1969-11-19 — Page 26

頁二第張七日十初月十年酉己曆夏

WAH KIU YAT: PO

170英文中學會考試題預習專欄

化學科

書院主編

CHEMISTRY (3)

SOLUTIONS FOR LAST WEEK

(=)

t. The only starting materials for the preparation of

hydrogen that meet the commercial requirements of low cost and great abundance are water and coke In the Bosch procesa, steam and heated coke are used to produce a mixture of hydrogen and carbon monoxide called water gas. This occurs by an endothermic reaction at about 10000

C + E¿O + heat = E ̧ † + 00↑

As the hot carbon removes the oxygen from San steam, it acts as a reducing agent. Since heat is absorbed, the temperature drope as the reaction proceeds, and the reaction slows down, The coke in Teheated to speed up the reaction again simply by blowing air through it, causing a rapid exothermic oxidation

C+02 = CO2 7 + heat

The carbon dioxide produced is permitted to escape. along with the excess air. Enough heat is evolved to increase the temperature of the coke so that it can further react with the steam. The endothermic and exothermic reactions are alternated in the process to yeidl intermittently hydrogen and carbon monoxize. The carbon monoxixe is capable of - reacting with additional steam to form more hydrogenj whilst itself is oxidised to carbon dioxide,

H

Co

But this occurs only at temperatures much lower than 100000. Thus the water gas is mixed with an excess of steam and passed at 500°C over a catalyst of an iron (III) oxide and chromium (III) oxide mixture. The presence of the catalyst causes the reaction to occur rapidly at this lower temperature, and additional, hydrogen and carbon dioxide are produced. To obtain pure hydrogen gas, the carbon dioxide must be removed. Carbon dioxide is 100 times more soluble in water than hydrogen. Furthermore, ita solubility increases significantly at low temperatures under high pressures.co Consequently the two gases can be separated-very effectively by passing them through cold water under high pressure. The carbon dioxide dissolves and the hydrogen gas bubbles up through the water and is collected.

(a) Hydrogen gas enters into direct companation

with chlorine gast

+

C12-

2H01

Light has a marked effeot on the speed at whion this reaction takes place. Thus a mixture of hydrogen and chlorine at room temperature reacts only slightly in the dark, but in the presence of direct sunlight react explosively to form HC1. Apparently the sunlight initiatde and accelerates the reaction. Similarly the Light from a burning magnesium ribbon will cause the mixture to react explosively,

At high pressures and at temperatures near 400 C in the presence of a catalyst, hydroger and nitrogen react to form anmonias

29H + 24 kcal.

this

382 + N2

chemical

The influence of pressure on. equilibrium is important, since the lesser number of molecules on the right hand side compared with the left indicates a diminution in volume. According to Le Chatelier1g. principle, as one of the conditions of the system in equilibrium is altered, the systen will adjust itself in such a direction as

partially to neutralise this change, so that an increase in pressure favours the forward reaction. More over, a large amount of heat is evolved in its formation. Thus the successful application of this reaction depends upon the use of high pressures and, of a catalyst which will bring a mixture of gases to the equilibrium point in a reasonable time and at as low a temperature as possible,

2. Temporary hardness of water is caused by pressende

of bicarbonates of calcium and magnesium, whereaн permanent hardness is due to their sulphates, Temporary hardness may be removed

by boiling, when the bicarbonate is changed

into the insoluble carbonate

2+

Ca

+

2003-0 Caco, ✓ ▼ ¤D2 } +

by addition of calculated quantity of slo

lime (Clark's method)

Ca(K¢O.)1⁄2

Ca(OH)2

20a002H 0

For magnesium bicarbonate, more lime is required, since the magnesing is precipitated as the hydroxide which is less soluble than the carbonate:

Mg(HCO3)2 + 20«(OH),

+ 28 0

Mg(OH)2 + 20a¢0 /

Both temporary and permanent hardness may be removed:

by addition of sodium carbonate:

Ca(HCO3)2 + Na2CO3

Caso + Na2 CO2

-

Caco + 2NaHCO

CaCO ✓ + Na2504

1964

三期星

Here, as in all cases of removal of permanent hardness, the water is not purified, i.e. there is merely a replacement of a multivalent ion by a univalent one which has no effect on Boa, (e.g. Ca2+ replaced by Na*).

ih, by the use of an ion-exchanger,

represents a cation

the shaded portion represents the solid part which consists of a very large anion carrying negative charges at intervals along it. This giant poly-ion can be a naturally occurring zeolite, ie, an aluminosilicate ion of a three-dimensional network of atome, with spaces in between occupied by cations. Alternatively, it can be a synthetic resin. or polymer. The important property is that the negative charge is static while the positive ions can move from their positions i these holes. Suppose that these cations are ions, when the ion-exchanger is shaken with hard water the sodium ions are

exchanged) with ions of greater.

oharge (Ca2+ and Mg2+); hence the latter remain in the exchanger and the water contains sodium ions. Representing the exchanger simply as za-, the process becomes i

nia* + Inca" - z′′-

nCa + nNa

Thus this type of exchanger is actually a cation-exchanger,

During titration n using methyl orange, the weakly basic bicarbonate is neutralized Ca(BCO2) + 2H

+220 + 200 1

Thus number of equivalents of Ca

18 0.06

in: 100:0.

1000

the number 0.0003 x *

moles of Ca

in 100 c.d

In 100,000 c.c. there would be 0.0

100,000 100

moles of Ca

in terms of weight of calcium carbonate per 100,000 0.0, of water of temporary hardness

- 0.15 x (40 + 12 + 48) gm. - 15 gm..

puring treatment of 200 c.c. water with sodium

bonate:

CaSO4 + Na¿¢¤ ̧ • CaCO2 √ + Na2SO4

Thus number of equivalents of calcium carbona

formed number of equivalents of sodium carbonate used excess of sodium carbonate

(25 x 0.12 - 22.0 x 0.1) E 0.0008

1000

Thus number of moles of calcium cargonave

x*

100,000 0.0. of water, number of moles of

100,000 calcium carbonate = 0.0008 x x

200

0.2

In terms of weight of calcium carbonata per 100,000 co. of water of permanent hardness -0.2 x (40+ 12 + 48) gm. 20

3. (a) zinc CSTOCIM . A KUTUMIA uontqining zine

ions is prepared by dissolving zino in dilute sulphuric acid

(b)

Zn + H230 « ZnS0 + Ef

Since the carbonate is insoluble it may be prepared through precipitation reactions. However, with carbonates of less electropositive metals such as zinc, sodium carbonate will produce a mixture of hydroxide and carbonats i.e. a basic carbonate. Therefore the zino carbonate must be prepared using sodium bicarbonate.

ZnSO

(notes

+2NaHCORE

Na,SO4 + B2O + CO2↑

this similarly applies to Mg, Fe, Fb,

Anhydrous zino chloride. The anhydrous Balt. cannot be prepared from aqueous solution because when evaporated, the hydrate Zn01, HO crystallises out; but this latter cannot be converted into anhydrous form by heating as de composition to basic chloride and hydrogen chloride occurs. Thus the anhydrous zino chloride has to be prepared by passing chlorina or hydrogen chloride over heated zinc. The zine chloride formed sublinea and should be kept in a sealed tube as it is deliquescent,

Zn + Cl2 = ZnCl2

(note: this also applies to Mg, Al, Fe, Pb,

Gu, Hg)

N.B. omission last week in problem 31 "The undissolved material after filtration and drying weighed 1.50 go.▾

Since "copper is not acted upon by dilute hydrochloric acid whilat zinc and aluminium are its weight would be given as the undissolved material, i.e. 1.50 gm.

日九十月一十年九六九一网公布八十五國民華中

Thus % composition of copper in the power

1.50

x 100 47.5.

3+

and Zn when the solution containing Al is treated with a slightly alkaline medium (ammonium chloride and ammonium hydroxide); only the aluminium is precipitated as the gelatinous hydroxide. Aluminium hydroxide o. ignition will be converted into A

Thus amount of aluminium

2.55-gm.

27

(2 × 27 + 3 x 16)

1.35 g.

Therefore composition of aluminiun

1.35

4

x 100 33.75

% composition of zinc = 100

(47.5+ 33.75)

28.75

4. In electrolysis, products formed at electrodes

depend upon the nature and concentration of the ions, nature of the electrodes and the physical conditions under which the electrolysis is taking place. In general, metallic ions lower in the electrochemical series will be deposited at the cathode whereas at the anode, electrons may be removed from anions in solution or metallic atom that make up the anode.

(a) Electrolysis of sulphuric acid with inert

felectrodes (9.8. Pt)

i, a dilute solution contains nydrogen one.

sulphate ions and hydroxyl ions as a resul of the following dissociations r

H2SO4

2- 28+ SO

4

#20 - £* + OH

During electrolysis, hydrogen lons are discharged at the cathode as they are the only species of cations, with the liberatio, of hydrogen. At the anode hydroxyl ions are preferentially discharged, because they have a lower discharge potential than the sulphate ions Thus I

at cathode R + • • H

at anode

2H

он 40H

ii. With a 40% ice-cold solution of sulphuric acid and a high current density, the acid sulphate ion HSO produced by incomplete dissociation of the ulphuric acid is discharged in preference to the hydroxyl ion. Thus perdisulphuric acid is formed at the anode s

ESO

4

0 SE - Ə.

2850

25208

(b) Electrolysis of silver nitrate solution

In solution, Ag H, NO, and OE are present, At cathode silver ions are discharged as they are less electro-positive than hydrogen ions resulting in deposition of silver,

+

At the anode hydroxyl is still prefered anion to be discharged, so that oxygen is liberated as previously.

2+

Electrolysis of copper sulphate solution

$2. Cuf In solution,

0 are present, Copper de deposited at cathode and oxygen liberated at anodes

Ca2+ + 28

OR

401 - 28 0

If in (b) a silver anode is used instead, then the removal of electrons from the metallic silver atoms are prefered than the discharge of the hydroxyl ions, Hence the anode process is different, and the silver anode gradually dissolves to give silver ions.

Age ="Ag

Similarly in case of changing to a copper anode in (a), the hydroxyl ions will no longer discharge to liberate oxygen, but instead copper ions will go into solution from the anode.. Furthermore, the copper ions so formed will migrate to the cathode and be deposited in preference to the hydrogen ions.

Cu - 20 - Cy2+

Thus in both cases, there is effectively and transportation of the corresponding metal from the anode to the cathode.

Hydrogen

Platinum

cathode

oxygen.

voltameter for collection of.. gaseous producta in electrocysig

Dil H 80 soln.

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