1970-02-12 — Page 23

CRENCE LIBRARY

42FEB 1970

每三第一八第日七初月正年戌庚需复 WAH KIU YAT PO 報日僑華

65 1955 1963 FEE

四类星:

日二十月二年〇七九一曆公年九十五國民营

However, if water is present, the reaction that takes place would be a

reduction oxidation process. wielding thi chloride and sulphate:

SO

+ 2НСГ

Number of moles of water in 0.9009 gm.

0.9009 + (2 * 16) 0.05003

Therefore number of moles of water per mole of

0.05003

8 (to closest whole number)

herefore formula of hydrate in K2SO 8H2O

179英文中學會考試題預習專欄

堅道英文書院主編

化學科

(十五)

CHEMISTRY (15)

SOLUTIONS FOR LAST WEE":

Any substance that can produce energy either alone (as for example, through radioactive decay or spontaneous decomposition) or by reaction with

another substance (e.g. by comburgh oxygen).

is a potential fuel.

Ta ordinary combustion, reaction of the fuel (e.g., hydrogen) with the oxidant (e.g., oxyge

releases energy in the form of heat. However, in the fuel cell, the fuel is ionized at an electrode, the electrons generated are transfered to the oxidant, thus the energy of reaction appears as current from the cell. However, the carbon- containing substances remained to be the. conventional type of fuels. They may be either solid fa.g., coal, coke, charcoal, wood), liqui (e.g., oil, petroleum, alcohol) or gaseous (e.g., satural gas, coal gas, producer gas, wat

Coal is formed by the gradual decay of the lulose and other constituents of vegetation and Gocure naturally Petroleum products are the most waful or liquid fuels. Alcohol is of some une' in tropical countries, where coal and petroleum are relatively scarce but carbonydrate crops "groW rapidly the alcohol being obtainable through farmentation.

For many industrial and domestic purposes gaseous fuels are more convenient, their combustions are much easier to control by varying the quantities of the gases burned and of the air nized with it.

Producer gan may be prepared by passing a tream of air over red-hot coks. Carbon dioxide. First formed, however, as there is excess coke, it is reduced to carbon monoxide, The issuing gan is thus a mixture of nitrogen and carbon monoxide, so that one third of the mixture is inflammables:

115 (02 + 4N2) + 20 - 200 ƒ + 4N2 ↑

As the overall process is exothermic, it is unnecessary to supply heat to the coke during the process once the reaction has started. Producer: gas 18 uced in heating retorta in which coal is onverted to coal gae, for heating lime-kilns, steel furnaces as well as glass furnaces......

Water gas is produced when steam is passed over red-hot coke: it is a mixture of carbon monoxide and hydrogen in aqual volumes.

C ‹ 820 - coƒ + 81⁄2 f

Water gan is a more useful fuel than producer gas, because both constituents can be buzat, thus contributing to a higher calorific value. Howevez the reaction involved in its preparation. 18 endothermic so that temperature falls rapidly unless heat is supplied. Usually producer gas and water gas are generated alternately using the same apparatus, the duration of the two processer is controlled so that it is not necessary to heat the ooke.

Coal gas 18 often manufactured commercially by high tomperature destructive distillation. The coal is beated to a temperatura of about 1000.0. by producer gas in large vertical retorts in the absence of air. The volatile produota are passed to a water main and ther sir- and water-cooled condenasza to resova the dár. Fszé, ammonia is removed from the gas by washing very thoroughly in water sorubbers. Hydrogen sulphide is removed by passing the gas over ferric oxide whenas iron

sulphide is formed. The purified seal gas may the be collected over water in large gas-holders and rent through pipes to consumers as required.

(a) Sulphur dioxide can be prepared by the action.

of hot concentrated sulphuric acid on copper whence sulphuric acid behaves both as an oxidising agent as well as an acids

.Cu

+

21,50 CuSO

44

28,090 f

the gas so obtained is usually pure and dry, because of the presence of the concentrated aoid.

Sulphur dioxide is also produced when mineral sulphides such as zino blende is roasted in a stream of air. However, under auch conditions oxygen and nitrogen would also be present.

122:13 26 2ZnO + 250, f

Reactions of sulphur dioxide are as followB3 ((1) Chlorina

* Sulphur dioxide reacts with chlorine an

the presence of charcoal to give sulphury] chloride, 50,C1, a colourless liquid. Thai 19 indicative of sulphur dioxide taking part in addition reactions.

Potassium dichromate

Wada Bulphur dioxide is in contact wita acidified potassium dichromate solution, the solution rapidly turned from orange- Jallow to gren. The green colour is caus by trivalent chromium

H2O - H203

592

8H* • 350,

This is also redox reaction, the dichromate being the oxidant and the

alphur dioxide the reducatnt.

(iii) Nitric-ava

48.0

Nitric acid being an exidising agent will also reuct rapidly with sulphur dioxide: in this case the product may be nitrio oxide or nitrogen dioxide depending on the hydrogen ion consentration

(iv) Sodium hydroxide

H50

+2NO

Sulphur dioxide reacts with aqueous alkali to form sulphites and bisulphites:

2NCH + JO2 = Na250,

Na 50

2N FSO

the laquad must not be heated because thy bisulphite wall decomposet

4ESO,

250

¿H20 + 502 / * $1,

When steam 19 passed over red hot carbon, it is reduced to carbon monoxids, and hydrogent

When steam 28 passed over red hot iron triferzio tetroxide and hydrogen are formed:

4820 + 378 *

Anhydroum

te abвozoR, WEMUA, KA MUNA air and forms the monohydrate, Na200,, E2Of this balt is also formed

the decahydrate is expose

to dry air. Aqueous solutions of the carbonate are alkaline owing to hydrolysis;

BO

- NaOB NaHCO

or B

ов

HCO.

When gypsum (the dihydrate of calcium sulphate) 19 heated, it first loser water to form the homihydrate, 2050, 0, this is known as plaster

47. of Paris. Plaster of Para ma

with water is rapidly raconverted to gypsum, the mass setting to a solic Slight expansion occurs on setting, and plaster of Pari is therefore used for taking cante,

Phosphorus pentachloride fumes strongly in moist air and is decomposed by water giving the oxychloride, POCI, and then phosphorio aoid,

PC1 H2O POOL

POCL

2HC1

H3PO4 SECL

Calcium carbide is rapidly decomposed by ter, forming ·

Caked lime and acetylene gast

+

|_ Ca(OH)2

Aluminium sulphide is completely hydrolysed in water, yielding the hydroxide and hydrogen sulphides

A1253 + 6850 • 241(08)3 + 3H23 7

Let the formula for the monovalent metal sulphate be X So

Since this salt contains 55.96% of water o crystallization, the actual amount of K,50

„e, ankydrous salt) in 1.61 g. of the by crystals

44.04 100

1.61

0.7091 80.

of water of crystallisation in this sample

0.9009 8

Now this amount of material is equivalent to 1.16 6. Ba30

thus the aumber of moles . 1.16 + (137.5 + 32 +64)

0.004968

4. (a) During the combustion of carbon containing

compounds, the first step is often the formation of carbon monoxide, this is because not enough oxygen can approach the site of the reaction. However, once carbon monoxide is formed, and sufficient oxygen is available in its surroundings, it will buza with a characteristic blue flame." Hence in a clear fire, the flame at top has almayı a blue colour, indicative of complate oxidation.

200+

+ 02 - 2002

(b) When hydrochloric acid is added to size, displacement reaction occurs and hydrogen gas 11 liberated, this nas cent) hydrogen has a strong reducing property. The aqueous solutions of compounds of iron usually have iron present a the ferrio ions or the ferrous ions. The former imparte a yellow coloration, and the latter a pale green coloration. Thus when hydrochloric acau reacte with zinc, the nascent hydrogen reduces ferrio chloride to ferrous chloride, effecting in the observed change of coloration.

Zn + 2E01 – ZaC1, + 25

2.

2PCL, 2E 2F C1, + 2HC1.

(c) Stalactites are downward growths of calcium carbonate found in limestone caves. During raining, the rain water dissolves some darbon dioxide and is thus slightly acidic. When the rain-water flows over limestone, it reacts to form a solution of calcium bicarbonate. This diffuses downwards, and when-it appears at the top of a cave, it drips. from the roof. During the process, the water evaporates slightly, and loses some darbon dioxide. Calcium carbonate is thus precipitated, and over hundreds of years can grow! to resemble an over-hang icicle."

CaCO2 + H2O *

Ca(HCO),

(d) Mortar used in building consists of slaked lime, stirred to a paste with water and three or four times sand thoroughly mixed in. The mortar first dries, and later hardens. The hardening process is interpreted as combination with carbon dioxide from air to form Calcium Carbonate, thip process may take centuriasi

Ca(OH), + CO2 - CaCO. - 8,0.

(a) When chlorine is dissolved is water, hypochlorous and hydrochloric acids are formed *he reversible reactions

■ -HOOL "+ HEL

Hypochlorous acid imparts a golden yellow colour ¿ to the solution, whilst hydrochlorio noid by itself is colourless. On exposure to bright sun- light chlorine water evolved oxygen, owing to the decomposition of hypochlorous acid by light, and to the notion of activated chlorine on waters

4801+

2H001 - 2001 + 02 ↑ 2012 + 2E20

Thus this provides en explanation of the dis-

af melour an exposure to light.

ap

(f) Coal gas is a mixture of variable composition containing about 50% hydrogen, 30% methane, and 10% carbon monoxide, moreover ganes such as t ethylene, benzene, are present in small quantities Hydrogen and carbon monoxide are both odourless, whereas the organic vapours contribute to the smell of the coal gas. After burning, all three are converted to carbon dioxide and water vapour both of which are odourless.

Why do papole think that (a) hydrogen (b) nitrogen and (o) oxygen are diatonio? What is the evidence. for the monoatomicity of neon? Demonstrate the use of atomicity in determinations of atomic weigh s of gaseous

2. Describe the manufacture of sodium hydroxid.

en electrolytic method. How, and under what conditions, does sodium hydroxide react wit (a) aluminium, (b) phosphorus, (o) ohlorin (d) eulphur?

3. Write an account of energy changes during ohydion

reactions. How can a study of these lead to deeper understanding of chemical reactione?

A tribasio organic acid was allowed to undergo series of reactions the silver salt was prepare und then ignited so that the acid radical part was completely oxidised, leaving only the silv metal behind..

The

following experimental data were obtainedis Weight of orucible and ad

16.520 gm.

Height of crucible and Lid and silver salt the acid......-17.854 gm.

Weight of orucible and lad and silver Calculate the molecular weight for the tribasio organic acid,