1989-03-02 — Page 41

41)日五廿月正年已己曆夏

1989 中學會考預習專欄

明德出版社,

MILL & DALE PRESS

Chemistry (23)

Unit 12 (Continued)

Dry cell

R. Chu

6.1 It serves as a convenient source of

energy.

6.2 The structure. of

shown below.

Pitch seal

Graphite rod (es. The evo

Mixture of moisted in.

(to oxidise H and C powder

to reduce Internal Pakistarcel

-Paste of ZnCl2 and Nigel

(as alectrolyte)

Zinc, as the ve poje)

6.3 The celi reactions iria Ary cell in

use are:..

At anode: Zn dissolves in the

Zn(s)

At cattiede:

electrolyte paste.

back

6.4 Differences in structures of

6.4 Differences in structures of a new

ane a used cry cell

New dry cell

(a) Zn case is.

thicker:

(b) The powder

Cmixture.

is black. (c) the paste

Used dry cell

(a) za case is

corraded and

(b). The powder

becomes thinner.

mixture becanes, grey,

(c) The paste may

dry-cup.dee

7. Corrosion of Metals

to less water.

7.1 Corrosion

the is

gradual destruction of any metals due to their reactions with supstances in their surroundings, e.g. substances in ang to form oxides or other compounds

7.2 Corrosion of a metal is primarily an oxidation of the metal to its

oxide. The cxice in most cases is

望你 tu powdery, permeable

and

moisture, and readily flakes off. e.g. rusting of iron.

7.3 In some cases, the oxide layer is tough and adheres. strongly onto the thus forming a protective

metal,

Layer.

8. Rusting of Iron

8.1 The corrosion

commonly known as rusting. The

must chemically

is

more

and

as

is reddish-brown

represented

8.2!ron rests unly in the presence of

both air and water.

B.3 Iron rusts

faster in salt water-

because salt water

话 better

electrical conducting medium.

8.4 Jron rusts faster im acidic :+(aq) solutions because.. accelerates the loss of clectrons by iron to form iron (II) ions.. 8.5 Iron does not rust if it is in contact with a metals above it in the E.C.5. e.g. Zinc..

8:6 Iron rusts

isin faster if it contact with a metal below it in Els, e.g.copper ..

8.7 The diagram below helps explain

what happens during iron rusting.. Oy content Oz. content

g的

Jov

Satnodic

arda (39

Anotic aten

(a) The

primary

rusting

process.

Cy conten

gr

Cathodic

area 78)

reaction

drop.o water!

Tron plate

in iron an electrochemical

{b}. The part of iron. -far inside the

water drop octs, as the anodic area) while. the part of iron near the edge of the water drop acts as the cathodic area because water hear the edge contains relatively more dissolved air.

(c) First stage reactions

At the anodic area:

Fe(s) re2+(aq)+2e" (oxidation) At the cathodic ana.

•°¿{9}+2H20{£¦ +46 ~~» 40+* (ag).

The

(reduction)

above.

W]

increased if there are dissolved ions

to increase

reactions

in, water

(conductivity,

報日僑華

(e). Which of them could have its oxide ireduced by carbon but not by hydrogen?

aluminium and (f) if mixtures of Tiron(11) oxide and of aluminium(11) oxide and iron are heated separately, thera bea in which mixture will

reaction and Why? Give an equation of the reaction..

(g) What would you expect to happen if a little powdered zinc is slowly added into an aqueous solution of silver nitrate and the mixture is well shaken? Give an ionic equation of the reaction. (h) Name a total in the above series which is usually extracted from its melten chloride by electrolysis.

(1) Ordinary aluminium metal shows Sittle chemical reactivity towards many of the reagents such as water, dilute acids, etc., although at has a rather. his position in the Reactivity Series. Explain briefly why this is so.

Question 13.2

In the experiment below. a zinc rod is placed in 1 zinc sulphate solution inside. a. porous pot. The pót hilth its content is then placed in a beaken containinja.copper plate in copper(17) sulphate solution. When the key is e'Ösed, a current is registered by the ameter.

Zinc rod

四期星日二月三(九八九一)年八十七國民華中

The blue colour fcdes off because there is

a decrease in the concentration of copper(II) ions due to its, depesition on the copper late.

After some time, the following results. (el Yes. were obtained."

Aron nal metai.

strips

slun

Onservation

Gel near magnesium stripl

Gel rear ne sl

(f) Yes. The overall reaction is: u2+(aq) →→2r2+(aq) +Cu(s) Zn(s)+Cu°

(g).M(Zn) -165,4 g mol

turns pink.

来

turns pink.

1 Trontieed

得Cél near Iron ̇nail-tip

turas lu

tron-cupp

Gel rear tron mall, Na turns blue?

(CJ) deposited 201

63.5

(a) What is the purpose of adding phenolphthalein indicator into the gel? (b) What is the purbose of adding potassium

hexacyanoferrate(11)

solution into, tre „gel? q

(c) Referring to the result in explain why the gal turns, pink.

(d) Referring to the result in IV, explain why the gel turns blue.

which case would the iron. (e) in corrode most readily?

心

Write. the ionic equations showing the changes of iron during its corrosion. (f) Describe how the zinc coated on iron (galvanized irca) portects' it from Corrosion even when the zinc cuatis broken.

Solution to Q.13.1

(a): Cu or Ag: because it is less reactive thar hydrogen.

(b) K; because it is the most reactive one of the elements listed.

least (c) Ag; because it is the. reactive one of the elements listed.

Copper plate-

t Zinc sulphate solution

Deater

its

1 M Copper sulphate solution

Porous pot

(d) Ag- (e) Zn.

(f): A reaction wilt.

in the aluminium and iron(II) oxide um, being more reactive reduce iron(II) oxide

occur

(d) Electrons flow from the anodic arec

to the casnodio area inside the iron plate.

(e) 2nd stage reaction

(1) Cxidation

4Fe2* (aq) +02(c)+244

茶

40H(aq)

on of iron(III) hydroxide

|-→→ Fe(OH).

{F} 3 ( 5 ) -

on handing

hyorated. iron(111) oxide (rust).

edds of Preventing Iron Rusting-

rease/ol

Paint/Enamel/Plastic coating-

-plating

(d). Galvanising iron (Zinc-plating)

crificial Corrosion snless steel containing à

Nb percentage of Cr. and yi.

Question 113.1

G+

K, Mg, Al, Zn, Fe, H, Cu, Ay are the

ants which appear. symbols for the

vity: Series. potassium silver

in this order

or Electrochemic Sepa being the mast reach being the least reactil

State the direction of the a

low the exteqyhe

your answer.

te the Tomc equations of the

Reactions at the zinc roc and the

copper plate respectively.

(c) fhy must the pot be porous?.

direction would

whist sulpoate ions, pass

(d) an

pot?

the

::hrough the porous

Explain your answer briefly..

(e) Would there be any change in the colour intensity nf copper(II) sulphate solution?

Explain your answer briefly.

(f) Do you think the reaction above is equivalent to placing a zinc red in copper (1) sulphate solution?.

(g) If the copper plate changes its the mass by 2 get the end of. experiment, what would be the change in hass of the zind rod?

(h) Describe THREE changes you would observe if the zinc. rod and 14 zing' sulphate salution are - replaced respectively by a silver oc and 1 M silver nitrate solution.

(Question: 13:3

Four pieces of iron nail's each coupled with a different metal strip were placed in separate pertridishes. containing gel with a few drops of phenolphthalein indicator and potassium suletion. The texecyanoferrate(i11)

the diagram

(g) znk powder di

grey crystals of s

This is because reactive silver.

Zn(s) + 2Ag+(aq) → Zn(ac)

· (6) K. --

Ivery ted:

the ess. ion.

(i) Freshly-prepared aluminium metel very reactive chemically." aluminium is usually covered wit layer of transparent, through aluminium oxide which acts as a protective layer on the metal.

Solution to Q.13.2

It

(a) The electrons flow from @to

because zinc S electropositive than copper and hence has 日 stronger electrons.

(b). At the zinc rod:

Zn(s) 22+

tendency

Haq).

At the copper plate:

Du2+(q).

(c) The pot must be two electrons contáčk.

(ba) 163,5 g mol.

According to thie following equations

fZn(s)→ Zn/*(aq)+2e7

(aq)+2e"> Quest

niZn) dissolved : ='n(Cu) deposited: change in mass 'of the zinc rod.

65.4 x

2.06 9

[h) The 3 changes observed are: y

(1) Copper plate is dissolved.

(2) Silver deposited on the silver

rod..

(3) [lections flow from copper

plate to the silver rod in the external circuit, .e., the current is reversed.

Solution to Q.13.3

(a) It is used to detect the presence- of the hydroxide ions (CHT), because "phenolphthalein turns pink presence of Oнions.

我

the.

(b) It is usec. to detect the presence.

ions (Fe2T) because of Iron(11)

hexacyanoferrate{ll)ions form a deep blue complex with iron(11) ions.

(c) This indicates that hydroxide ions are present near the magnesium strip. It is formed when magnesium practs with mator and the dissolved oxygen.

*(aq)+40f(aq)

(d). This indicates that iron(11) icis are aresent near the iron nadt..

is formed when iron.reacts with

and the dissolved oxygen.

•2Fe2+(aq)+40H-(aq)

(e) from corrode most readily in IV. because iron is attached to a metal lower in ECS, i.e. Copper. The ionic equations of the reactions

involved are:

Fels)→ Fe(aq)+2e.

2: Fe2+(aq)+02(g)+2H20(X)- 4Fe3+{an}+40H(aa)

ARIES

to

lectrical

NG PUBLIC LIBRA

(a) Which of them will NOT di

hydrogen from dilute acids?

(b) which of them will form the mos stable hydroxide?

(c) which of them is most likely to

form the lease stable oxide?

(d) Which then s most likely found free in nature?:

$

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152257 STATE ALLEMANA SAG. 52

iron natl

Was

shown as 10

strip

gel plate

would pass from inside of the porous.

Because there is an excess of positively-charged

Tons (Zn2+) zinc

inside the pot which thus attract the

sulphate negatively-charged

(so?") 'intg the pot.

ions

+(aq)430H (aq)→Fe(OH)g(s)

( standing.

rust(hydrated

iron(III) oxide)

(f) When the zinc coat is broken Iron will be exposed to air. However, since zinc is more electropositive than iron and: has a stronger tendency to se electrons to form ions than iron. so, iron - remains unchanged while zinc is

·xide zed to zinc{1t} ions, Zn2*(aq),

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