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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