Metals

Unit 10 Occurrence and extraction of metals

Unit 11 Reactivity of metals

Unit 12 Reacting masses

Unit 13 Corrosion of metals and their protection

Topic 3

KeyC o ncepts

Reactivity of metals• Comparing the reactivityofmetals• The reactivity series• Writing chemical and ionic equations• Displacement reactions• Prediction ofmetal reactionsusing the

reactivity series

Metals

Corrosion of metals and their protection

• The necessary conditions for rusting• Factors that speedup the rusting

process• How toprevent rusting• Corrosion resistanceof aluminiumand

aluminiumanodization

Reacting masses• Calculations involvingmoles and

masses• Determining thepercentagebymass

of an element in a compound• Determining the empirical formulaof

a compound• Calculations involving chemical

equations and reactingmasses

Occurrence and extraction of metals

• Uses andproperties ofmetals• Different metal extractionmethods• Importance of recycling• Conservation ofmetals

Topic 3� Metals �Unit 10  Occurrence and extraction of metals

10.1 – 10.9

Summary

1 Aluminiumand iron are the twomost abundantmetals in theEarth’s crust.

2 Thedifferentmethodsofmetal extraction include

• heating theore alone;

• reductionwith carbon;

• electrolysis.

3 a) The sequence of discovery of various metals relates closely with the ease ofextracting the metals from their ores. Metals that are difficult to extract werediscovered later.

b)Aluminium is the most abundant metal in the Earth’s crust. The extraction ofaluminiumoccurredmuch later than thatof ironbecausealuminium isdifficultto extract. Electrolysis is required for its extraction.

c) Thefollowingtablesummarizestheyearsofdiscoveryandextractionmethodsofsome commonmetals.

Metal Year of discovery Ore

Main metallic compound in the

oreExtraction method

Gold (Au)

about8000B.C.

as freeelement free elementof gold mechanical separation

Silver (Ag)argentiteor as freeelement

silver sulphide(Ag2S) or freeelementof silver

displacement from solutionormechanical separation

Mercury(Hg) cinnabar mercury(II) sulphide

(HgS)

HgS(s) +O2(g)heatin air

Hg(l) + SO2(g)

Copper(Cu)

about6000B.C.

copperpyrite

copper(II) iron(II)sulphide (CuFeS2) sulphide

controlledheat in air

metal

Lead (Pb) about3500B.C. galena lead(II) sulphide

(PbS)

2PbS(s) + 3O2(g)heat

2PbO(s) + 2SO2(g)

2PbO(s) +C(s)heat

2Pb(s) +CO2(g)

Iron (Fe) about1200B.C. haematite iron(III) oxide

(Fe2O3)

carbonmonoxide reactswithiron(III) oxide in ablastfurnace, producing iron.

Fe2O3(s) + 3CO(g)heat

2Fe(l) + 3CO2(g)

Zinc (Zn) 1746 zincblende zinc sulphide (ZnS)

2ZnS(s) + 3O2(g)heat

2ZnO(s) + 2SO2(g)

2ZnO(s) +C(s)heat

2Zn(s) +CO2(g)

10.1 Metals inour lives

10.2 Uses ofmetals and their properties

10.3 Metals in theEarth’s crust

10.4 Occurrence ofmetals innature and their extractionmethods

10.5 Year ofdiscoveryofmetals and the easeof extraction

10.6 How longwillmetal resources last?

10.7 The life cycleofmetals

10.8 Importance of recycling

10.9 Conservation ofmetal resources

Unit 10 Occurrence and extraction of metals

Topic 3� Metals �Unit 10  Occurrence and extraction of metals

Example

Carbon canbeused to extractmetals fromcertainmetal oxides.

a) Suggest how copper can be extracted from copper(II) oxide using carbon. State theexpectedobservations. (2marks)

b)Explain whether the method of extraction by carbon can be used to distinguishbetween copper(II) oxide and silver oxide. (2marks)

➤Justmixingcopper(II)oxideandcarbondoesNOTproducecopper.Heatingis required.

➤Besides using a method that only one substance will react, we can alsousethedifferentresultsproducedbycopper(II)oxideandsilveroxideinthesame test to distinguish between them.

➤The following methods can also be used to distinguish between the twooxides:

– heating each strongly (only silver oxide decomposes to give silver andoxygen);

– heating each with dilute sulphuric acid (both react with the acid, butonly copper(II) oxide gives a blue solution).

➤Studentsshouldunderstandthebehaviourofdifferentoxidesuponheatingwith carbon powder.

e.g.

Carbon CANNOT extract magnesium from magnesium oxide becausemagnesium is high in the metal reactivity series and magnesium oxide isvery stable.

RemarksRemarks*

ExamtipsExamtipsExamtipsExamtips ♦ Questions often ask about theuses of titanium.

– Titanium is light but very strong. It resists corrosion and has a veryhighmelting point.

– Titanium iswidelyused inaerospace industryand joint replacementparts.

– Titaniumisalsofoundinmanyconsumerproducts,suchasjewellery,watch cases, glasses, bicycles and clocks.

♦ Gold ornaments are usually made from gold alloys instead of puregold because the alloys are stronger and harder than pure gold. Theproportionofgoldinitsalloyisgivenbyacaratnumber.24-caratgoldis pure gold. 18-carat gold contains 18 parts by mass of gold in 24parts.

♦ Cadmiumisusedtomakerechargeablecells(nickel-cadmiumsecondarycells).

Metal Year of discovery Ore

Main metallic compound in the

oreExtraction method

Aluminium(Al) 1826 bauxite

hydratedaluminiumoxide(Al2O3•2H2O)

electrolysis ofmoltenore

Magnesium(Mg) 1808 magnesite magnesium

carbonate (MgCO3)

Sodium(Na) 1807 rock salt sodiumchloride

(NaCl)

Potassium(K) 1807 carnallite

hydratedpotassiummagnesiumchloride(KCl•MgCl2•6H2O)

4 Recycling is very importantbecause

a) metal resources are limited;

b)it can save fuel andother resources;

c) it can reduce environmental impact; and

d)it can raisepublic awarenessof conservation.

5 Metal resources are limited. The conservation methods are Reuse, Reduce, RecycleandReplace.

Answer

a) Mix carbonpowderwith copper(II) oxide, andheat themixture strongly. (1)

A reddishbrownpowder is obtained. (1)

b)Yes (1)

Both copper and silver can be extracted from their oxides by heating with carbonpowder.

Copper(II) oxide produces a reddish brown powder while silver oxide produces agreypowder. (1)

Topic 3� Metals �Unit 11  Reactivity of metals

11.1 Comparing the reactivityofmetals

11.2 How dometals reactwithoxygen?

11.3 How dometals reactwithwater or steam?

11.4 How dometals reactwithdilute acids?

11.5 The reactivity series

11.6 What is a chemical equation?

11.7 How towrite balanced chemical equations

11.8 What determines the reactivityof ametal?

11.9 Displacement reactions

11.10 Ionic equations

11.11 Relationship between the extraction method and position ofmetals in the reactivity series

11.12 Prediction ofmetal reactionsusing the reactivity series

Unit 11 Reactivity of metals 11.1 – 11.4

Summary

1 Metals can be arranged in a list of decreasing reactivity. This order is called thereactivity series.

2 The following table summarizes the reactionsofmetals in the reactivity series.

Metal

Reaction of metal with air

or oxygen

Reaction of metal with water or steam

Reaction of metal with dilute acids (dilute HCl and

H2SO4)

Displacement reaction

Potassium burnvigorously

reactwith coldwater

metal +watermetalhydroxide+

hydrogen

react explosively K,Na andCareactwithwaterto giveoffhydrogengas

Sodium

Calcium

reactwithdecreasingvigour

metal +dilutehydrochloric acid

metal chloride+hydrogen

metal +dilutesulphuric acid

metal sulphate+hydrogen

Magnesiumreactwith steam

metal + steammetal oxide+

hydrogen ametalwillalwaysdisplacea less reactivemetal fromasolutionof thecompoundofthe less reactivemetal, butnotvice versa

Aluminium

Zinc

Iron

Lead a layerof oxideformsonthe surface

no reaction

Copper

no reaction

Mercury

Silver

noreactionPlatinum

Gold

3 Balancedchemicalequations for reactionsof somecommonmetalswithoxygen inthe air:

Potassium 4K(s) +O2(g) 2K2O(s)

Sodium 4Na(s) +O2(g) 2Na2O(s)

Calcium 2Ca(s) +O2(g) 2CaO(s)

Magnesium 2Mg(s) +O2(g) 2MgO(s)

Aluminium 4Al(s) + 3O2(g) 2Al2O3(s)

Zinc 2Zn(s) +O2(g) 2ZnO(s)

Iron 3Fe(s) + 2O2(g) Fe3O4(s)

Lead 2Pb(s) +O2(g) 2PbO(s)

Copper 2Cu(s) +O2(g) 2CuO(s)

Mercury 2Hg(l) +O2(g) 2HgO(s)

Topic 310 Metals 11Unit 11  Reactivity of metals

ExamtipsExamtipsExamtipsExamtips ♦ There isNO reactionbetween silver anddilute sulphuric acid. There isNO reaction between silver and concentrated hydrochloric acid.

♦ There is NO reaction between copper and dilute ethanoic acid.

♦ Questions may give a list of processes and ask which of them giveshydrogen gas as themajor product.

e.g.

– Adding zinc to dilute nitric acid does NOT give hydrogen. Nitrogenmonoxidegas isevolved.This isbecausedilutenitricacidactsasanoxidizing agent.

3Zn(s) + 2NO3–(aq)+8H+(aq) 3Zn2+(aq)+2NO(g)+4H2O(l)

– Adding copper to dilute sulphuric acid does NOT give hydrogen.There is NO reaction.

– Electrolyzing brine using mercury as the cathode does NOT givehydrogen. Sodium amalgam is formed at the cathode.

– Passingsteamoverheatedzincgranules/ironpowdergiveshydrogengas as themajor product.

– Electrolyzing potassium bromide solution using carbon electrodesgives hydrogen at the cathode.

– Adding zinc granules to dilute ethanoic acid gives hydrogen.

4 Balanced chemical equations for reactions of some common metals with water orsteam:

Potassium 2K(s) + 2H2O(l) 2KOH(aq) +H2(g)

Sodium 2Na(s) + 2H2O(l) 2NaOH(aq) +H2(g)

Calcium Ca(s) + 2H2O(l) Ca(OH)2(s) +H2(g)

Magnesium Mg(s) +H2O(g) MgO(s) +H2(g)

Aluminium 2Al(s) + 3H2O(g) Al2O3(s) + 3H2(g)

Zinc Zn(s) +H2O(g) ZnO(s) +H2(g)

Iron 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)

5 Balanced equations for reactions of some common metals with dilute hydrochloricacid anddilute sulphuric acid:

Metal Chemical equation Ionic equation

CalciumCa(s) + 2HCl(aq) CaCl2(aq) +H2(g)

Ca(s) + 2H+(aq)Ca2+(aq) +H2(g)

Ca(s) +H2SO4(aq) CaSO4(s) +H2(g) —

MagnesiumMg(s) + 2HCl(aq) MgCl2(aq) +H2(g)

Mg(s) +H2SO4(aq) MgSO4(aq) +H2(g)Mg(s) + 2H+(aq)

Mg2+(aq) +H2(g)

Aluminium2Al(s) + 6HCl(aq) 2AlCl3(aq) + 3H2(g)

2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)2Al(s) + 6H+(aq)

2Al3+(aq) + 3H2(g)

ZincZn(s) + 2HCl(aq) ZnCl2(aq) +H2(g)

Zn(s) +H2SO4(aq) ZnSO4(aq) +H2(g)Zn(s) + 2H+(aq)

Zn2+(aq) +H2(g)

IronFe(s) + 2HCl(aq) FeCl2(aq) +H2(g)

Fe(s) +H2SO4(aq) FeSO4(aq) +H2(g)Fe(s) + 2H+(aq)

Fe2+(aq) +H2(g)

Example

Calciumreactswithcoldwatertogiveagas. Inacertainexperiment,astudentaddedsomenewlybought calciumgranules into abeakerof coldwater.

a) Write a chemical equation for the reactionbetween calciumandwater. (1 mark)

b)Drawa labelleddiagramof theset-up forcarryingout thereactionofcalciumwithcoldwater in abeaker,with the collectionof the gas. (2marks)

c) The student added a few drops of methyl orange indicator to the content of thebeaker after the experiment.

State your expectedobservation. Explainyour answer. (2marks)

d)SuggestTWOpotentialhazards inperforming the above experiment. (2 marks)

e) Thestudentrepeatedtheexperimentwiththesamemassof‘expired’calciumgranulesthathadbeen exposed in air for a long time.

Suggest why less gas was collected in this second experiment compared with thefirst experiment. (1mark)

Topic 312 Metals 13Unit 11  Reactivity of metals

➤Questions often ask about the reaction between calcium and water, andthe set-up for collecting thehydrogengas evolved.

➤Calcium sinks inwater.

➤Todrawexperimental set-upsaccurately, studentsneed topayattention inpractical lessons.

RemarksRemarks*

Answer

a) Ca(s) + 2H2O(l) Ca(OH)2(s) +H2(g) (1)

b)

(2)

hydrogen

water

inverted funnel

calcium

(1 mark for correct set-up; 1 mark for correct labels; 0 mark if the set-up is notworkable)

c) Themethylorange indicator turnedyellow. (1)

This is because the calciumhydroxide formed is alkaline. (1)

d)Any twoof the following:

• Thehydrogengasproduced is explosive / flammable. (1)

• Calcium is corrosive. (1)

• Heat is givenoff from the reaction. (1)

e) Mostof the ‘expired’ calciumhadbeenoxidizedby air to formcalciumoxide. (1)

11.5 – 11.12

Summary

1 Atoms of more reactive metals lose outermost shell electrons to form cations morereadily.

2 Ionic equations for somedisplacement reactions:

Metal Reaction with solution of Ionic equation

Magnesium

aluminiumcompound

e.g.AlCl3,Al2(SO4)3,Al(NO3)33Mg(s) + 2Al3+(aq) 3Mg2+(aq) + 2Al(s)

zinc compound

e.g. ZnCl2, ZnSO4, Zn(NO3)2Mg(s) +Zn2+(aq) Mg2+(aq) +Zn(s)

iron(II) compound

e.g. FeCl2, FeSO4, Fe(NO3)2Mg(s) + Fe2+(aq) Mg2+(aq) + Fe(s)

copper(II) compound

e.g.CuCl2,CuSO4,Cu(NO3)2Mg(s) +Cu2+(aq) Mg2+(aq) +Cu(s)

silver compound

e.g.AgNO3Mg(s) + 2Ag+(aq) Mg2+(aq) + 2Ag(s)

Zinc

iron(II) compound Zn(s) + Fe2+(aq) Zn2+(aq) + Fe(s)

copper(II) compound Zn(s) +Cu2+(aq) Zn2+(aq) +Cu(s)

silver compound Zn(s) + 2Ag+(aq) Zn2+(aq) + 2Ag(s)

Ironcopper(II) compound Fe(s) +Cu2+(aq) Fe2+(aq) +Cu(s)

silver compound Fe(s) + 2Ag+(aq) Fe2+(aq) + 2Ag(s)

Copper silver compound Cu(s) + 2Ag+(aq) Cu2+(aq) + 2Ag(s)

3 The lower the position of the metal in the reactivity series, the more easily it canbe extracted from its ores.

4 Compoundsof ametal lower in the reactivity serieshave lower stability.Thus, thereductionof theoxideof themetal is easier.

ExamtipsExamtipsExamtipsExamtips ♦ When sodium is added to copper(II) sulphate solution, a colourlessgasevolves. This isbecause sodium reactswith thewater in copper(II)sulphate solution.

♦ Excess zincgranules are added to a solution containing copper(II) ionsand magnesium ions, the reaction mixture is filtrated. The residuecontains copper metal and the filtrate contains zinc ions.

Thisisbecausezinccandisplacecopper(II) ionsfromthesolutionwhileitCANNOTdisplace magnesium ions from the solution.

Zn(s) +Cu2+(aq) Zn2+(aq)+Cu(s)

♦ Displacement reactions can also be used to distinguish between twosolutions.

e.g.

Zincstripcanbeusedtodistinguishbetweenmagnesiumnitratesolutionand silver nitrate solution. Zinc can displace silver from silver nitratesolution but NOTmagnesium from magnesium nitrate solution.

Topic 31� Metals 1�Unit 12  Reacting masses

➤Displacement reactions canbeused topredict the relative reducingpowerofmetals.

➤WhenmagnesiumribbonsareaddedtoFeSO4(aq),adisplacementreactionoccurs. We say that the Fe2+(aq) ions act as the oxidizing agent. They arereduced. On the other hand, magnesium acts as the reducing agent. It isoxidized.

Mg(s) + Fe2+(aq) Mg2+(aq)+ Fe(s)

RemarksRemarks*

Example

The tablebelow showswhetherdisplacement reactionoccursbetweenmetalsW,X,YandZwith their ions.

W X Y Z

W2+(aq) ✘ 4 4

X+(aq) 4 4 4

Y2+(aq) ✘ ✘ 4

Z+(aq) ✘ ✘ ✘

‘4’ representsdisplacement reactionoccurs

‘✘’ representsdisplacementdoesnotoccur

a) Arrange the fourmetals inorderof increasing reactivity. Explainyour answer. (3marks)

b)Whichof themetals is the strongest reducing agent? Explainyour answer. (2marks)

Answer

a) Reactivity:X<W<Y<Z (1)

Metal Z can displace the other three metals from solutions of theirions. Thus, it is themost reactive.

Metal Y can displace metals W and X from solutions of their ions.Thus, it ismore reactive thanmetalsWandX.

Metal W can displace metal X from a solution of its ions. Thus, itismore reactive thanmetalX.

b)An atomofmetal Z loses electronsmost easily. (1)

Thus,metal Z is the strongest reducing agent. (1)

(2)(2)

12.1 Calculating the amountsof substances

12.2 Counting numbersbyweighing

12.3 What is amole?

12.4 What ismolarmass?

12.5 Calculations involvingmoles andmasses

12.6 Percentage bymassof an element in a compound

12.7 Determining the empirical formula of a compound fromexperimental data

12.8 Mole ratio in abalanced chemical equation

12.9 Chemical equations and reactingmasses

12.10 Limiting reactants

12.11 Theoretical yield andpercentage yield

Unit 12 Reacting masses

Topic 31� Metals 1�Unit 12  Reacting masses

12.1 – 12.5

Summary

1 Onemole(symbol:mol)istheamountofasubstancethatcontainsthesamenumberofparticles as there are atoms in12.0gof carbon-12 (12

6C).

2 Experimentsshowthat12.0gofcarbon-12(126C)contains6.02x1023atoms.Chemists

call thisnumber theAvogadro constant (symbol: L).

L = 6.02 x 1023mol–1

3 Numberofparticles in a substance=numberofmolesof the substance x L

4 The mass of one mole of a substance is called its molar mass. The units of molarmass are grampermole (gmol–1).

5 Numberofmolesof a substance= massof a substance (g)molarmassof the substance (gmol–1)

6 The following diagram summarizes the steps in calculations involving the numberof particles in a substance, the number of moles of the substance and the mass ofthe substance.

number of particles in a substance

÷ Avogadro constant

x Avogadro constant

number of moles of the

substance

x molar mass

÷ molar mass

mass of the substance

Example

The total volume of 20 drops of water is 1.00cm3. What is the number of moleculesin1dropofwater.

(Avogadro constant =6.02 x 1023mol–1; densityofwater = 1.00gcm–3; relative atomicmasses:H=1.0,O=16.0) (2marks)

Answer

Massof 1.00cm3ofwater = 1.00gcm–3 x 1.00cm3

= 1.00g

Numberofmolesofmolecules in1dropofwater = 120

x 1.00g18.0gmol–1

= 0.00278mol (1)

Numberofmolecules in1dropofwater = 0.00278mol x 6.02 x 1023mol–1

= 1.67 x 1021 (1)

ExamtipsExamtipsExamtipsExamtips ♦ Onemoleofethaneandonemoleofethenehasthesamenumberofmolecules, butDIFFERENTnumber of atoms.

♦ When calculating the molar masses of crystals, remember to includethewater of crystallization.

♦ Questions may list some gases and ask which of them contains thegreatest / smallest number of molecules / atoms. Read the questioncarefully.

e.g.

Which of the following gases contains the greatest number ofmolecules?

50.0g of ammonia

50.0g of oxygen

50.0g of carbondioxide

➤Thequestionasksaboutthenumberofmolecules in1dropofwater,NOTin 1.00cm3. Read thequestion carefully.

✘

RemarksRemarks*

12.6 – 12.7

Summary

1 Percentagebymassof elementA in a compound

= numberof atomsofA in formula x relative atomicmassofAformulamassor relativemolecularmassof the compound

x 100%

2 The empirical formula of a compound gives the simplest whole number ratio ofatomsor ionspresent in the compound.

3 The followingdiagramsummarizes the steps fordetermining theempirical formulaof a compound.

mole ratioof atoms

thatcombine

simplestwhole

number ratio of atoms that

combine

empiricalformula of

thecompound

masses of elements in a compound

Topic 31� Metals 1�Unit 12  Reacting masses

ExamtipsExamtipsExamtipsExamtips ♦ To deduce the empirical formula of a compound, the followinginformation is required:

– percentage bymass of each constituent element;

– relative atomic mass of each constituent element.

♦ To deduce the molecular formula of a compound from its empiricalformula, the following information is required:

– relative molecular mass of the compound;

– relative atomic mass of each constituent element.

Example

An oxide of metal M reacts completely with carbon to give 12.6g of metal M and4.36gof carbondioxide.What is the empirical formulaof theoxide?

(Relative atomicmasses:C=12.0,O=16.0,M=63.5) (3marks)

Answer

Massof oxygen in theoxide= 4.36g x 32.044.0

= 3.17g (1)

Metal M Oxygen

Massof element in thecompound 12.6g 3.17g

Relative atomicmass 63.5 16.0

Numberofmolesofatoms that combine

12.6g

63.5gmol–1 = 0.198mol3.17g

16.0gmol–1 = 0.198mol (1)

Mole ratioof atoms0.198mol

0.198mol=1.00

0.198mol

0.198mol=1.00 (1)

Simplestwholenumberratioof atoms 1 1

∴ the empirical formulaof theoxide isMO.

➤When tackling questions on empirical formula / mole ratio, it is importantto knowhow to convertmass tomole.

➤The relative atomicmass of an element bearsNOunit.

RemarksRemarks*

12.8 – 12.11

Summary

1 The followingdiagram summarizes the steps for findingout the reactingmassesofsubstances.

number of moles of A

numberof moles of substance

required in question

mass of substance

required in question

known mass of A

use coefficients in the balanced equation

to find the mole ratio

2 Percentage yield= actual yieldtheoretical yield

x 100%

ExamtipsExamtipsExamtipsExamtips ♦ Forcalculations,readquestionscarefullytoseewhetherthemassesaregiven in g or in kg.

♦ Suppose equal masses of different metals, such as Mg and Zn, areadded separately toexcessdiluteH2SO4 /HCl.Oneof themetals (Mgin this case) produces a greater amount of hydrogen. Questions mayaskwhat canbededuced from that.

Thismaybeexplainedusingaconcreteexample.Suppose100gofMgand 100g of Zn are added separately to excess dilute H2SO4.

Mg(s) +H2SO4(aq) MgSO4(aq)+H2(g)

Zn(s) +H2SO4(aq) ZnSO4(aq)+H2(g)

Number ofmoles ofMg in 100g=mass ofMg

molar mass ofMg

=100g

24.3gmol–1

= 4.12mol

Number ofmoles of Zn in 100g =mass of Zn

molar mass of Zn

=100g

65.4gmol–1

= 1.53mol

100g of Mg will produce 4.12 moles of hydrogen while 100g ofZn will produce 1.53 moles of hydrogen. Hence the difference in theamountofhydrogenproducedbyMgandZn isdue to thedifferencebetween themolar masses of the twometals.

♦ Questions may NOT tell students when a limiting reactant is involved.Be careful when thequestion gives thequantities of both reagents.

Topic 320 Metals 21Unit 12  Reacting masses

Answer

a) Fe2O3(s) + 3H2(g) 2Fe(s) + 3H2O(l) (1)

b)Themetal obtained can conduct electricity. (1)

c) Fe2O3(s) + 3H2(g) 2Fe(s) + 3H2O(l) 7.18g ?g

Numberofmolesof Fe2O3used = 7.18g159.6gmol–1

= 0.0450mol (1)

According to the equation, 1moleof Fe2O3 canproduce2molesof Fe.

∴ numberofmolesof Feobtained = 2 x 0.0450mol = 0.0900mol (1)

Massof Feobtained = 0.0900mol x 55.8gmol–1

= 5.02g (1)

d)Hydrogen gas is flammable and explosive. Burning the excess hydrogen avoidsdangerous situation. (1)

e)

(1)

f) 2Ag2O(s) 4Ag(s) +O2(g) (1)

g) No. The reactivity of iron and silver can only be compared using the samereaction. (1)

♦ Questionsmaygivethemassofmetalobtaineduponthedecompositionofametaloxideandaskstudentstocalculatetherelativeatomicmassof themetal.

e.g.

In an oxide of metal M, the ion of M carries +1 charge. Completedecomposition of 3.48g of the oxide gives 3.24g of metal M. Whatis the relative atomic mass ofM?

(Relative atomicmass:O=16.0)

Let x be the relative atomic mass ofM.

The chemical formula of theoxideofM is M2O.

2M2O(s) 4M(s) +O2(g) 3.48g 3.24g

Accordingtotheequation,2molesofM2O(i.e.[4x+32.0]g)decomposeto give 4moles ofM (i.e. 4xg).

4x + 32.0

4x=

3.48g3.24g

x= 108

Example

The following experimental set-upwasused to reduce7.18g iron(III) oxide to iron.

(Relative atomicmasses: Fe =55.8,O=16.0)

hydrogen

heat

flame

iron(III) oxide

a) Write a chemical equation for the reaction thatoccurred in the experiment. (1mark)

b)SuggestONEway to show that ametal is formed in this experiment. (1 mark)

c) Calculate themassof ironobtained. (3marks)

d)Explainwhy the excesshydrogenwasburnt. (1mark)

e) Drawahazardwarning label that shouldbedisplayedon a cylinderofhydrogen. (1mark)

f) Uponheating, silveroxidedecomposes togive silverandoxygen.Writeachemicalequation for thedecomposition. (1mark)

g) Is it possible to deduce from the results of the experiments of iron and silver thatiron occupies a higher position in the reactivity series than silver? Explain youranswer. (1mark)

➤Students should be able to draw thehazardwarning labels properly.

➤Questions often ask about the decomposition of silver oxide uponheating.

➤Studentsshouldmakeuseoftheconceptof‘fairtest’whenansweringpart(g).

RemarksRemarks*

Topic 322 Metals 23Unit 13  Corrosion of metals and their protection

13.1 What is corrosion?

13.2 Corrosion of iron: rusting

13.3 What factors can speedup the rustingprocess?

13.4 To observe rustingusing a rust indicator

13.5 How toprevent rusting?

13.6 Socioeconomic implicationsof rusting

13.7 What is themost suitablemethodtoprotectaparticular ironarticle from rusting?

13.8 Corrosion resistanceof aluminium

Unit 13 Corrosion of metals and their protection 13.1 – 13.8

Summary

1 Thereactionofametalwithoxygen intheair,moistureorothersubstances intheenvironment is called corrosion.

2 The corrosion of iron is called rusting. Rust is the powdery reddish brown coatingwhich forms when iron corrodes. Its chemical name is hydrated iron(III) oxide(Fe2O3•xH2O).

3 The following ionichalf-equations are involvedwhen iron rusts:

Fe Fe2+ + 2e–

Fe2+ Fe3+ + e–

4 For rusting tooccur, two thingsmustbepresent: oxygen andwater.

5 Factors that speedup the rustingprocess include:

a) presenceof ionic substances;

b)presenceof acidicpollutants;

c) higher temperature;

d)scratchingor bending the surface; and

e) attachment to a less reactivemetal.

6 Rusting can be observed using a rust indicator, a mixture of potassiumhexacyanoferrate(III) (K3Fe(CN)6) and phenolphthalein. Fe2+ ions turn potassiumhexacyanoferrate(III) blue.

7 Methods toprevent rusting include:

a) coatingwithpaint, oil, greaseorplastic;

b)tin-plating;

c) zinc-plating;

d)electroplating;

e) sacrificial protection;

f) protectionby thenegative terminal of ad.c. source; and

g) using corrosion resisting alloysof iron.

Topic 32� Metals 2�Unit 13  Corrosion of metals and their protection

ExamtipsExamtipsExamtipsExamtips ♦ Theterm‘rusting’onlyreferstothereactionofironwithairandwater.Use the term ‘corrosion’ for othermetals.

♦ Students should be able to spell the term ‘electroplating’ correctly.

♦ Galvanized iron is NOTused formaking food cans. ✘

♦ The exhaust pipeof a petrol-driven car corrodes easily because

– there are nitrogen oxides (i.e. acidic gases) passing through theexhaust pipe;

– the exhaust pipe usually becomes hot when the car engine isrunning.

♦ Chromium is one of the components of stainless steel. Chromium isoxidized more readily than iron. It can prevent iron in stainless steelfrom rustingby sacrificial protection.

♦ The followingdiagramshowsa systemused in somepiers for slowingdown the rusting of steel pillars:

sea waterplatinum alloy

d.c. power supply

sea bed

steel pillar

– +

sea bed

Electrons flow from the d.c. power supply to the steel pillar and thussteel / iron cannot beoxidized to iron(II) ions.

8 a) When aluminium reacts with oxygen in the air, an even coating of aluminiumoxide forms.

b)The thickness of the oxide coating can be increased by aluminium anodization.This gives the surfacemoreprotection.

aluminium object to be anodized (as the positive electrode)

aluminium sheet (as the negative electrode)

dilute sulphuric acid

6V

Duringaluminiumanodization,thealuminiumobjectisconnectedtothepositiveterminal of thepower supply.

♦ The following are advantages of using anodized aluminium to makedrink cans:

– the cans canbedyedmore easily;

– the hardness of the cans canbe increased;

– the corrosion resistance of the cans canbe enhanced.

♦ Questions often ask students to compare aluminium and iron, forexample:

– explain why aluminium is discovered later than iron although it ismore abundant than iron in the Earth’s crust;

– advantages / disadvantages of using aluminium over iron;

– explainwhyrustingisadestructiveprocessbutcorrosionofaluminiumis a useful process.

♦ Increasingthethicknessofaluminiumoxideonthesurfaceofaluminiumwill enhance corrosion resistance, but NOT its strength.

Increasing the thickness of iron oxide on the surface of iron CANNOTenhance its corrosion resistance.

♦ Anodized aluminium is more commonly used than iron for makingwindowframes.This isbecauseanodizedaluminium ismorecorrosionresistant than iron.

Example

Four iron rods areplaced separately in gelwith a rust indicator containingpotassiumhexacyanoferrate(III),andallowedtostandinairforsometime.Completethefollowingtable by writing down the observation and giving the relevant explanation for eachcase. (5marks)

Case Observation Explanation

a) Iron rod fullyplatedwithchromium

b)Iron rod fullyplatedwith tin

c) Iron rod fullyplatedwithchromium;butpart of thechromium scratched to exposethe ironunderneath

Topic 32� Metals

➤Zinc canprotect iron from rusting in twoways:

– For an ironobject fullyplatedwith zinc, the zinc layerprotects the ironfrom oxygen andwater.

– If part of the zinc layer is scratched to expose the ironunderneath, thezincstillprotectstheironbysacrificialprotection.Zincpreventsironfromlosing electrons.

Students shouldbe able to distinguish between these twoways.

➤Questionsoftentestwhetherstudentsknowthat‘tinCANNOTpreventironfrom rusting via sacrificial protection’ as tin is less reactive than iron.

RemarksRemarks*

Case Observation Explanation

d)Iron rod fullyplatedwith tin;butpart of the tin scratchedto expose the ironunderneath

Answer

Case Observation Explanation

a) Iron rod fullyplatedwithchromium

Noobservable change

The iron rod does not rust asthe chromium coating preventsoxygen and water from reachingthe iron. (1)

b)Iron rod fullyplatedwith tin

Noobservable changeThe iron roddoesnot rustas thetin coating prevents oxygen andwater from reaching the iron.(1)

c) Iron rod fullyplatedwithchromium;butpart of thechromium scratched to exposethe ironunderneath

Noobservable changeThe iron rod does not rust asthe chromium provides sacrificialprotection. (1)

d)Iron rod fullyplatedwith tin;butpart of the tin scratchedto expose the ironunderneath

Ablue colour appearsnear the scratchedarea

The exposed iron rusts (1)

as iron is in contact with tin, aless reactivemetal. (1)