1973-02-21 — Page 17

21 FEB1973

買一第張五第日九十月正年丑癸层复

35

【1973英文中學會考試題預習專欄

堅道英文院生編

化學科

Chemistry (18)

Solution for last we

Part I

25

5.3

Part II

E

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10. T

A mixture of pure alumini -un power and sulphur (9:16) is heated in a test tube. When the mixturò begins to glow it is re- moved from the flame, but continu→ es to clow vigoroundy until the reaction is complete. After co01- ing, the tube is broken and the solid which remains is extracted, mass is aluminium sulphide,

WL + 35

AT253

në aluminium sulphide is then ansferred to a flask which is fitted with a two holed stopper carrying a tap funnel and a de- livery tube. Water is dropped on to the solid aluminium sulphide from the top funnel. Effervescenc occurs and the colourless gas of hydrogen sulphide is to pass through the U-tube containing calcium chloride. Dry and pure hydrogen sulphide is collected by downward delivery

(b)

241(OH) + 3H2S↑

JES

Sulphide repour

A Kipp's, apparatus is

shown in the diagram. The middle bulb contains nome ferrous sulphid With the tar open, dilute hydro- chloric acid is poured down the upper bulb, until the acid covers the sulphide. Hydrogen sulphide is produced immediately,

Fes + 2HC1

CHAST

PeCl2

festous

Mijdrechlinic and

hen the tap is closed the gas, continues to be produced in the riddle bulb until the builds up sufficient pressure to push the acid back into the lower bulb, and then the production of hydrogen sulphide atops.

c) A strip of filter paper soaked with lead nitrate solution, held in the gas, is imrediately turned to silvery black.

HAS + Pb(NO),

Pbs + 2HNO

This is the confirmatory test for hydrogen sulphide,

(a) (1)

H2S + 2HNO

2H 0+2NO

A pale yellow precipicate of sulphur and reddish brown fumes of

rogen dioxide form.

(11) H2S + C1 -→2HC1 + S

A pale yellow precipitate of sulphur and hydrochloric acid formed.

· 2H2O + 35 ↓

(111) 2HAS SO,

A pale yellow deposite of sul- phur and water vapour form on the Vessel.

(iv)

H2 + 2FeCl →→2Fe01 + 2HC1

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The yellow solution is reduced to pale green ferrous chloride, and a pale yellow precipitate of sulphur and hydrochloric acid is formed./

Sulphur is obtained as a by-product from both the petroleum industry and the coal gas industry Sulphur is extracted from crude petroleum as hydrogen sulphide gas The hydrogen sulphide is burnt in i a limited supply of air to give sulphur..

→211,0 + 25}

CHAS 0

Sulphur may also be obtained from the spen oxide which is recovered from the purifiers in the gas works. During manufacture of coal gas, hydrogen sulphide is removed in the purifier by react-

on with moist ferric oxide:

FeS + 3H201 Fe + 3H2S

When all the oxide is used up, the spent oxide is exposed to air and moisture. It is reconvert- ed to ferric oxide and sulphur is, formed:

2reu + 30

2Fe 0+ 63

(b) (i) By the equations FeS + 2HC1

HS+ FeCl

2.2 Em. mol.wt. 23.

by direct propertion we

88.34

et 2.2:x

2-2 X 34 = 0.85 cm. H

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Its density is 0.001365. gm. per cc. at 27°0. and 75 cm.

0.85 0,85 gm. occupied

0.00135600 PV Boyle and charles-law:

75

559 cc.

The N.T.P. volume of the H gas evolved - 559.co..

Sulphur dioxide.

Preparation o

(1) By the action of concentrated sulphuric acid on copper.

Cu + 2H SO

Criso +2H OSO

Copper Turnings are first plac- ed in a flask and concentrated sulphuric acid is added. After the mixture is heated, rapid efferves- cence takes place. The gas can be purified by passing it through little water rid of its sulphuric acid vapour, dried by conc. Has and collected by upward displace- ment of air or over mercury.

A dark brown mixture is left in the flask. It contains anl.ydrous copper sulphate mixed with cuppro- us sulphide Cu S formed by a side reaction.

50+ 4H SO ->Cu2S + 3Cu50 +4H2O (2) Another method used in the laboratory is to pour cold dilute hydrochloric acid on any sulphide or bisulphite;

2HC1+ Na2SO3

HCL + NaHSO3

2NaCl + H2O + SO2 NaCl + H2O + 502

Test (1) Reduces Kano solutivn

from purple to colourless 5502 + 2Kmn0 + 2H O

4

→K2SO + 2MSO + 2H SO

4

(ii) Reduces Cr2O solution)

yellow to green. K2Cro + 3502 + H250

→ K2SO4 + Cr2 (504)3 + H2O

Properties:

It is colourless, pungent odour smell of burning sulphur, much heavier than air, readily soluble in water.

1) Will not burn or support com.). bustion, but burning magnesiu continues to burn in the gas to, form sulphur. 2Mg + SO2

2MgO + S

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2) As an acid, sulphurous acid, SO2 + 120

This acid is unstable and connot exist out of solution.

is a

reducing

is slowly

3) Moist S02 19

agent, eig

1) Sulphur dio

oxidised by oxygen of the air 2502

· + 2H2O + 02

2112504

ii) Chlorine is reduced to hydro -gen chloride,

So + 24,0 + 01 Ho~04 +2H01

iii) Sulphur dioxide reduces concentrated nitric acid to: nitrogen peroxide

SO2 + 2HNO3 --

HAS

SO A

+ 2002

iv) Sulphur dioxide reduces ferric sulphate to ferrous su Thate

502 + Je2 (504)3

2H

2H SO +2TeSO

v) Sulphur dioxide reduces potassium permanganate solution, from purple to colourless. 2KM10 ▲ + 5502 + 2H220

> K2SO + 2MnSO4 + 2H2 50 vi) Sulphur dioxide reduces potassium dichromate solution form orange colour to green. K2Cr2Oŋ + 380, + H250

A

cral (50413

if

As a bleaching agent, Dye

MS + HOT MESO4 + H2

c) liberates carbon dioxide.

from carbonate;

1) with hases forms salt and

water

2NaOH + H2SO

a 50 + 2H

ii) Concentrated

1150 as oxidizing agent.

Hot, concentrated sulphuric acid oxidises substances while it is itself reduced;

Cu + 2H SO

Cuso, + 21,0+ SO2

CO2+20 +250

2H2SO

H50

iii) Concentrated as a dehydrating

agent

132211 - 115 O

sugar

(120

60

CO

5H0

CuSO4

White

1005 100

cotton

H2C204 - H2

oxalic acid

4

blue

Uses of sulphuric acid'

a) for the manufactire of super-

phosphate of lime and other Fertilisers; w

b) in the refining or petroleum

for the preparation of nitric acid, hydrochloric acid and organic compounds →

d) in the lead accumulator

Test for sulphates

A little dilute hydrochloric acid is added to an aqueous solut

【Dye + hydrogen]+ 504 ion of a sulphate. When an ageous

(coloured

substance) colourless

Wool, silk, straw are all bleach -ed by sulphur dioxide due to the reduction of coloured substance to a colourless compound, possibly by nascent hydrogen.

5) As an oxidising agent

With hydrogen sulphide, sulphur dioxide acts as an oxidising a7 agent.

Uses:

35 + 2H 0

1. In the manufacture of sulphur

acid.

2. As a refrigerant.

In the manufacture of calcium bisulphite for the treatment of wood-pulp in the early stages of paper manufacture. As a fumiganmt. 1

As a bleaching agent.}

Tests: for sulphites

Whenever a dilute acid is added to any sulphite and warm, sulphur dioxide is evolved which can be identified by its smell and action in potassium permanganate and dichromate.

Sulphuric acid Manufacture

The contact pro-

cesa

1) 50, is prepared by roasting

Pyrites ro sulphur in air

[4FeS2 + 1102 >2Fe203 + 850 The mixture of 502, nitrogen| and excess of air is freed from dust by passing it through chambers packed with quartz and then washing it with steam or very dilute caustic soda solut ion. It is then dried by con-

centrated HSO4

3) The dry dust-free gas passes

into contact chambers heated to 45000

250, + 02 7500 250

Catalyst commonly used are platinum and oxides of vanodium The $0 is absorbed in 97.7%

{H2SO A and water is run in at

the same time to keep the con- centration constant. H, SO

HO+ SO

Properties Physical- heavy, oil liquid,)

miscible with water in all. proportions. S.G. 1.8. Much heat is evolved when the acid and water are mixed, So the

acia must always be added tol water. Chemical ---- 1) ÁÐ an acıa' it turns litmus red;

reacts with metals to liberate hydrogen:

solution of barium chloride is we then added to the mixture, a white precipitate of barium sulphate wLI be formed,

Na, 50 + Bacl, 2Na01 BaSO

The addition or hydrochlorio acid distinguishes a sulphate from other acid radicles which also ( produce a precipitate with barium phloride,

Na2SO + BaC1→ BaSO + 2NaC1

Sodium sulphite will give 1⁄2 --- White precipitate of barium sul phite with barium chloride solut- ion, but no precipitate will appear if the sulphite solution is first acidified with dilíta hydrochlaric acid.

Exercise Part I

1. When a mixture of conc. sulphur -ic acid and solid potassium bromide is heated-

A. only pure bromine is formed B. only hydrogen bromide is

formed

[C. A mixture or promine and

hydrogen bromide is formed D. A mixture of bromine and

sulphur dioxide is formed A mixture of bromine, hydrae gen bromide and sulphur dioxide is formed.

2. When sulphur dioxide is bubbled into a solution of ferric chloride. ferric ions are oxidized

2. sulphuric acid is formed 3. hydrogen sulphide is evolved 4. sulphur is precipitated 5. ferrous chloride is formed

Which of the following alternat -ive is true.

A. 1 and 2 only

B. 1 and 4 only

C. 2 and 5 only

D. 3 and 5 only E. 4 and 5 only.

3. When sulphur dioxide is passed into a solution of caustic soda. for a prolonged period,"

A sulphuric acid is formed B. sodium sulphite 1b farmed. C. sodium sulphate is formed

Bodium bisulphite is formed)] sodium bisulphate is formed

4. When sulphur dioxide 19 buopred into a solution of potassium di- chromate, the solution turns:

A

colourless

B. yelloW

c. orange)

D. green

E. no change in coloration.

5. When excess sulphur aloxias' passed into sodium hydroxide, the product formed is

A. sodium sulphite

B. ‘sodium hydrogen sulphite,

0. sodium sulphide

D. sodium hydrogen sulpNATY,

F. sodium thiosulphate

完明日瓶)