1973-02-01 — Page 27

PEROVCE LIBRARY

- 2 FEB 1973

育教伊 頁三筅張七第F九廿月二十年子壬所夏

WACITY HA

PO

報日橋

1973英文中學會考試題預習專欄

化學科

(十五)

Chemistry (DOJE

Solution for last week

(a) A saturated solution 15 defined as one which contains. the maximum amount of solute which can be dissolved at the temperature concerned, and in the presence of excess solute. (b) The solubility of a salt is defined as the number of grams of solute required to satruate 100 gm of solvent, at a particular temperature. At 3500, 30 gm. of saturated salt solution contain 5 gm of slat. 1.e. (30-5) gm. of water dissolve 5 gm. of salt,

The solubility of the salt at 3500 is

25

X-100

20.

2. A graph of solubilities of KC1 and KNO in water at different temperatures as constructed:

Solubility in gm./100 gm. of water

Temperature °C.

*

(a) Temperature at which botn KC1 and KNO possess the same solubility is 24oC. (b) Range temperature at which the chloride is more soluble than the nitrate is from 0°C. to just below 2400.. (c) Solubility of nitrate at 2500 18 37.5 gm per 100 gm. OT water and at 5500 is 97.5 gm. per 100 gm of water.

(d) Solubility of nitrate at 7000 110 gm./100 gm. water Solubility of nitrate at 1000. = 20.9 gm/100 gm. water Thus if there is 100 gm. water, weight of crystals = 89.

`gm. Actually the saturated solution contain only 30 gm water, therefore weight of crystal formed = 0.3 x 89.1 = 26.73 gm.

% of this to the total salt originally dissolved

$89.10

110

x 100 = 84

(e) Solubility of calori e au 70°C 45.5 gm./100 gm, water Solubility of chloride at 1000. - 31.0 gm./100 gm. water. Thus if there is 100 gm. water weight of crystal = 14.5 gm. Actually the saturated solutiur therefore weight of crystal formed - 0.3 x 14 5 = 4.35 em

of this to the total salt originally dissolve

14:5 45.5

x 100 = 31.9

Since the pertentage yield of crystals is much greater in the case of the nitrate, the nitrate is the more suitable: salt for this type of crystall -ization.

At

(f) At 18°C., the chloride is more soluble than the nitrate by 5 gm/100 gm. of water. 2800 the nitrate is more soluble than the chloride by 5 gm. /100 gm. of water. (g) In the process of forming a solution of the salts, i.e. dispersing the particles of solid, energy changes take place, and heat is usually absorbed. It is for such a reason that the solubility or most solids increase with temperature, as a consequence of Le Chatelier's principle. As the increase in solubility

of potassium chloride witri

temperature is much less thai

that of the nitrate, it may be.. concluded that the heat of solution for the former is much less than that for the latter.

8. Carbon and Its compo

occurrence

Carbon is found in nature Lu both the free and the combined state, (e.g. carobhydrates; hydro- carbons, and limestone etc.

Allotropes.

Allotropy applies to a pheno- menon in which the same chemical element exists in more than one

in/m physical form in the same state. These forms are called allotropes or allotropic modifications. In carbon, there are three o different allotropes.

A. Diamond 19 a crystalline for

of carbon, its value is due to its great hardness the dazzl ing brilliancy of its lustre and its permanency. Apart from its use as a gem stone, diamond is used for making rock-drills glass cutting, its powder form used for polishing other gems, Graphite is a crystalline (flat plates), soft, greasy, grey-black solid, which is smooth and soapy to the touch.. It is a good conductor of heat and electricity,

Graphite is used in the manufacture of lead pencils, as a lubricant for heavy machinery. and for coating iron works to prevent rusting. It is also used for making crucibles for smelting and electrodes.

Amorphous carbon:

Wood charcoal This is made by the destructive distillation of wood, it is a black, porous. substance of very low-apparent specific gravity. It has a great power of absorbing large quantity of gas, therefore, charcoal finds great uses in the making of gas masks for the protection against toxic or poisonous gases, other uses are making gum powder, in water filter, as a crayon, and for

purifying oris,

Animal, charcoal This is the residue left after the destruct. eive distillation of bones, which contains about 10 20% of carbón and about 80% of calcium and magnesium phosphat- es. Its chief use is for deco- lorizing raw sugar solution during sugar refining.

Lampblack Lampblack or scot is obtained by burning substan- ces rich in carbon in an in- sufficient supply of air. It is used in making paint, printer's ink and boot polish.

Coal Coal is a mixture of compounds of carbon, hydrogen______ and oxygen, together with small amounts of nitrogen, sulphur and phosphorus compounds. It was found about 300 million years ago, plants of great. forests died and were covered with sand and mud. Bacteria. caused the vegetation to decay, and then heat and pressure changed the decayed material to coal.

Plant matter Lignite

Peat

Bituminous Coal

Anthracite

Peat is a brown, very light substance, it is formed by the slow decay of mosses, reeds etc. Lignite is a soft coal, it contains about 66% of carbon.

Bituminous Coal-is the -dinary coal, it contains about 88% of carbon.

Anthracite contains about

94% of carbon. It is the hard- est form of coal, and produces most heat on burning

Chemical properties:

(1) Carbon burns. in oxygen air, forming carbon dioxide

C+02

If the supply of air is limited carbon monoxide will be formed. 2002

200

sulphuric

(2) With not conc. acid, producing carbon dioxide, water and sulphur dioxide. C2H2S

SOCO + 2H2O + 250,

四期星

(3) With hot cone. nitric acid,

carbondioxide, water and nitro-

gen peroxide formed.

+ 4HNO →→ CO2+ 2H20

400

(4) With sulphur vapour, rea hot carbon can for a volatile liquid called carbon disulphide 0+ 28

(5) As an reducing agent: Pbo + C

Cuo

C.

Pb+ CO Cu + Co (6) Combilics with certain elements, when beated together, foming carbides. e.g. Cà0 + 30 2000od.

$102 30

DaC, +

60

1800°C PU + 200 Oxides of carbon

Three oxides of carbon, all of which are gaseous at ordin ary temperatures, are definite- Ly known: carbon dioxide, car- bon monoxide and carbon sub- Oxide

Carbon monoxide.

Carbon monoxide seldom vours free in nature and then only in small quantities. Minute traces are found in air. volcanic gases and tobacco smokes, and also some quantiti es are found in chenney gases and in the exhaust gases of internal combustion engines and about 10% in coal gas.

It is a colourless, odour less, very poisonous gas, in- soluble in water, alkaline solution but soluble in a solution of cuprous chloride. in ammonia to form CuCl. COHO

therefore,

this solution can."

be used as a good absorbent for the gas.

Chemical properties:

(1) A non-supporter of combust ion, but burns with a blue flame foming carbon dioxide: 200+02

→200,

(2) A good reducing agent owing to its tendency to turn into carbon dioxide, carbon monoxide is a powerful reducing agent, eg.

Guð + 60

ilarly, Fe + 300

CC.

ou + co2

2Fe + 300,

(3) Poisonous nature

The poisonous nature of carbon monoxide is due to the absorption of the gas by the haemoglobin of the blood tö for the bright red and stable carboxyhaemoglobin to the exclusion of oxyhaemoylobin. Carbon monoxide thus slowly displaces oxygen from the blood and the victim eventually dies from oxygen lack.

The gas methane, CH, occurs

in some mines and can cause. explosions. Usually, an explos- ion itself does not kill many miners. However, it forms car- bon monoxide which poisons many miners and people who try save them!

2CHA

+30 → 200 + 4H,O.

Uses or carbon monoxide

Carbon monoxide is an im- portant constituent of coal gas water gas and producer gas. It is also used in the refining of nickel,

Exerci Part I

1. Carbon compounds are mostly covalent. This 19 because

A. carbon is a non-metal

B. carbon is a reducing agent. C. All organic compounds contain

carbon

D. carbon has 1uur electrons

its outer shell

carbon exhibits allotropy.

Allotropes or an element have 1ifferent atomic volumes because A. Their crystal structure are

different

B. Their atomic structure are

different

They have different heata! of combustion

D. They have different melLINK

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經濟援助浸會書院問題

| 香港能仁學院

演獲贈獎學額

教育司謂在考慮中

(EE) RES

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該校「原子吸收分光儀一舉行啓 招來械强龙是仁公报

E. They have different numbers

of neutrons in their nuclei

3. Which one or the ipfiowing

substances can absorb coloured substances from solution?

Acoke

B. coal

C. wood charcoal

D. animal charcoal E. graphite

學院

Diamond and graphite are different forms of the element, Which of the following are eviden ce of the truth of this statement

1. Both conduct electricity 2. Both are similar in appearan

ce

3. Both or the same melting

point

Both have the same pasit crystalline structure

mass of either will

Siven ma

yield the same mass of car bon dioxide on complete combustion.

Which substance is the absorb. ent of carbon monoxide?

A. Caustic soda

B. lime water

C. cuprous chloride in ammonia

Cone. Sulphuric acid. water

Diamond and graphite are both

hard and brittle.

Q. soft and lustrous R. amorphous and pure S. crystalline and pure Tamorphous and impure

7. Diamond and graphite do not differ in

P. refractive index

Q. colour, when they are pure R. conductivity

S. crystal shape

T. physical state

8. Which of the following amorph-

ous forms of carbon as least pure?

P. animal charcoal

Q. lampblack

R.

Wood charcoal.

soot

coal

9. Carbon monoxide is a very poisonous gas. It is particularly dangerous because

P

it paralyses the nervous system

2. it destroys the liming of

the lungs :

H. it produces a poisonous gas

on combustion

S. it is tasteless, colourless. and odourless as well as being poisonous

T. it coagulates the b1000. 10. Carbon black is an important constituent of motor tyres. It ds used

P. to improve the colour

Q. to increase the flexibility

of the rubber.

R. to improve the air-noldina.

power

S. to improve the wearing

qualities

to make the tyres quieter on the road.