Book 3 ans

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HKDSE CHEMI

HKDSE CHEMISTRY –

STRY – A Modern View

A Modern View

(Chemistry)

Coursebook 3

Suggested answers

Chapt

Chapter er 2525 Simple Simple molecumolecular lar substsubstances ances with with non-onon-octetctet structures and shapes of simple molecules

structures and shapes of simple molecules

Page Page Number Number

 Class PracticeClass Practice 11

 Chapter ExerciseChapter Exercise 22

Chapter

Chapter 2266 BBoonnd pd poolalarritityy

Cha

Chaptepter r 2727 IntIntermermoleolecucular lar forforcesces

Chapter

Chapter 2828 StrucStructures tures and and propproperties erties of of molecmolecular ular cryscrystalstals



 Part ExercisePart Exercise 1414

Cha

Chaptepter 2r 299 CheChemicmical cal cellells in s in dadaily ily lifelife

Chapter

Chapter 3030 SiSimpmple cle chehemimicacal cel cellllss

Ch

Chapapteter 3r 311 ReRedodox rx reaeactctioionsns

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Chapter

Chapter 3232 RedRedox ox reareactioctions ns in in chechemicmical al celcellsls

Chapter

Chapter 3333 EElelecctrtroollyyssiiss

Chapt

Chapter 34er 34 ImporImportance tance of reof redox rdox reactioeactions in ns in modermodern wayn ways of lis of livingving



Chapter

Chapter 3535 EneEnergy rgy chachangenges in s in chechemicmical ral reaeactioctionsns

Chapter

Chapter 3636 StaStandndard enthard enthalpalpy y chachangnge e of combuof combustistion, neuton, neutraralizlizatioation,n, solution and formation

solution and formation

Chapter

Chapter 3377 HHeessss’’s s LLaaww

 Part ExercisePart Exercise 5353

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Chapter

Chapter 3232 RedRedox ox reareactioctions ns in in chechemicmical al celcellsls

Chapter

Chapter 3333 EElelecctrtroollyyssiiss

Chapt

Chapter 34er 34 ImporImportance tance of reof redox rdox reactioeactions in ns in modermodern wayn ways of lis of livingving



Chapter

Chapter 3535 EneEnergy rgy chachangenges in s in chechemicmical ral reaeactioctionsns

Chapter

Chapter 3636 StaStandndard enthard enthalpalpy y chachangnge e of combuof combustistion, neuton, neutraralizlizatioation,n, solution and formation

solution and formation

Chapter

Chapter 3377 HHeessss’’s s LLaaww

 Part ExercisePart Exercise 5353

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HKDSE CHEMISTRY

HKDSE CHEMISTRY   _A_{A Modern}_{Modern View}_{View (Chemist}_(Chemistry)_ry) _Coursebook_{Coursebook 3}₃

Chap

Chapter ter 2525 SimplSimple molecue molecular sular substanbstances witces with non-oh non-octet stctet structuructures anres andd shapes of simple molecules

shapes of simple molecules Class Practice Class Practice A25.1 A25.1 ((aa)) BBCCll33 NCl NCl33 IF IF33

((bb)) BBCCll33. The central boron atom has . The central boron atom has only six outermost shell electrons.only six outermost shell electrons.

IF

IF33. The central iodine atom has . The central iodine atom has 10 outermost shell electrons.10 outermost shell electrons.

A25.2 A25.2 M Moolleeccuullee NNoo. . oof f electron electron pairs pairs Spatial Spatial arrangement arrangement of of electron electron pairs pairs No. of lone No. of lone pairs pairs N Noo. . oof f bond pairs bond pairs Shape Shape CH

CH44 44 TTeettrraahheeddrraall 00 44 TTeettrraahheeddrraall

NH

NH33 44 TTeettrraahheeddrraall 11 33 TrigonalTrigonal

pyramidal pyramidal H

H22OO 44 TTeettrraahheeddrraall 22 22 V-shapedV-shaped

©

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HKDSE CHEMISTRY  _{A Modern View (Chemistry)} _{Coursebook 3}

Chapter 25 Simple molecular substances with non-octet structures and shapes of simple molecules

Chapter Exercise 1. eight 2. eight 3. six 4. 10 5. 12 6. non-octet 7. linear, 180° 8. trigonal planar, 120° 9. tetrahedral, 109.5° 10. trigonal bipyramidal, 90°, 120° 11. octahedral, 90° 12. 2, 2 13. 1, 3 14. shapes 15. D 16. B 17. C 18. A 19. D 20. A 21. (a) A: ; B: (b) A: WZ 3 B: YZ 3

(c) A. The central atom (W ) has only six electrons in its outermost shell. 22. (a) X : Trigonal pyramidal

Y : Trigonal bipyramidal (b) 107°

(c) 90°(for Cl(axial) −P−Cl(equatorial) bond angles) and 120° (for Cl−P−Cl bond angles

within the plane of triangle)

23. (a) (b)

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24. (a) SO2:

SO3:

(b) SO2: V-shaped

SO3: Trigonal planar

(c) Since there are three groups of electrons around the central atoms (sulphur) in both SO2and SO3, all the O=S=O bond angles are about 120°.

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Chapter 26 Bond polarity Class Practice A26.1 (a) (b) (c) A26.2

(a) NCl3 has three polar N−Cl bonds and is trigonal pyramidal in shape. As there is a

resultant dipole moment arising from the three polar bonds, the molecule is polar. BCl3 has three polar B−Cl bonds and is trigonal planar in shape. As the dipole

moments of the three polar bonds cancel out each other, the molecule is non-polar. (b) As the order of electronegativity is F > N > Br, the resultant dipole moments of

NBr 3and NF3are pointing to different directions. The situations are shown below:

In a non-uniform electrostatic field, the nitrogen end of NBr 3 will point to the

positive pole while the nitrogen end of NF3will point to the negative pole.

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Chapter 26 Bond polarity Chapter Exercise

1. attract 2. polar 3. non-polar

4. larger, less, smaller 5. polar, cancel out 6. polar, symmetrically 7. deflection 8. C 9. B 10. C 11. B 12. C 13. A

14. (a) The electronegativity values of elements do not follow the trend of changing masses. (b) It increases. (c) It decreases. (d) 2.5–3.5 (e) 0–2.8 (f) C and F (g) Cl and Br 15. (a) (b) (c) 16. (a) Y , Z , X . (b) (c) (d) 17. (a) Mistakes:

1. ‘covalent bond involving fluorine is polar in nature’ 2. ‘all compounds of fluorine must be polar’

(b) 1. The covalent bond in a fluorine molecule (F−_{F) is non-polar.}

2. Polar X−_{F bonds may cancel out their individual dipole moments to give}

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a non-polar molecule e.g. CF4, SF6, etc.

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Chapter 27 Intermolecular forces Class Practice

A27.1

(a) When the electron distribution around the two iodine nuclei is uneven, instantaneous dipole is formed in which the side with more electrons will carry a partial negative charge.

(b)

A27.2

1. (a) Non-polar molecules (b) Dispersion forces

(c) (i) The statement is correct as butane has a higher boiling point than that of propane. The strength of dispersion forces increases with increasing

molecular size as there is a greater chance of uneven distribution of electrons in a larger molecule.

(ii) The statement is incorrect as butane has a higher boiling point than that of 2-methylpropane. Butane (straight-chain hydrocarbon) has a long, thin shape. This contributes to a larger contact surface area between butane molecules, resulting in larger dispersion forces.

2. ClF and CH2Cl2 are polar in nature and their molecules are attracted by both

dipole-dipole forces and dispersion forces. On the other hand, F2 and Cl2 are

non- polar in nature and their molecules are attracted by dispersion forces only. As a result, ClF and CH2Cl2have higher boiling points than those of F2and Cl2.

Since the molecular size of CH2Cl2is larger than that of ClF, the dispersion forces

between CH2Cl2molecules are larger. So CH2Cl2has a higher boiling point.

Similarly, since the molecular size of Cl2 is larger than that of F2, Cl2 has a higher

boiling point.

3. (a) CH3F is polar in nature and its molecules are attracted by both dipole-dipole

forces and dispersion forces. On the other hand, C2H6 is non-polar in nature

and its molecules are attracted by dispersion forces only. As a result, CH3F

has a higher boiling point than that of C2H6.

(b) Although Cl2is non-polar and its molecules are attracted by dispersion forces

only, the larger dispersion forces in Cl2 outweigh the dipole-dipole forces in

HCl. Thus, Cl2has a higher boiling point.

A27.3

(a)

CH

3OH

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(b)

(c)

A27.4

(a) In glucose, the hydrogen atoms and oxygen atoms of the five hydroxyl groups (−_{OH) on the molecule can form hydrogen bond with the oxygen atoms and}

hydrogen atoms of water molecules respectively. As a result, glucose is very soluble in water. On the other hand, 1,2,4-trichlorobenzene cannot form any hydrogen bond with water.

(b) Glucose molecules are attracted to each other by extensive hydrogen bonds. A considerable amount of energy is needed for separating glucose molecules in melting. On the other hand, 1,2,4-trichlorobenzene molecules are attracted by dipole-dipole forces. Less energy is needed to melt 1,2,4-trichlorobenzene.

A27.5

(a) In diamond, carbon atoms are held together by strong covalent bonds and much energy is needed for separating the atoms during melting. In ice, water molecules are held together by weak intermolecular forces (hydrogen bonds and van der Waals’ forces), so less energy is needed for separating molecules during melting. (b) Water molecules are held together by hydrogen bonds while oxygen molecules are

held together by dispersion forces only. Therefore, more energy is needed to separate water molecules during boiling.

(c) Hydrogen chloride molecules are held together by van der Waals’ forces but hydrogen fluoride molecules are held together by hydrogen bonds. As the intermolecular forces in hydrogen fluoride are stronger than those in hydrogen chloride, it is more difficult for hydrogen fluoride molecules to escape into the atmosphere.

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Chapter 27 Intermolecular forces Chapter Exercise

1. molecules, electrostatic

2. an der Waals’ forces, ydrogen bonding 3. Dipole-dipole forces

4. Dispersion forces, induced 5. molecular, surface, olarity 6. hydrogen, electrons

7. energy

8. surface, iscosity 9. hydrogen bonds 10. high

11. proteins, deoxyribonucleic acids 12. D 13. A 14. A 15. A 16. B 17. A 18. (a) GroupVIII/0 (b) Dispersion forces

(c) The intermolecular forces arise from the uneven distribution of electrons within the atoms.

(d) The boiling point increases down the group.

(e) The larger the atom, the higher is the chance of uneven distribution of electrons. This gives rise to a more prominent instantaneous dipole and larger dispersion forces.

19. (a) Dipole-dipole forces and dispersion forces (b) Dispersion forces

(c) Since the molecular sizes of HCl and F2 do not differ much, the dispersion

forces among their molecules are similar in strength. However, the presence of dipole-dipole forces in HCl strengthens the attractions among molecules of HCl and so HCl has a higher boiling point.

20. (a) A and B. Their molecules are held together by hydrogen bonds.

(b) Lower. As there is no hydrogen bond between molecules of C , less energy is needed to separate the molecules in boiling.

21. Ethanol and ethane-1,2-diol have higher boiling points as their molecules are held together by hydrogen bonds. Among them, each ethane-1,2-diol molecule can form two hydrogen bonds per molecule while each ethanol molecule can form one hydrogen bond per molecule. Therefore, ethane-1,2-diol has a higher boiling point than ethanol. Chloroethane has a lower boiling point than ethanol because there are dipole-dipole forces but not hydrogen bonds between chloroethane molecules. Ethane is non-polar. It has the lowest boiling point.

22. − _The _{melting point}_and_{boiling point}_{of water are} _{much higher}_{than those of}

simple molecular substances without hydrogen bonds. This is because much

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energy is needed to overcome the strong hydrogen bonds between water molecules and separate them.

− _{The existence of}_{extensive hydrogen bonds} _{makes the} _{surface tension of}

water exceptionally high. That is, water molecules are held strongly together to form an unstretchable surface.

− _{Regarding to its viscosity, water is} _{more viscous} _{than most molecular}

liquids. This is because the strong hydrogen bonds hold water molecules together and do not allow them to move past one another easily.

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Chapter 28 Structures and properties of molecular crystals Class Practice

A28.1

(a) Liquid water

(b) The regular open network structure of ice allows the formation of maximum number of hydrogen bonds (four for each water molecule).

(c) To overcome a certain amount of hydrogen bonds and separate the water molecules.

(d) During boiling, all hydrogen bonds have to be broken before the water molecules can escape as steam.

A28.2

(a) Diamond and graphite were discovered before fullerene. (b) Buckminsterfullerene

(c) Covalent bond

(d) Dispersion forces (or van der Waals’ forces)

A28.3

(a) Both of them have molecules held together by dispersion forces.

(b) Buckminsterfullerene is spherical in shape but carbon nanotube is cylindrical. In buckminsterfullerene, pentagonal patterns of atoms are found between the hexagonal patterns. In a carbon nanotube, pentagonal patterns of atoms are found only near the two ends of molecules.

A28.4

1. (a) In diamond, carbon atoms are held strongly by covalent bonds but the buckminsterfullerene molecules are held by weak dispersion forces.

(b) Graphite has delocalized electrons in its structure but diamond and buckminsterfullerene do not.

(c) As buckminsterfullerene molecules are held by dispersion forces, less energy is needed during melting. In melting diamond and graphite, strong covalent bonds have to be overcome.

2. (a) Carbon nanotubes are electrically conductive because of the presence of delocalized electrons in their structures.

(b) Carbon nanotubes have very high tensile strength.

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Chapter 28 Structures and properties of molecular crystals Chapter Exercise 1. order 2. four, hydrogen 3. oxygen 4. open 5. hydrogen 6. carbon, hollow

7. spherical, benzene, close, strong, hard, insulator

8. cylindrical, melting, solvents, carbon, tensile, conductors 9. D 10. B 11. D 12. B 13. A 14. C 15. (a)

(b) Covalent bond between O atom and H atom

(c) Hydrogen bond between O atom of a water molecule and H atom of another water molecule

16. (a) Ice has a lower density than water.

(b) Ice floats on water. This prevents heat loss from water and helps maintain the water temperature.

(c) Ice possesses an open structure in which H2O molecules are separated farther

and there are more spaces between molecules.

17. (a) It is an element as it is composed of carbon atoms only. (b) 12 pentagons and 0 hexagon

(c) Other than pentagons, there are hexagons over the surface of C60.

(d) C60 has a larger molecular size than C20. Hence, C60 has larger dispersion

forces between its molecules and thus a higher boiling point than that of C20.

18. (a) X −_{Carbon nanotube;}_Y−_Diamond;_Z−_{Buckminsterfullerene}

(b) Carbon nanotubes are good electrical conductors due to the movement of delocalized electrons. Furthermore, each carbon atom in a carbon nanotube is covalently-bonded to three neighbouring carbon atoms. Because of the strong

* Label all other bonds accordingly.

a a b b b a 12

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covalent bonds holding carbon atoms along the tube axis, carbon nanotubes can withstand high tension.

(c) Some metal (e.g. potassium) atoms

(d) No. There are no strong covalent bonds holding the carbon atoms along a specific direction.

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Part VI Microscopic world II Part Exercise 1. C 2. A 3. C 4. A 5. C 6. B 7. A 8. C 9. C 10. A 11. (a) WY 2

(b) The electron diagram of WY 2is:

There are two double bonds surrounding the central atomW . The molecule should adopt a linear shape to minimize the repulsion between the two groups of electrons.

(c) X and Z .

Due to the repulsion between the lone pair of electrons and the three bond pairs of electrons, the molecule adopts a trigonal pyramidal shape with a bond angle of about 107°.

12. (a)

(b) C – B – B – C

(c)

13. (a)

(b) SF2−105°; SF6−90°

(c) SF2is polar while SF6is non-polar.

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HKDSE CHEMISTRY

(d) (d) (e

(e)) It iIt is nons non-po-polar lar as tas the ehe effeffects octs of all f all popolar lar SS−−_{F bonds cancel each other.}_{F bonds cancel each other.}

1

144.. ((aa)) ((ii)) NNoonn--ppoollaar r ((iiii)) NNoonn--ppoollaar r ((iiiii)i) PPoolalar r (b

(b)) IoIodidine ne is is nonon-n-ppololarar. . It It didissssololveves s wewell ll in in tetetrtracachlhlororomometethahane ne bebecacaususee tetrachloromethane is also non-polar. However, iodine dissolves slightly in tetrachloromethane is also non-polar. However, iodine dissolves slightly in water which is polar.

water which is polar.

(The amount of energy released from the formation of intermolecular forces (The amount of energy released from the formation of intermolecular forces (dispersion forces) between iodine

(dispersion forces) between iodine and tetrachloromethane molecules is largeand tetrachloromethane molecules is large enough to compensate for that required to break the intermolecular forces enough to compensate for that required to break the intermolecular forces (dis

(disperspersion ion forceforces) s) betwbetween een tetrtetrachloachloromeromethane moleculethane molecules. s. On On the the otheother r hand, the amount of energy released from the formation of intermolecular hand, the amount of energy released from the formation of intermolecular forces (dispersion forces) between iodine and water molecules is not large forces (dispersion forces) between iodine and water molecules is not large enough to compensate for that required to break

enough to compensate for that required to break the hydrogen bonds betweenthe hydrogen bonds between water molecules.)

water molecules.) (c)

(c) TTetraetrachlochloromeromethanthane cae cannot nnot dissdissolve olve in win waterater.. (d)

(d) TTwo sepwo separaarate layete layers will be seen. Thrs will be seen. The e yelyellow collow colour of the our of the aquaqueoeous layeus layer r fades while the tetrachloromethane layer turns violet.

fades while the tetrachloromethane layer turns violet. 15

15.. (a(a)) In iceIn ice, , wwatater moer molelecuculeles s arare e ararrarangnged orded ordererly in ly in a a reregugulalar r opopeen n nenetwtworork k structure because of the extensive hydrogen bonding. In this case, water structure because of the extensive hydrogen bonding. In this case, water molecules are farther apart than in liquid water and thus ice takes up a larger molecules are farther apart than in liquid water and thus ice takes up a larger volume.

volume.

((bb)) EEaacch h HH22O molecule can form two hydrogen bonds O molecule can form two hydrogen bonds but each HF molecule canbut each HF molecule can

form one hydrogen bond

form one hydrogen bond onlyonly. Thus, more energy is needed to . Thus, more energy is needed to separate water separate water molecules in the boiling process.

molecules in the boiling process. (c

(c)) AlAlththouough both comgh both compopoununds can ds can foform hydrm hydrorogegen n bobondnds, s, CHCH33CHCH22CHCH22CHCH22OHOH

has a larger size than CH

has a larger size than CH33CHCH22OH and it has larger dispersion forces betweenOH and it has larger dispersion forces between

mol

molecueculesles. . ThThus, us, momore re eneenergrgy y is is neeneeded ded to to seseparparate ate CHCH33CHCH22CHCH22CHCH22OHOH

molecules in the boiling process. molecules in the boiling process. (d)

(d) HexaHexane is non-pne is non-polarolar. It canno. It cannot dissolt dissolve in wateve in water whicr which is polarh is polar..

(Hexane cannot form strong hydrogen bonds with water and thus it is not (Hexane cannot form strong hydrogen bonds with water and thus it is not soluble in water.)

soluble in water.) 1

166. . ((aa)) HH22O and CHO and CH33CHCH22CHCH22CHCH22CHCH33

(b) (b)

H

H22OO HHyyddrrooggeen n bboonndd

NH

NH33 Hydrogen bondHydrogen bond

Cl

Cl22 DispersioDispersion n forceforce

CH

CH33CHCH22CHCH22CHCH22CHCH33 DispersioDispersion n forceforce

((cc)) EEaacch Hh H22O molecule can form two hydrogen bonds but each NHO molecule can form two hydrogen bonds but each NH33 moleculemolecule

can form one hydrogen bond only. Thus, more energy is needed to separate can form one hydrogen bond only. Thus, more energy is needed to separate water molecules in the boiling process.

water molecules in the boiling process.

((dd)) CCHH33CHCH22CHCH22CHCH22CHCH33should have a higher boiling point as it has a larger sizeshould have a higher boiling point as it has a larger size

than Cl

than Cl22and thus larger dispersion forces between molecules.and thus larger dispersion forces between molecules.

©

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HKDSE CHEMISTRY

1 177.. ((aa)) (b) (b) (c) (c) (d) (d) (e)

(e) ThThe roots of the ple roots of the plantants can absos can absorb the ferrb the fertiltilizeizer dissor dissolvelved in d in wawater frter from theom the soil.

soil. 1

188.. ((aa)) CC6060 has a higher boiling point than that of Chas a higher boiling point than that of C2828 since Csince C6060 has a larger size andhas a larger size and

so larger dispersion forces between molecules. More energy is needed to so larger dispersion forces between molecules. More energy is needed to separate the molecules in boiling.

separate the molecules in boiling. (b)

(b) AmAmmonia hamonia has a higher boils a higher boiling poiing point than nitrnt than nitrogen. ogen. AmAmmonimonia molea moleculecules ares are att

attracracted ted by by hydhydrogrogen en bobonds nds whwhile ile nitnitrogrogen en molmolecuecules les are are attattracracted ted byby weaker dispersion forces. More energy is needed in boiling ammonia.

weaker dispersion forces. More energy is needed in boiling ammonia. 19

19.. (a)(a) EitEither bher by usy using ving verery higy high prh pressessureures to cos to compmpreress thss the hye hydrodrogegen gas on gas or verr very lowy low temperatu

temperatures to turn the res to turn the hydrogen gas into a liquid.hydrogen gas into a liquid. (b

(b)) DDisispepersrsioion fon forcrceses (c)

(c) HydHydrogerogen in mon in moleculecular folar form carm can be reln be releaseeased easid easily and ly and used aused as fuels fuel..

©

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HKDSE CHEMISTRY

Cha

Chaptepter 29r 29 CheChemicmical cal cells ells in din dailaily liy lifefe Class Practice

Class Practice

A29.1 A29.1

A litt

A little water should be present so that le water should be present so that the ions present in the electrolyte would becomethe ions present in the electrolyte would become mobile to allow flow of electricity.

mobile to allow flow of electricity.

A29.2 A29.2

1.

1. ItIts s elelecectrtrololytyte e is potis potasassisium hyum hydrdroxoxidide, whe, whicich h isis alkalinealkaline. Its positive electrode. Its positive electrode consists of

consists of manganesemanganese(IV) oxide (mixed with a (IV) oxide (mixed with a little powdered graphitelittle powdered graphite).). 2.

2. HeHeariaring aidng aids are sms are small aall and thend they neey need very sd very smamall celll cells. Sls. Silvilver oxer oxide ceide cells (slls (shaphapeded like a button)

like a button) would serve the purpose.would serve the purpose. 3.

3. SiSilvelver and ir and its cots compmpounounds (eds (e.g. .g. silsilver over oxidxide) are) are expe expensensiveive.. 4

4.. HHKK$$11000 0 000000.. Explanation: Explanation:

Each silver oxide cell produces 2.79

Each silver oxide cell produces 2.79 ×× ₁₀₁₀−−44 _{kWh (=180}_{kWh (=180} ×× ₁₀₁₀−−33 ×× _1.55_1.55 ×× ₁₀₁₀−−33

kWh) electricity. To produce one ‘unit’ of electricity (1 kWh), about 3585 (= kWh) electricity. To produce one ‘unit’ of electricity (1 kWh), about 3585 (=

4 4 10 10 79 79 .. 2 2 1 1 − − ×

× ) cells are needed. Therefore, the approximate cost is HK$30) cells are needed. Therefore, the approximate cost is HK$30×× 35853585

= HK$107 550 (about HK$100 000). = HK$107 550 (about HK$100 000).

A29.3 A29.3

(a

(a)) A A flflat dat disischchararge ge cucurvrve.e. (b

(b)) AAlklkalalinine e mmananggananeese se cecellll/s/sililvever r oxoxidide e cecellll/n/nicickekel-l-cacadmdmiuium m rerechcharargegeabablele cell

cell/lith/lithium-ium-ion ion recharechargergeable able cellcell/nick/nickel-mel-metal etal hydrhydride ide recharechargergeable able cell cell (any(any ONE)

ONE)

©

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Chapter 29 Chemical cells in daily life Chapter Exercise

1. electricity

2. electrolyte, paste

3. inc-carbon, lkaline manganese, ilver oxide, ithium primary

4. nickel-cadmium rechargeable, lithium-ion rechargeable, nickel-metal hydride rechargeable, lead-acid accumulator

5. zinc-carbon, alkaline manganese, nickel-metal hydride rechargeable, 3.7 V, Lead-acid accumulator, 12 V

6. voltage

7. alkaline manganese 8. (a) size of cell

(b) shape of cell (c) type of cell

9. nvironment, mercury, rechargeable, recycling 10. D 11. A 12. B 13. B 14. A 15. D 16. C 17. D 18. B 19. C 20. A 21. B 22. C 23. B 24. B 25. C

26. (a) Primary cells are not rechargeable whereas secondary cells can be recharged and used again.

(b) Primary cells: zinc-carbon cells, alkaline manganese cells, silver oxide cells, lithium primary cells (any TWO)

Secondary cells: nickel-cadmium rechargeable cells, lithium-ion rechargeable cells, nickel-metal hydride rechargeable cells (any TWO)

(c) Zinc-carbon cell is more commonly used because its price is low and a portable radio does not draw a large current, and can work even when the

voltage and current are not steady. (d) Electrodes: (+) graphite; (−_{) zinc}

Electrolyte: ammonium chloride 27. (a) B

New cells may have a higher voltage than the old cells, thus charging the old cells during operation. Charging old zinc-carbon cells may cause cells to overheat or even explode.

(b) A

The acidic/alkaline electrolyte of the cells may corrode the metal case

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(negative electrode) of the cells. The electrolyte leaks out when holes appear in the metal case. The acidic/alkaline electrolyte can damage metal contacts/parts of the appliance. The electrolyte can also cause skin burns. (c) C

Heat of fire may cause chemicals in cells to expand. Most cells are tightly sealed to prevent leakage, so the expansion of chemicals under pressure may lead to explosion. Chemicals inside cells may be flammable (hydrogen produced at electrodes during discharging), or oxidizing (depolarizer added)

and can hence cause fire to burn more fiercely. (d) C

Alkaline manganese cells cannot be recharged. During recharging, heat and hydrogen may be produced and hence lead to explosion.

(e) D

Small children may mistake small-sized button cells as sweets. 28. (a) Silver oxide cell/lithium button cell

These cells are small enough to fit into a watch. Since a watch consumes little energy, a silver oxide cell can power a watch for about 1–2 years, while a lithium button cell can power a watch for about 5 years.

(b) Lithium-ion rechargeable cell

It has a high charge capacity but lightweight, and has a high voltage. It can be recharged for about 1200 cycles and a life span of about 3 years, and has no memory effect. It is most suitable for daily recharge irrespective of whether it has been fully discharged, and meets the demands for steady high-current, high-voltage of mobile phones.

(c) Zinc-carbon cell/alkaline manganese cell/nickel-metal hydride rechargeable cell (any ONE)

Zinc-carbon cell is cheap./Alkaline manganese cell is more expensive but more durable./Nickel-metal hydride rechargeable cell is more environmentally friendly. (Any ONE)

(d) Zinc-carbon cell/alkaline manganese cell/nickel-metal hydride rechargeable cell (any ONE)

Zinc-carbon cell is cheap./Alkaline manganese cell is more expensive but more durable./Nickel-metal hydride rechargeable cell is more environmentally friendly. (Any ONE)

29. (a) A silver oxide cell is normally made into a button-sized cell. It is lightweight and small so is suitable for use in watches. Most other cell types are normally larger in size and too heavy for use in watches.

(b) Calculators are always used when there is light. Watches, however, must continue to run even in the dark, so watches powered by solar cells are rare. Measures must be taken to prevent the watch to stop when there is no light. (c) A solar cell powered watch must have another secondary chemical cell inside.

The solar cell produces electricity when there is light. The secondary cell stores the excess electrical energy and powers the watch when there is no light.

(d) The solar cell is not a chemical cell. The energy change involved in a solar cell is a physical change instead of a chemical change. The electrical energy is not stored in the form of chemical energy.

30. (a) Lithium-ion cells are rechargeable. They can provide a large and steady current, and are lightweight. These are advantageous when they are used in

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portable electronic devices like mobile phones, notebook computers and digital cameras.

(b) Lithium-ion cells are expensive (each costs at least HK$200). For use in low-end electrical appliances like portable radios, hand torches (flashlights) and motorized toys, standard-sized and low-priced zinc-carbon cells or alkaline manganese cells are more affordable and more readily available.

31. Advantages of a lead-acid accumulator:

− _{It provides a}_{large current}_{and a} _{steady voltage.}

− _{It is} _rechargeable_{, so can be used for many years without the need of}

replacement.

Disadvantages of a lead-acid accumulator:

− _{It is} _heavy_and_{large in size}_, _{difficult to carry}_.

− _{It is a wet cell, turning the cell upside down may cause}_{leakage of acid}_.

Suitability of using it as a car battery:

− _{It is very suitable for use as a car battery, since starting the car needs a large}

and steady current. Besides, frequent replacement of battery is inconvenient particularly when the car is travelling a long journey between cities.

− _{The disadvantages do not affect its suitability for use as a car battery. The car}

is powerful enough to carry the battery along. The acid electrolyte does not leak out unless the car overturns.

32. − _{Dry cells}_{contain toxic chemicals} _{which may pollute the environment. These}

may include mercury in low-priced zinc-carbon cells, and cadmium in nickel-cadmium rechargeable cells.

− _{Other remaining chemicals in spent dry cells, when disposed of, are wasted}

andcause burden to landfill sites.

To minimize pollution, the following measures can be taken:

− _{Use mains electricity instead of cells whenever possible.}

− _{Do not use mercury-containing zinc-carbon cells or alkaline manganese cells.} − _{Use rechargeable cells instead of disposable cells.}

− _{Use nickel-metal hydride rechargeable cells or lithium-ion rechargeable cells}

instead of nickel-cadmium rechargeable cells.

− _{Return expired rechargeable cells to the manufacturer or the Environmental}

Protection Department for recycling of the remaining chemicals. (Any FOUR)

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Chapter 30 Simple chemical cells Class Practice

A30.1

(a) Copper to silver. Copper is more reactive than silver, thus it loses electrons more readily.

(b) Copper foil

(c) Yes. Electrons flow from zinc to copper. Zinc is more reactive than copper, thus it loses electrons more readily.

A30.2

Mg/Ag > Zn/Ag > Fe/Ag > Pb/Ag > Cu/Ag

A30.3

(a) Negative electrode. It is because copper is higher than silver in the E.C.S. Copper would lose electrons to silver when they are connected together.

(b) From left to right. It is because electrons flow from copper rod to silver rod in the external circuit. (c) Copper half-cell: Cu(s)→_Cu2+_{(aq) + 2e}− Silver half-cell: Ag+_{(aq) + e}− →_Ag(s)

(d) Cu(s) + 2Ag+_(aq)_→_Cu2+_{(aq) + 2Ag(s)}

(e) If the porous pot is not used to separate the electrolytes, the silver ions can come into contact with the copper electrode and a direct displacement reaction occurs on the copper surface. A grey deposit forms on the copper rod and no voltage can be recorded in the multimeter.

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Chapter 30 Simple chemical cells Chapter Exercise

1. metals, electrolyte, metals, wire 2. negative, positive

3. electromotive force (or e.m.f.), higher 4. voltmeter, igital, ultimeter

5. voltage (or e.m.f.), Electrochemical Series 6. (a) similar

(b) lose, cations, aqueous solution (c) reactivity, air, dilute acids, losing 7. salt bridge, complete, charges

8. porous pot 9. D 10. C 11. C 12. C 13. B 14. A 15. C 16. C 17. A

18. (a) Dilute sulphuric acid (or any other dilute acid, or an acidic salt solution like ammonium chloride.)

(b) Positive electrode: copper electrode Negative electrode: zinc electrode

(c) From zinc electrode to copper electrode (or from left to right). (d) (i) Bubbles appear at the copper electrode.

(ii) Bubbles appear at the zinc electrode and the zinc electrode dissolves gradually.

(e) (i) The copper electrode is quickly covered with a brown deposit. (ii) The zinc electrode dissolves gradually.

19. (a) To complete the circuit by allowing ions to move from one half-cell to another cell, and to balance the charges in the solutions of the two half-cells.

(b) Positive electrode: copper electrode Negative electrode: iron electrode

(c) (i) A brown solid deposit forms on the copper electrode and the blue colour of the copper(II) solution becomes paler.

(ii) The iron electrode slowly dissolves and the green iron(II) solution becomes darker.

(d) Fe(s) + Cu2+_(aq)_→_Fe2+_{(aq) + Cu(s)}

(e) From iron electrode to copper electrode (or from left to right).

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20. (a) Zn(s) →_Zn2+_{(aq) + 2e}−

(b) Copper container Cu2+_{(aq) + 2e}−

→_Cu(s)

(c) The porous pot is used to separate the two electrolyte solutions and prevent them from direct mixing. Besides, it allows ions to pass through to complete the circuit.

(d) There would be a displacement reaction occurring at the surface of the zinc electrode immediately: Zn(s) + Cu2+_(aq) _→_{Cu(s) + Zn}2+_{(aq). Once the zinc}

rod is coated with copper, both electrodes become the same (copper electrodes) and the electrolyte is also the same, so the voltage of the cell drops to zero.

(e) Yes. The zinc strip loses electrons and these electrons pass through the external circuit to the copper container. The zinc strip is therefore the negative pole and the copper container is the positive pole.

21. (a) A metal couple is a combination of two different metals connected to an external circuit.

(b) The filter paper separates the two metal strips, preventing a short circuit. Besides, the salt solution allows movement of ions to complete the circuit. (c) From copper to silver.

(d) The voltmeter would give a positive reading.

(e) Removing the filter paper would cause a short circuit, thus the voltmeter reading drops to zero.

(f) The voltage would be higher. This is because magnesium is in a higher position than copper in the Electrochemical Series.

22. − _{Connect the metals aluminium, copper, iron, lead, magnesium and zinc one}

by one in turn to the circuit as shown below:

− _{The metal strip under test and the silver sheet is separated by a piece of}_filter

paper moistened with sodium chloride solution.

− _{Measure and record the voltage} _{read out from the digital voltmeter for each}

metal tested.

− _{Since the same reference electrode (silver) is used, the Electrochemical Series}

of these metals can be obtained by arranging the metals in the order of voltage reading .

− _The _{higher the voltage reading}_{, the} _{higher position} _{the metal is in the}

Electrochemical Series.

clip metal strip

under test

filter paper moistened with sodium chloride solution

silver sheet

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Chapter 31 Redox reactions Class Practice

A31.1

(a) It involves a transfer of electrons (from Fe(s) to Cu2+_(aq)).

(b) Fe(s) is being oxidized. Fe(s)loses electrons to others.

(c) Cu2+_{(aq) is the oxidizing agent. Cu}2+_(aq)_gains_{electrons (or Cu}2+_{(aq) is}_reduced_).

A31.2 (a) O.N. of S = 0 (b) (+2)× _{3 + (O.N. of N)} × _{2 = 0} ∴ _{O.N. of N =} −₃ (c) (+1)× _{2 + (O.N. of S) + (}−₂₎ × _{4 = 0} ∴ _{O.N. of S = +6} (d) (+1)× _{2 + (O.N. of S) + (}−₂₎ × _{3 = 0} ∴ _{O.N. of S = +4} (e) (+1)× _{2 + (O.N. of Cr) + (}−₂₎× _{4 = 0} ∴ _{O.N. of Cr = +6} (f) For NH4+, (O.N. of N) + (+1) × _{4 = +1} ∴ _{O.N. of N =} −₃ (g) (+1) + (O.N. of C) + (−₂₎× _{3 =}−₁ ∴ _{O.N. of C = +4} (h) (O.N. of Cr) × _{2 + (}−₂₎× _{7 =}−₂ ∴ _{O.N. of Cr = +6} (i) (O.N. of N) + (−₁₎× _{3 = 0} ∴ _{O.N. of N = +3} A31.3 1.

Defined in terms of Oxidation Reduction

oxygen +O O

electron e +e

oxidation number increases decreases

2. (a) (i) Redox (ii) O2

(iii) CH4

(iv) Carbon (b) (i) Not redox

(ii), (iii) & (iv): not applicable (c) (i) Redox

(ii) Cl2

(iii) FeSO4(or Fe2+)

(iv) Iron

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3. Both statements are correct. However, the second statement is not a correct explanation of the first. A correct explanation would be: ‘The oxidation number of nitrogen in ammonia can be increased when ammonia reacts with a strong oxidizing agent.’

A31.4

(a) (i) Cl2(g) + 2e

−

→_2Cl−_(aq)

(ii) 2Br −(aq) →_Br₂_{(aq) + 2e}−

(iii) Cl2(g) + 2Br

−

(aq) →_2Cl−_{(aq) + Br}₂_(aq)

(b) (i) MnO4

−

(aq) + 8H+_{(aq) + 5e}−

(aq) →_Mn2+_{(aq) + 4H}₂_O(l)

(ii) Fe2+_(aq)_→_Fe3+_{(aq) + e}−

(iii) MnO4

−

(aq) + 8H+_{(aq) + 5Fe}2+_(aq)_→_Mn2+_{(aq) + 4H}

2O(l) + 5Fe3+(aq)

A31.5

(a) K +_(aq)

(b) F−(aq) (c) Fe2+_(aq)

A31.6

(a) Ag(s) + 2HNO3(aq) →AgNO3(aq) + NO2(g) + H2O(l)

(b) Ag(s) + 2H+_{(aq) + NO} 3

−

(aq)→_Ag+_{(aq) + NO}₂_{(g) + H}₂_O(l)

A31.7

(a) No reaction.

(b) C(s) + H2SO4(l)→CO2(g) + 2SO2(g) + 2H2O(l)

A31.8

1. Fe2+_{(aq) + Ag}+_(aq)_→_Fe3+_{(aq) + Ag(s)}

2. (a) Zn to Cu (b) Mg to Ag (c) Zn to Pb

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Chapter 31 Redox reactions Chapter Exercise

1. transfer 2. loses, gains

3. (a) oxidizes, accepting (b) reduces, donating 4. (a) 0 (b) ionic charge (c) 0 (d) ionic charge 5. (a) increases (b) decreases 6. Electrochemical 7. (a) weak (b) increases (c) strong (d) oxidizing (e) reducing (f) strong (g) decreases (h) weak 8. oxidizing, NO2(g), oxidizing, SO2(g) 9. B 10. C 11. D 12. D 13. C 14. C 15. C 16. C 17. D 18. B 19. B 20. B 21. D 22. D 23. B 24. C 25. B 26. C 27. C 28. A

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HKDSE CHEMISTRY  _{A Modern View (Chemistry)} _{Coursebook 3} 29. (a) 0 (b) +2 (c) +3 (d) −₃ (e) +4 (f) +6 (g) +7 (h) +5 (i) +1

30. (a) Redox reaction. Oxidizing agent: O2; reducing agent: NO; nitrogen monoxide

is oxidized.

(b) Redox reaction. Oxidizing agent: HNO3; reducing agent: C; carbon is

oxidized.

(c) Not a redox reaction.

(d) Redox reaction. Oxidizing agent: CuSO4; reducing agent: Zn; zinc is

oxidized.

(e) Redox reaction. Oxidizing agent: Cl2; reducing agent: Cl2; chlorine is

oxidized. (Chlorine is both oxidized and reduced in this reaction.) 31. (a) (i) MnO4

−

(aq) + 8H+_{(aq) + 5e}−

→_Mn2+_{(aq) + 4H} 2O(l)

+ 5 × _(Fe2+_(aq)→_Fe3+_{(aq) + e}−₎

____________________________________________________________ MnO4

−

(aq) + 8H+_{(aq) + 5Fe}2+_(aq) _→_Mn2+_{(aq) + 5Fe}3+_{(aq) + 4H} 2O(l)

(ii) Cr 2O72

−

(aq) + 14H+_{(aq) + 6e}−_→_2Cr3+_{(aq) + 7H} 2O(l) + 3 × _(2Br−_(aq) →_Br₂_{(aq) + 2e}−₎ ____________________________________________________________ Cr 2O72 − (aq) + 14H+_{(aq) + 6Br}−

(aq) →_2Cr3+_{(aq) + 3Br}₂_{(aq) + 7H}₂_O(l)

(b) (i) The purple permanganate solution is decolorized, and a yellow solution is formed.

(ii) The orange dichromate solution becomes green, and bromine is formed which is yellow. The resultant solution looks yellowish-green.

32. (a) Colourless bubbles are evolved from the copper turnings, and the solution turns blue.

(b) 3Cu(s) + 8HNO3(aq)→3Cu(NO3)2(aq) + 2NO(aq) +4H2O(l)

(c) Dilute nitric acid is the oxidizing agent. It is reduced as the oxidation number of nitrogen changes from +5 to +2. Copper is the reducing agent. It is oxidized as the oxidation number of copper changes from 0 to +2.

(d) If the plug of cotton wool is removed, brown fumes appear at the mouth of the test tube.

(e) 2NO(g) + O2(g) →2NO2(g)

(f) The cotton wool plug prevents oxygen in air from entering the test tube, so the nitrogen monoxide formed will not be converted to nitrogen dioxide.

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33. (a) Bubbles form immediately, and green fumes are evolved. (b) 2MnO4

−

(aq) + 16H+_{(aq) + 10Cl}−_(aq)_→_2Mn2+_{(aq) + 5Cl}

2(aq) + 8H2O(l)

(c) Permanganate ion is the oxidizing agent. It is reduced as the oxidation number of manganese changes from +7 to +2. Hydrochloric acid is the reducing agent. It is oxidized as the oxidation number of chlorine changes from –1 to 0.

(d) The stopcock should be closed. Otherwise, the chlorine gas would leave the flask through the funnel rather than through the side arm to react with the potassium iodide.

(e) The water in the moistened broken porcelain chips can absorb unreacted hydrochloric acid vapour (hydrogen chloride gas), which is highly soluble in water.

(f) The potassium iodide crystals turn black/dark brown. (g) Cl2+ 2I

−

→_2Cl−_{+ I}₂

(h) Chlorine is the oxidizing agent. It is reduced as the oxidation number of chlorine changes from 0 to –1. Potassium iodide is the reducing agent. It is oxidized as the oxidation number of iodine changes from –1 to 0.

(i) The experiment should be carried out in the fume cupboard as chlorine is a poisonous gas.

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Chapter 32 Redox reactions in chemical cells Class Practice

A32.1

(a) Anode: zinc electrode Cathode: silver electrode (b) Silver electrode (cathode) (c) Zinc electrode (anode)

(d) Ionic half equation for the oxidation: Zn(s)→_Zn2+_{(aq) + 2e}−

Ionic half equation for the reduction: Ag+_{(aq) + e}−

→_Ag(s)

A32.2

(a) Electrons flow from electrodeU to electrodeV in the external circuit. It is because the oxidizing agent MnO4

−

(aq) will gain electrons while the reducing agent Br −(aq) will lose electrons.

(b) Oxidation takes place at electrodeU . Hence,U is the anode andV is the cathode. (c) V is the positive electrode and U is the negative electrode. Br −(aq) loses electrons

at electrode U and is oxidized to Br 2(aq). Electrons thus move in the external

circuit fromU toV . Therefore, U is the negative electrode. (d) At electrodeU : solution around electrode turns yellow/brown.

At electrode V : solution around electrode becomes pale purple (partially decolorized).

(e) Ionic half equation for reaction at electrodeU : 2Br −(aq)→_Br₂_{(aq) + 2e}−

Ionic half equation for reaction at electrodeV : MnO4 − (aq) + 8H+_{(aq) + 5e}− →_Mn2+_{(aq) + 4H}₂_O(l) (f) 2MnO4 −

(aq) + 10Br −(aq) + 16H+_(aq)_→_2Mn2+_{(aq) + 5Br}

2(aq) + 8H2O(l)

A32.3

1. Zn(s) + 2H+_(aq)_→_Zn2+_{(aq) + H} 2(g)

2. (a) A paste of ammonium chloride is used so that ions are mobile and can conduct electricity.

(b) If the cell is dry, the ions inside are no longer mobile. Electricity cannot pass through and so the cell will not produce electrical energy.

(c) No. As shown by the half equation, zinc participates in the electrode reaction. (d) Yes. As shown by the half equation, carbon does not participate in the

electrode reaction.

(e) At the zinc cup (the anode), zinc dissolves to form zinc ions. A metal-metal ion system is set up.

A32.4 (a) At anode: CH3OH + H2O→6H++ CO2+ 6e − At cathode: 3O2+ 12H++ 12e − →_6H₂_O

(b) Methanol is a liquid which is easier to handle than gaseous hydrogen during refilling./Methanol poses a lower risk of explosion than hydrogen. (Any ONE) (c) Methanol is flammable, if carelessly handled, it may catch fire. Furthermore,

methanol is a colourless liquid like water, yet it is highly poisonous. If it is not stored or labelled properly, there is a danger of accidental poisoning.

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Chapter 32 Redox reactions in chemical cells Chapter Exercise 1. oxidation, cathode 2. negative, cathode 3. Zn(s)→_Zn2+_{(aq) + 2e}− 2MnO2(s) + 2H+(aq) + 2e − →_Mn₂_O₃_{(s) + H}₂_O(l) 4. secondary Pb (s) + SO42 − (aq) ⇌PbSO4(s) + 2e − PbO(s) + 4H+_{(aq) + SO} 42 −

(aq) + 2e−⇌ PbSO4(s) + H2O(l)

5. primary 6. hydrogen, oxygen 7. oxidant, catalyst 8. B 9. A 10. D 11. D 12. D 13. D 14. C 15. C 16. B

17. (a) Dilute sulphuric acid (b) Cr 2O72

−

(aq) + 14H+_{(aq) + 6e}−_→_2Cr3+_{(aq) + 7H} 2O(l)

(c) The colour of the acidified potassium dichromate solution slowly changes from orange to green.

(d) Fe2+_(aq)_→_Fe3+_{(aq) + e}−

(e) The colour of the iron(II) sulphate solution slowly changes from pale green to yellow.

(f) Electrons flow from the right electrode to the left electrode in the external circuit.

18. (a) 2MnO2(s) + 2H+(aq) + 2e

−

→_Mn₂_O₃_{(s) + H}₂_O(l)

(b) Zn(s)→_Zn2+_{(aq) + 2e}−

(c) Ammonium chloride

(d) After a zinc-carbon cell has been used for a certain time, all the NH4+(aq) ions

will be used up. No more H+_{(aq) ions are available. Therefore, the reaction at}

the cathode cannot take place. The cell will stop functioning.

(e) During storage, the zinc casing of the cell will react with H+_{(aq) ions present}

in the electrolyte. Therefore, zinc can be used up even when the cell is not being used.

19. (a) At the zinc electrode (anode): Zn(s)→_Zn2+_{(aq) + 2e}−

At the copper electrode (cathode): Cu2+_{(aq) + 2e}−_→_Cu(s)

(b) Zn(s) + Cu2+_(aq)_→_Zn2+_{(aq) + Cu(s)}

(c) Zn(s) + Cu2+_(aq)_→_Zn2+_{(aq) + Cu(s)}

(d) Yes, they are the same.

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(e) (1) In the Daniell cell, Cu2+_{(aq) and Zn(s) are separated by a porous}

partition. There is no direct mixing of the chemicals. Solid copper deposits on the surface of the copper container. In the beaker, the Cu2+

(aq) and Zn(s) are in direct contact and solid copper deposits directly on the surface of the zinc rod.

(2) In the Daniell cell, the reaction generates energy mainly in the form of electricity. In the beaker, the reaction generates energy mainly in the form of heat.

20. (a) Hot, concentrated potassium hydroxide solution. (b) As a catalyst.

(c) H2(g) + 2OH

−

(aq)→_2H₂_{O(l) + 2e}−

(d) Oxidation. It is because the oxidation number of H changes from 0 to +1. (e) O2(g) + 2H2O(l) + 4e

−

→_4OH−_(aq)

(f) 2H2(g) + O2(g)→2H2O(l)

(g) The electrons flow from the left hand side to the right hand side (clockwise) in the external circuit.

(h) The fuel of fuel cells can be refilled easily. They can be used basically forever (by refilling the fuel). Conventional cells either run out (primary cells) and need to be discarded, or need recharging (secondary cells) which is time-consuming.

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Chapter 33 Electrolysis Class Practice

A33.1

(a) (i) At cathode: magnesium; at anode: chlorine (ii) No electrolysis takes place.

(b) Molten magnesium chloride contains mobile ions that conduct electricity while solid magnesium chloride does not conduct electricity.

(c) For (a)(i),

At cathode: Mg2+_{(l) + 2e}−_→_Mg(l)

At anode: 2Cl−(l) →_Cl₂_{(g) + 2e}−

A33.2

H+_{(aq), OH}−

(aq), cation, metal, H+_{(aq), hydrogen, OH}−

(aq), oxygen, halide ions, concentration

A33.3

(a) Cations: Na+_{(aq), H}+_{(aq); anions: OH}−

(aq) (b) (i) Hydrogen

(ii) Oxygen

(c) Volume ratio of hydrogen : oxygen = 2 : 1

A33.4

1. (a) (i) At cathode: Cu2+_{(aq) + 2e}−_→_Cu(s)

At anode: Cu(s) →_Cu2+_{(aq) + 2e}−

(ii) Overall equation: not applicable

(iii) The intensity of blue colour of the solution remains unchanged. (b) The blue colour of the solution becomes paler.

2. (a) chlorine (b) sodium amalgam (c) oxygen (d) ionizes (dissolves) (e) Cu2+_(aq) A33.5

(a) At copper cathode: hydrogen At platinum anode: oxygen (b) At graphite cathode: hydrogen

At graphite anode: oxygen (c) At graphite cathode: copper

At graphite anode: chlorine

(d) Electrolysis products cannot be predicted because the concentration of solution and material of electrodes used are not specified.

A33.6

1. Remove the metal ions from effluents by chemical treatment, if it is economical to do so.

2. Acidic effluents usually contain sulphuric acid. The calcium sulphate formed is only slightly soluble in water, so it would prevent the neutralization reaction from going on. Besides, it is difficult to remove the insoluble calcium sulphate.

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HKDSE CHEMISTRY  _{A Modern View (Chemistry)} _{Coursebook 3} Chapter 33 Electrolysis Chapter Exercise 1. decomposition 2. chemical, electricity 3. cathode, anode 4. positive

5. ions, cathode, anode, cathode, gain, anode, lose, anode, cathode 6. E.C.S., concentration, electrodes

7. electroplating, electroplated, corrosion 8. cathode

9. anode 10. thicken

11. lkalis, heavy metal, rganic, health 12. Environmental Protection 13. C 14. A 15. B 16. B 17. A 18. D 19. A 20. B 21. (a) Carbon

(b) Lead(II) ions and bromide ions

(c) At the cathode: A silver colour appears.

At the anode: Some brown gas bubbles appear. (d) At the cathode: Pb2+_{(l) + 2e}−_→_Pb(l)

At the anode: 2Br −(l)→_Br₂_{(g) + 2e}−

22. (a) Ions present: H+_{, OH}−

, SO42

−

.

Product at cathode: H2; product at anode: O2

(b) Ions present: H+, Na+, OH−, Cl−.

Product at cathode: H2; product at anode: O2

(c) Ions present: H+_{, Na}+_{, OH}−

, Cl−.

Product at cathode: H2; product at anode: Cl2

(d) Ions present: H+_{, Cu}2+_{, OH}−

, Cl−.

Product at cathode: Cu; product at anode: O2

(e) Ions present: H+_{, Cu}2+_{, OH}−_{, SO} 42

−

.

Product at cathode: Cu; product at anode: O2

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23. (a) Graphite (carbon) electrodes should be used. During electrolysis of halides, if platinum electrodes are used, they can be easily corroded by the respective

halogens produced at the anode. Carbon is used as electrodes for electrolysis of halides as there is no direct reaction between carbon and the halogens. (b) The left electrode. During electrolysis, the electrode connected to the positive

pole of the d.c. source is the anode. (c) In electrolytic cell A:

4OH−(aq) →_O₂_{(g) + 2H}₂_{O(l) + 4e}−

In electrolytic cell B:

Cu(s) →_Cu2+_{(aq) + 2e}−

(d) This is due to the difference in material used to make the electrodes. When an inert electrode (graphite or platinum) is used, OH−(aq) is preferentially discharged as it is a stronger reducing agent than Cl−(aq). When copper metal is used as the anode, Cu(s) is preferentially discharged. Cu(s) is a stronger reducing agent than OH−(aq) and Cl−(aq).

(e) The copper anode becomes smaller.

(f) There would be colourless bubbles (of oxygen) appearing on the surface of the graphite anode.

24. (a) At electrode D: 2Cl−(aq)→_Cl₂_{(g) + 2e}−

At electrode E :

2H+_{(aq) + 2e}−_→_H 2(g)

(b) Judging from the equations in (a), when 2 moles of electrons pass through the electrolytic cell, 1 mole of chlorine is liberated at the anode and at the same time 1 mole of hydrogen is liberated at the cathode. The theoretical ratio of the volumes of gases collected over the electrodes should be 1:1.

(c) The relative volume of gases shown in the diagram is reasonable. Since hydrogen is insoluble in water whereas chlorine is quite soluble in water, the actual volume of chlorine collected is less than the volume of hydrogen collected.

(d) At the anode, when the chloride ions are discharged, chlorine gas is produced. Since chlorine water is acidic and bleaching, the solution near the anode turns red and then colourless.

At the cathode, when hydrogen ions are discharged, an excess of hydroxide ions are present. The solution near the cathode turns blue.

25. (a) The three mistakes are:

(1) The door handle should be connected to the negative terminal (cathode). (2) The piece of nickel metal should be connected to the positive terminal

(anode).

(3) The electrolyte should be an aqueous solution of nickel(II) sulphate instead of dilute sulphuric acid.

(b) Two methods to shorten the time needed for electrolysis: (1) Use a larger current (higher voltage).

(2) Increase the surface area of the piece of nickel metal connected to the anode.

(3) Increase the concentration of the electrolyte. (Any TWO)

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26. Similarities:

− _{Both electroplating and anodizing are industrial processes.}

− _{Both processes involve electrolysis with the purpose of beautifying and/or}

strengthening the surface of metal items. Differences:

Electroplating Anodizing

Can be applied to most metal items (e.g. iron or copper items) and non-metal items.

Can only be applied to metal items made of aluminium.

The item to be electroplated is connected to the cathode during electroplating.

Aluminium item to be anodized is connected to the anode during anodizing. A thin layer of another metal (e.g. nickel

coating on iron) is coated on the item.

A thin layer of protective aluminium oxide is formed on the aluminium item.

The item acquires the colour of the coating metal (e.g. the golden colour of gold plating). The item cannot be further dyed

to another colour.

The oxide layer is too thin to be seen and the aluminium item retains its own silvery colour. The oxide layer can be dyed to a new colour if necessary.

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Chapter 34 Importance of redox reactions in modern ways of living Class Practice

A34.1

1. (1) A diesel generator has a lower efficiency than a fuel cell system. In other words, a diesel generator consumes more fuel to produce the same quantity of heat and electricity as compared to a fuel cell.

(2) A diesel generator causes pollution to the environment, producing smoke, bad smell, and a lot of NOx and SO2. A fuel cell system is clean and the exhaust is

non-polluting, so it is more suitable for on-site energy production for a block of flats.

(3) A diesel generator is very noisy while a fuel cell operates quietly. This again is better for on-site power production.

2. (a) At cathode: O2(g) + 4H+(aq) + 4e

−

→_2H₂_O(l)

At anode: 2H2(g) →4H+(aq) + 4e

−

(b) Overall equation of the cell reaction: 2H2(g) + O2(g) →2H2O(l)

A34.2

Lithium metal, like other alkali metals (sodium, potassium, etc.) reacts vigorously with water to produce hydrogen and a corrosive, strongly alkaline solution LiOH. If the seal of a cell with a lithium metal anode is broken, water or even moisture in the air may react with lithium, causing hydrogen and alkaline solution to leak out. Hydrogen may cause explosion and the alkaline solution can cause severe skin burns.

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Chapter 34 Importance of redox reactions in modern ways of living Chapter Exercise

1. hotosynthesis, etabolism

2. ermentation, ntioxidants, reathalyzers, leaching

3. Alkaline Fuel Cells, Proton Exchange Membrane Fuel Cells, Phosphoric Acid Fuel Cells, Molten Carbonate Fuel Cells, Solid Oxide Fuel Cells

4. lithium, lithium-ion rechargeable, lithium-ion polymer rechargeable 5. oxidized, reduced

6. lithium-carbon compound, metal oxide, lithium, organic solvent,

discharging CoO2+ xLi++ xe − ⇌ Li xCoO2 charging discharging Li xC6 ⇌ 6C + xLi++xe − charging discharging CoO2+ Li xC6 ⇌Li xCoO2+ 6C charging

7. harge density, oltage, rain capacity, recharged, nvironmentally preferred 8. A

9. A 10. D 11. A 12. A

13. (a) The statement is half correct and half incorrect.

A fuel cell can in theory work forever because it can continue to work as long as the fuel is replenished. A fuel cell is not a secondary cell because it cannot be recharged by using an external power source.

(b) A fuel cell is better than a secondary cell in the way that recharging secondary cells usually takes hours (2 hours for lithium-ion cells, and about 8 hours for nickel-cadmium cells). Instead, refilling the fuel of a fuel cell may just take seconds.

Besides, a small bottle of fuel (say, methanol for DMFC) can be easily carried around whereas electrical power for recharging secondary cells may not be available in remote locations.

(c) (1) High efficiency.

(2) Environmentally friendly (for hydrogen fuel cells, the only exhaust is water).

(3) Convenient (easy to refill the fuel).

14. (a) It is commonly used to measure the amount of ethanol present in the breath of drivers to help police collect evidence for drink driving prosecutions.

(b) A conventional breathalyzer contains a solution of acidified potassium dichromate. If alcohol is present in a driver’s breath, the dichromate is reduced and changes from orange to green:

2K 2Cr 2O7(aq) + 3CH3CH2OH(aq) + 8H2SO4(aq)

→_2Cr₂_(SO₄₎₃_{(aq) + 3CH}₃_{COOH(aq) + 2K}₂_SO₄_{(aq) + 11H}₂_O(l)

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(c) The alcohol in the breath of the driver, when blown into the intoximeter, is used as the fuel in an ethanol fuel cell. A higher concentration of ethanol produces a larger current, thus the intoximeter can give a reading that is proportional to the breath alcohol concentration of the driver.

(d) (1) An EC/IR intoximeter can give a continuous, linear reading of the breath alcohol concentration of a driver. The colour change of the conventional breathalyzer can only give an approximate data, which may not be good

enough for legal procedures.

(2) The EC/IR intoximeter is more environmentally friendly (no need to dispose of spent chemicals).

(e) An infrared (IR) sensor

15. (a) At cathode: O2(g) + 4H+(aq) + 4e

−

→_2H₂_O(l)

At anode: H2(g)→2H+(aq) + 2e

−

(b) 2H2(g) + O2(g) →2H2O(l)

(c) To start up a cold PAFC, it must first be heated to a temperature well above 40°_{C (the melting point of phosphoric acid) before the PAFC can function.}

(d) The exhaust of this fuel cell is steam (at 150°_{C to 200}°_{C). It can be used to}

heat up water (for hot water supply) or heat up the air (for warming air in winter).

(e) Generating great power/providing power at a relatively low cost/high efficiency/non-polluting/providing hot water or heat on-site (any TWO)

16. Advantages of lithium-ion cells:

− _{Lithium-ion cells are}_rechargeable _{and can provide}_{high voltage} _{(3.7 V) and}

large current .

− _{Lithium-ion cells are}_{comparatively lightweight}_{(high charge density).}

− _{Lithium-ion cells} _{do not contain/contain less heavy metals} _{that pollute the}

environment like zinc-carbon cells or nickel-cadmium cells.

− _{Lithium-ion cells have} _{no memory effect}_{, so discharging fully before}

recharging is unnecessary. This makes the charging process more economical and more environmentally friendly.

(Any THREE)

Disadvantages of lithium-ion cells:

− _{Lithium-ion cells cannot be used for appliances that need a voltage lower than}

3.6 V.

− _{The unit cost of lithium-ion cells is currently several times higher than that of}

other cell types.

− _{Lithium-ion cells must be} _{recharged with a dedicated charger}_{, and if the}

charger is not available, the lithium-ion cell cannot be recharged.

− _{After recharging for more than 1200 cycles, using a lithium-ion cell may}

cause the cell to heat up, electrolyte to leak and the cell may even explode. They may also explode if overheated or if charged to an excessively high voltage.

(Any THREE)