2]

(iii) Name the acid formed in this electrolysis.

....................................................................................................................... [1]

31) Nov 2011 [31] Q (3_a, b)

3 Aluminium is extracted by the electrolysis of a molten mixture of alumina,which is aluminium oxide, and cryolite.

(a) (i)Alumina is obtained from the main ore of aluminium.Name this ore.

.................................................................................................................................. [1]

(ii) Explain why it is necessary to use a mixture, alumina and cryolite, ratherthan just alumina.

.......................................................................................................................................

.................................................................................................................................. [2]

(iii) Copper can be extracted by the electrolysis of an aqueous solution.Suggest why the electrolysis of an aqueous solution cannot be used to extractaluminium.

.......................................................................................................................................

.......................................................................................................................................

.................................................................................................................................. [2]

tin

4 OH - 4 e O + 2 H O

sulfuric acid

-2 2

Bauxite

to decrease the melting point of the mixture and make it good conductor

because aluminium is more active than hydrogen


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(b) The ions which are involved in the electrolysis are Al3+ and O2. Theproducts of this electrolysis are given on the diagram.

Explain how they are formed. Use equations where appropriate.

.......................................................................................................................................

.......................................................................................................................................

.......................................................................................................................................

.................................................................................................................................. [4]

32) Nov 2011 [31] Q (4_b)

(b) Iron has two oxidation states +2 and +3. There are two possible equationsfor the redox reaction between iron and bromine.

Fe + Br2Fe2+ + 2 Br2 Fe + 3 Br22 Fe3+ + 6Br

(i) Indicate, on the first equation, the change which is oxidation. Give a reasonfor your choice.

.......................................................................................................................................

.................................................................................................................................. [2]

(ii) Which substance in the first equation is the reductant (reducing agent)?

.................................................................................................................................. [1]

33) Nov 2011 [33] Q (6_b, iv)

(b) Nickel ores are converted into nickel(II) oxide. This can be reduced toimpure nickel by heating with carbon. The nickel is purifi ed by the followingreversible reaction.

Ni(s) + 4CO(g) Ni(CO)4(g)

nickel carbonyl

(iv) Suggest another method of refining nickel. Give a brief description of themethod which you have suggested. A labelled diagram is acceptable.

[4]

at cathode Al + 3e Al

at anode 2 O O + 4e

oxygen reacts with carbon (anode) to form carbon dioxide

3+

- 2

C + O CO22

because its oxidation number changed from 0 to 2+

Fe

.......................................................

nickel sulfate solution

anode

impure nickel pure nickel

cathode

.......................................................

.......................................................

.......................................................

.......................................................

.......................................................

.......................................................


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34) June 2012 [31] Q (4_d)

(d) The oxidation states of vanadium in its compounds are V(+5), V(+4),V(+3) and V(+2). The vanadium(III) ion can behave as a reductant or anoxidant.

Indicate on the following equation which reactant is the oxidant.

3 2 22 2V Zn V Zn [1]

Which change in the following equation is oxidation? Explain your choice

3 3 4 22 2V Fe V Fe ............................................................................................................................

........................................................................................................................[2]

35) June 2012 [31] Q (4_a, i, ii, b)

4 The ore of aluminium is bauxite which is impure aluminium oxide. Alumina,pure aluminium oxide, is obtained from bauxite.

Aluminium is formed at the cathode when a molten mixture of alumina andcryolite, Na3AlF6, is electrolysed.

(a) (i) Name two products formed at the anode in this electrolysis.

....................................................................................................................... [2]

(ii)All the aluminium formed comes from the alumina not the cryolite.

Suggest two reasons why the electrolyte must contain cryolite.

............................................................................................................................

....................................................................................................................... [2]

(b) The purification of bauxite uses large amounts of sodium hydroxide.

(i) Describe the chemistry of how sodium hydroxide is made fromconcentrated aqueous sodium chloride. The description must include at leastone ionic equation.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

....................................................................................................................... [5]

(ii) Making sodium hydroxide from sodium chloride produces two otherchemicals.

Name these two chemicals and state one use of each chemical.

chemical .............................................................................................................

use .....................................................................................................................

3+ 4+V to V

because it loss electrons

oxygen and carbon dioxide

because cryolite decrease the melting point of mixture and it improve the

conductivity of the mixture

by electrolysis of concentrated aqueous sodium chloride using two graphite electrodes

where chlorine formed at anode (2Cl - 2e Cl ) and hydrogen gas formed at

cathode, solution remaining contains Na and OH which combine to produce

sodium hydroxide which left behind in the solution

- 2

hydrogen

making ammonia


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chemical .............................................................................................................

use ................................................................................................................ [2]

36) June 2012 [32] Q (4_a, i, ii, b)

4 The ore of aluminium is bauxite which is impure aluminium oxide. Alumina,pure aluminium oxide, is obtained from bauxite.

Aluminium is formed at the cathode when a molten mixture of alumina andcryolite, Na3AlF6, is electrolysed.

(a) (i) Name two products formed at the anode in this electrolysis.

....................................................................................................................... [2]

(ii)All the aluminium formed comes from the alumina not the cryolite.

Suggest two reasons why the electrolyte must contain cryolite.

............................................................................................................................

....................................................................................................................... [2]

(iii) The major impurity in bauxite is iron(III) oxide. Iron(III) oxide is basic,aluminium oxide is amphoteric. Explain how aqueous sodium hydroxide canbe used to separate them.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

(b) The purification of bauxite uses large amounts of sodium hydroxide.

(i) Describe the chemistry of how sodium hydroxide is made fromconcentrated aqueous sodium chloride. The description must include at leastone ionic equation.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

....................................................................................................................... [5]

(ii) Making sodium hydroxide from sodium chloride produces two otherchemicals.

Name these two chemicals and state one use of each chemical.

chemical .............................................................................................................

use .....................................................................................................................

chemical .............................................................................................................

use ................................................................................................................ [2]

chlorine

making bleach / treatment of water

oxygen and carbon dioxide

because cryolite decrease the melting point of mixture and it improve the

conductivity of the mixture

aluminum oxide react with sodium hydroxide gives salt and water while iron (III) oxide

does not react with sodium hydroxide so it can be separated by filtration

by electrolysis of concentrated aqueous sodium chloride using two graphite electrodes

where chlorine formed at anode (2Cl - 2e Cl ) and hydrogen gas formed at

cathode, solution remaining contains Na and OH which combine to produce

sodium hydroxide which left behind in the solution

hydrogen

making ammonia

chlorine

making bleach

2-

+ -


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37) Nov 2012 [32] Q (6)

6 During electrolysis, ions move in the electrolyte and electrons move in theexternal circuit.

Reactions occur at the electrodes.

(a) The diagram shows the electrolysis of molten lithium iodide.

(i) Draw an arrow on the diagram to show the direction of the electron flow 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 whenmolten it is a good conductor.

............................................................................................................................

....................................................................................................................... [1]

(b) The results of experiments on electrolysis are shown in the following table.Complete the table. The first line has been done as an example.

[4]

from battery

from negative electrode of battery to external circuit

in solid ions cannot move in solid but in molten ions can move in liquid

copper

hydrogen

sulfuric acidformed

potassiumhydroxide


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(c) The diagram below shows the electrolysis of dilute sulfuric acid. Hydrogenis formed at the negative electrode (cathode) and oxygen at the positiveelectrode (anode) and the concentration of sulfuric acid increases.

The ions present in the dilute acid are ( )aq

,( )aq

OH

and2

4( )aqSO

.

(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).

( ) 2( ) 2 ( )4 ..... .......

aq g l OH O H O

[1]

(iii) Suggest an explanation of why the concentration of the sulfuric acidincreases.

....................................................................................................................... [1]

38) Nov 2012 [33] Q (4_b, i, iv)

4 Zinc alloys have been used for over 2500 years.

(b) Zinc metal is made by the reduction of zinc oxide. The major ore of zinc iszinc blende,

ZnS. Zinc blende contains silver and lead compounds as well as zinc sulfide.

Zinc blende is converted into impure zinc oxide by heating it in air.

2 22ZnS + 3O 2ZnO + 2SO

(i) Describe how zinc oxide is reduced to zinc.

....................................................................................................................... [1]

2H + 2e H+

2

2 4 e

because water used up

heat zinc oxide with carbon or carbon monoxide


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(iv) Describe how zinc metal can be obtained from zinc sulfate solution byelectrolysis.

A labelled diagram is acceptable. Include all the products of this electrolysis.The electrolysis is similar to that of copper(II) sulfate solution with inert

electrodes.

[4]

39) June 2013 [31] Q (5_a, iii, iv, v)

5 The reactivity series shows the metals in order of reactivity.

(a) The reactivity series can be established using displacement reactions. Apiece of zinc is added to aqueous lead nitrate. The zinc becomes coated witha black deposit of lead.

Zn Pb Zn Pb

Zinc is more reactive than lead.

The reactivity series can be written as a list of ionic equations.

........ ...... + ...... most reactive metal : the best reductant (reducing agent)

(iii) Explain why the positive ions are likely to be oxidants (oxidising agents).

....................................................................................................................... [1]

(iv) Deduce which ion is the best oxidant (oxidising agent).

....................................................................................................................... [1]

(v) Which ion(s) in the list can oxidise lead metal?

....................................................................................................................... [1]

carbon cathode

zinc deposited

electrolyte becomes sulfuric acid

oxygen formed

carbon anode

.......................................................

.......................................................

.......................................................

.......................................................

.......................................................

.......................................................

H+ SO42-

Mg Mg 2e

because they can gain electrons

Ag or silver+

Ag and Cu or silver and copper+ 2+


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40) June 2013 [32] Q (6)

6Aluminium is an important metal with a wide range of uses.

(a)Aluminium is obtained by the electrolysis of aluminium oxide dissolved inmolten cryolite.

(i) Solid aluminium oxide is a poor conductor of electricity. It conducts eitherwhen molten or when dissolved in molten cryolite. Explain why.

............................................................................................................................

............................................................................................................................

....................................................................................................................... [2]

(ii) Why is a solution of aluminium oxide in molten cryolite used rather thanmolten aluminium oxide?

....................................................................................................................... [1]

(iii) Explain why the carbon anodes need to be replaced periodically.

....................................................................................................................... [1]

(iv) One reason why graphite is used for the electrodes is that it is a goodconductor of electricity. Give another reason.

....................................................................................................................... [1]

(b)Aluminium is used to make food containers because it resists corrosion.

Explain why it is not attacked by the acids in food.

............................................................................................................................

....................................................................................................................... [2]

(c) Aluminium is used for overhead power (electricity) cables which usuallyhave a steel core.

because no free ions in solid state while there are free ions in liquid state

the process carried out at lower temperatures and the conductivity was improved.

because the oxygen given off at the anode reacts with the graphite anodeforming carbon dioxide.

graphite was unreactive

due to the presence of the protective layer of aluminium oxide which prevents

the aluminium beneath from reacting


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M f B k ( 6 6 )

90

(i) Give two properties of aluminium which make it suitable for this use.

............................................................................................................................

....................................................................................................................... [2]

(ii) Explain why the cables have a steel core.............................................................................................................................

....................................................................................................................... [1]

1) good conductor of electricity

2) low density

steel core increased strength

5 Electrochemistry

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