日汽廿月三年七六九一曆公年六十五安民楼
tube containing the sulphate will give a heavy
white precipitate of barium sulphate which is
insoluble in hydrochloric acid.
Na 2 SO + BaCl2 = BaSO4 + 2NoɑÇ1.
The other test-tube will not give a precipitate,
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日九十月二年未丁层复
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英文中學會考試題預習專欄
育僑華
物理科
(廿二)
·梁海明·
PHYSICS (22) K.N_Leuñe
solution
41. (a) Faraday's law of electrolysatı
年
化學科 ( 廿二 )
D
王錦釗。
(i) The mass of any substance liberated during electrolysis is proportional to the quan' tity of electricity passed, through the electrolyte.
(1) When the same quantity of electricity
passes through various electrolytes, the masses of the different shastänges liber ated are proportional to their cheminial! equivalents.
To verify the first law," "copper" voltaarter) consisting of twa copper plates dipping vatoi a strong solution of copper sulphate slightly acidified with sulphuric acid. The circuit is shown in the following diagrHİNS
oppin Cathode
Copper
Sulphate
Solution
A
Copper Anoda
The results are recorded as follows:
mass of cathode before paasang current-gm mass of cathode after passing current 2 mass of copper deposited.
KTD
gm
हंगा
t
ãeo
I
amp.
- Q
coulomb
It coulomb
tar !!
true.we should find that
time
current
quantity of electricity
If the first
m/Q is constant
To verify the second law, a water voltameter, a copper voltameter, and a silver voltameter, (electrolyte silver nitrate, electrodes, silver are connected in series. The same current will flow through all three voltame- ters. Pass the current for a definite period of time. Measure the volumes of hydrogen and oxygen liberated, and hence calculate the masses of these gases. Find, by weighing, the mass of the silver and copper. deposited. It will be found that the masses liberated are In the rat Lo
Silver
Hydrogen 1. go
Oxggen 8 gm
Copper
31.75 108 gm
ne
1041 equivalent on element are the chem
weighta
|| batteries
Voltamuler
・Water:
copper silver
DOO DD Do
1英文中學會考試題預習專欄:
CHEMISTRY
Q.1.Sulphur dioxide may be prepared in the laboratory by
the action of hydrochloric acid on sodiup_sulphite,
HCL
No. 50,
CONE
HSO
A fairly strong solution of hydrochloric acid is } dropped from a funnel on to some crystals of sodium sulphite. Double decomposition will take place, and
The gas contains as sulphur dioxide is given off. impurities, hydrochlorid vapour and water vapour. The impurages is ürist babbled through a small quan- tity of water contained in a Drechsel bottle which will remove the acid vapour The gas is then passed through a drying
such as concentrated su phuric acid, remove the water vapour. The gas which is collected gas jars is reasonably pure sulphur dioxide.
+ 2601 - 2NaCl Naz SO. Sodium sulphite
to
By
•
502
+
820-
(1) when burning magnesium is put into a gas jar con-
taining sulphur dioxide, the magnesium will con- tinue to burn in the gas with the formation of a white solid, magnesium oxide, and a yellow deposit of sulphur. Sulphur dioxide is not a supporter of combustion, but, as in the case of carbon dioxide, if the burning substance such as magnesium has a strong enough flame, it will remove the oxygen from the sulphur dioxide, and this will enable it to continue burning.
2MB 302 2Mg0 • $
(11) Sulphur dioxide in solution will serve as a bleach-
ing agent. It converts some coloured substances inta colourless compounds by reduction.
S02 H2 O
wet sulphur
dioxide
(sulphurous acid)
H2 SOL
oxygen remo ved
sulphuric
acid
from coloured
compound
When damp blue litmus paper 19 put into a
gas jar of sulphur dioxide, the litaus will first turn red, and will then be slowly bleached,
HONGY
KO
Apparatus for Use in Demonstrating. Faraday's Second Law of Electrolysia,
(b) maes of copper, m
current
gm
0.01 x 10 x Y 0.9
I -2
go amp,
0.0003
e.c.e. of copper, 2 = From m - ZIL
t
ir
0.0003x2 25
Bec.
gm/coulomb
Answer: The time required to deposit a
layer of copper 0.1 mm thick and
(c) Current.
42.(a)
time.
10 sq.om in area 1a 25 minutes.
1
1
1 +
amp. hr.
- 3600 sec.
gm/coulent
• 54.3
I'm
E.C.E. of copper, Z = 0.000326-
chemical equivalent
of copper
chemical equivalent
of zinc
• 32.5 Em mase of copper deposited - Zit
=0.000326x1x3600
1.1736x32.5/31.
mass of zing used
-1.1736
-1.212
gm
go.
Answer: Mass of copper deposited is 1,1736
grams and mass of zinc used 18
1.213 grams.
Current, 1
amp ohms' Volta
3.
2
ohms.
(shme.
Realstance in series
eum.f. of battery realetance of battery resistance of plating
bath
Electrochemical equivalent
of copper
·Hase of copper
0.00033 gm/coulomb, 0.2x30x8.9
11:05amp
24 035
E
I
From Ohm's Law #R »
where E is the e.n.f. of batteries, I the current in amp, and f, the total resistanc
the circuit. P
(111) When sulphur dioxide is bubbled into chlorine water,
it will reduce the chlorine to hydrochloric acid, the resulting solation being a mixture of hydrochloric and sulphuric acids.
+ H2
502 sulphurdus
Vicid
C12 H2O chlorine
water
=
as barium sulphite la soluble in dilute hydrochlorie acid,
Naz SO,
BaCl2
RASO NaC1.
+
((b) In order to distinguish between ferrous sulphate
and ferric sulphate, again make an aqueous solution of each solid in separate test-tubes. To each solution, add ammonium hydroxide solution
The ferrous salt will give a dirty green gelatinous
with the alkali.
ppt
Fe SOL 2NH4OH = Fe(OH)2✦ ✦ (NH4)2 SO,
The ferric salt will give a brown gelatinoua ppt with the alkali
Fez (30) 6NH2OH ̃» ̃2Fe(OH)21 + ̃3(NH4)2° SON (c), in order to distinguish between zinc sulphate and
aluminium sulphate, make an aqueous solution of each solid in separate test-tubes. To each solution add ammonium nydroxide solution.( The aluminium salt gives a white gelatinous ppt of! aluminium hydroxide, insoluble in excess of ammonia
A12(50) 6NHOH 2A1(OH); 3(NH4)2SO The zinc salt first gives a white gelatinous ppt. of zine hydroxide which is soluble in excess of ammonia with the formation of the complex compound. [Zn(NH3)4] (OH)2
[Zn 50% * 2NH4OH '*
Zn ̈ (OH)2 + (NH4)2 SON soluble in,
excess of/ ammon la
Q.3.The most important about the bleaching action of
sulphur dioxide and chlorine is that neither gas will bleach when dry. Moisture must be present before bleaching will take place.
(a) The bleaching action of sulphur dioxide is a reduc- ing action. In the presence of water, sulphu”” dioxide becomes sulphurous acid."
302 N20 H2S03
It is the sulphurous acid which will remove the oxygen from many dyes and natural colouring matter, and turn them into colourless compounds./
42 50. 10) H2 SO4.
oxygen
from dye
Sulphur dioxide is to bleach fabrics or substances. that would be damaged by chlorine or bleaching powder. The reduction does not in general disintez, grate the molecules of the colouring-matter, Ás a result, the bleaching action is not permanent,. and atmospheric oxidation may slowly restore the original colour.
1 The bleaching action of chlorine is an oxidising
action. In the presence of water, chlorine becomes a mixture of two acids, hydrochloric and hypochlorous acids.
C12 82 0 HC → HC10
Hypochlorous.
acid
The hypochlorous acid is very wistable and it re- leases oxygen which reacts with the colouring: matter or dye to form a colourless compound
HC20
Dŷe * 0 - (Coloured
compund)
HC O
Colourless compound
Chlorine is used to bleach coarse fabrice, such as cotton and liven goods. The bleaching action. is stable.
QUESTIONS FOR NEXT WEEK (23) Describe in detail the laboratory preporation and collection of dry ammonia. How and user what conditions does emmonia react with (1) chlorine, (11) ferric chloride, (iii) copper sulphate. (iv) hydrogen chloride? 2.2.(a) A substance X, on gentle heating, evolved a gar
which turns damp litmus blue. Addition of barium chloride solution ot the solution of K in the presence of hydrochloric acid gives a white ppt.
JBRA
H2SO
When some brown lead dioxide is heated in a defle- grating spoon, and the spoon is then lowered into a gas jar of sulphur dioxide, the gas will reduce the lead dioxide to whi lead sulphate.
Pb 0-
Q.2(-). In order to distinguish between sodium sulphite
and sodium sulphate, first make an aqueous solution of each solid in separate test-tubes. To each solution, add some dilute nydrochloric acid, followed by barium chloride solution. The teal-
From
m/21
0.2x30x8.9
0.00033x0.5
2.3 ohme? shres.
ser,
» dy br. 5) min. 56 690- *Anewer The resistance erould be 4.3 ohms
The time required to deposit the copper is 89 br. 53 min. 56 sec,
Electrochemical equivalent of silver"
2 = 0.001! gm/coul.)
- 2,5
amp. 1.32
21 10 x 60 sec.
[b]
Current of ammeter mass deposited
Time required.
Actual Curren
1.32
amp
2
Error of the ammeter * 2.5 −2
*0.5
апр. amp,
amp.
Answer: The ammeter reade 0.5 amp. Lagher,
than the correct value.
Topics for revision this week: Potential difference; electromotive force. The beating effect of current. The volt, the watt and the kilowatt-hour- Joule's Law,
Questions
43)(a) Two coils of resistance 20and SZ respect-
Name the substance X, the gas evolved, and the white precipitate formed. Account for the formation of the precipitate.
(b) Name the drying agents you have used in the
laboratory for drying gases. Draw diagrams which show two different ways in which gaseD may be brought into contact with the drying material.
Specify a suitable drying material for E2 S1 CLS 60, NH3
ively are connected in parallel, and ce thus formed is joined in series to another 1 of 42 resistance. Find the resistance of the complete circuit. When 80 watts are being dis- sipated in the 562 coil, calculate:(a) the cur- rent in the 5Rcoil; (11) the total current through the circuit. (1) the supply voltage: (b) Describe a simple experiment showing the heating effect of a current. On what factors does the heating effect of a current depend? (c) An electric lamp is marked 12 volts, 20: watts. Calculate: (1) the resistance of the lamp when in normal use; (11)the cost of running the lamp for 100 hours at a cost of 30 cents per unit.
44. (a) Briefly describe the connection between the
volt and joule.
(b) What current is taken by a 2 KW fire work- ing on a 250 volts circuit? If all the heat developed is used to heat water, what mass of water in kilograms would have is amperature. raised from 10°C to 60°C in 7 hours?{ J-4.2 Joules per calorie)
..
(c) A hot water cylinder of water equivalent 2000 gm contains 28000 gm of water at 10°C. It is heated by an electric immersion heater rated at 2.1 kilowatt, 200 volts, Assuming that no heat is lost and that the heater 18 being used at its correct voltage, find(i) the c rent of the heating coil(21) the time requ to heat the water to 70°C (111) the cost of heating the water to 70°c if the prize of electricity 18 20 cents per unit.(take J-4.2 Joules/calorie.)
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