1966-12-07 — Page 14

化文俊華育 健康 「四第

7日六廿月十年午丙屬室

英文中學會考試題預習 專欄

化學科

(六)

·王錦釗

„CHEMISTRY (6),

Q.),(8) An acid salt is formed when only part of the

replacable hydrogen atoms in an acid is replaced? by metal atoms or, the ammonium radical. Example: Sodium picarbonate, NaHCOy (5)1A dehydrating agent is a substance that can ex-) tract the elements of_water_from_compounds_con= taining them.

可

Example:e~ Concentrated sulphuric acid,H2SO

An efflorescent substance is a substance which loses all or part of its water of crystallisad tion on exposure to the air

Example:-- washing soda crystals, Na2C0.10 H20)

A oeliquescent substance is a substance which f

absorbs so much water from the atmosphere that

it dissolves in the water to form a concentrated solution.

Example:- Caustic soda,” Nauh'

When & salt is formed by neutralising`a strong) acid such as hydrochloric acid with a strong (base such as sodium hydroxide, a salt,, sodium ahloride is formed which is neutral to litmus

HGIY

+

dilute Aydrochloric]

acid

+

NaCl +

sodium

H20

(sodium

hydroxide) chlorioo!

solution

solut ion

They PH value of the resulting solution is 7,1,0 the concentration of the hydrogen inna, H' and hydroxyl ions, OH","in the solution is exact

exactly the same as that of pure water, namely,107 gramwlons per litreY

(b). When a salt is formed by neutrallsing a weak'

acid, such as carbonic acid, with a strong base such as sodium hydroxide, a salt, aqdium car bonate is formed which is alkaline to Iitous.

CO2 庐 tarbonic (acid

420*

2NaOH

Wa2CO3.+_2H20.

(cauatio) Bodium

soda solution

The explanation is a follows:{

carbonato, soüution

Sodium carbonate itself is completely ionised in, water, but water is only very feebly lonised.

Na2CO3 2Na co 37

H2O

Carbonzovacid) is a weak acid, so that there

tendency for the Co ions and Hions_to form molecules of soarbonic acid/

+ 003E2x2Hg = H2CO3

[Caustic soda jie strong-base and does not foma molecules so that the Naions from the sodium carbonate and the OH, ions from the water rems in as_separateįions in the solution.

more H ions a

More water, molecules will now ionise inj order to help restore the equilibrium between the Hions and. OH ions. But as soon as they are immediately res moved to form more molecules of undissociated carbonic acid.. Soon, there is a huge excess of OH ions over Hions in the solution, and con sequently, the solution is alkaline in reactions

LONG

When a salt is formed by neutralising a strong

voldvutuch.as.hydrochlaria.acid.awit)

好?

WAH KIU YAT"

T PO

case such as ferric nyároxide, s sait,

三期

日七月二十年六六九一层公年五十五國民路中

errid

chloride, is formed which is, acid, to litmus.

Fe(OH)3. FeC1: férric Uferric hydrochlorid hydroxide (chloride)

[dilute

acid

3420

solution

The explanation is as follows:"

Ferric chloride is completÉly" ionised in water, but water itself is only very feebly tonised,

FeC15 Hz O

Fe

H

•

5017

* OR*%

Ferric hydroxide is a weak base, so that there' is a tendency for the Fettf ions and OH Lons to combine to form molecules of ferric hydroxide.

Fetre + 3 OH = Fe(OH)

Hydrochloric acid is a strong, acid so that the

ions from water and the Cl- ions from the Terr rir chloride do not combine to form molecules, but remain as separate ions in the solution:

More water molecules will at once ionise Co try to restore the equilibrium between the H ang the OH” Long. Hut as soon as more On Long appear, they are immediately removed to form more molecules of undissociated ferric hydroxide. As a result, Lhe h' ions are far in excess of the

ions in the solution, and consequently, the solution is acid in reaction.

OH

(1) Characteristic properties of an acids.

(a) all acids taste sour.

(b) all acids in solution will turn blue liumus red. (c) all acids will neutralise bases to form a salt

and water only.

(d) all acids (apart from oxidising acids such as

nitric acid and hot, concentrated sulphuric acid) will react with metals more electropositive than/ hydrogen, liberating hydrogen.

'(e) all acids will react with carbonates in the co¡M

with the evolution of carbon dioxide: (11) Characteristic properties of a base;"

Bases are substances which are paleo neu

tralise acids to fom galt and water only. Bases are oxides and hydroxides of metals. Soluble, bases are

led alkalis. Alkaline solutions in water have a soapy feel: when rubbed between the thumb and the forefinger, and will cum red 111

Preparation of lead monoxide from lead

Add "concentrated nitric acid little by litere! to some lead foll contained in an evaporating dish, until all the lead has dissolved. The lead is cons, Lverted into lead nitrate.

Pb lead

mus blue.

nitrie acid/

Pb(NO3)2 lead nitrate

2N027

-2H20)

brown

fumes of

nitrogen

¡(111) Prepration Laulphate

To a solution

fin a beaker, add ammons

precipitate appears, stir

Filter and wash the residue w of distilled water.

112(504)3

sulphate

ONAL OHY

anmonium

hydroxide

white gela-

tinous ppt)

peroxide

from aluminium

hate containedy

til no more

re thoroughiy. mall quantities

241(OH),*

aluminium

SINH 4 ) 250,

ammonium

hydroxide sulphate

育僑華

'Remove Le toriums va an evaporating dish and】

heat until a constant weight is obtained. The white

infusible powder remaining in the dish is alumándum oxide.

2Al(OH) a

A1203 * 3420,

aluminiu

oxide

COLLS-

'The solotion obtained is heated until a

The residue is lead mono- tant weight is obtained. xide, which is a fusible yellow-powder

2PL(NO)2

* 2Pb0

lead monoxide

4NO2

(v) Preparation of ferric oxide from lon-

Put some lon nails in a beaker and add dilute sulphuric acid little by little until all the iron has dissolved

Fe +

iron

H2SOL dilute sulphuric (acid

FeSOL *

H2

ferrous

sulphate

solution

solution:

The resulting solution is evaporated until crystals start of appear.

The solution is then cooled, The / crystals are then removed, washed with a small quantity of distilled water and then dried between blotting -paper.

FeSOL + HO FeSOv 7H20

ferrous sulphate crystala (green)

The crystals are then heated in e hard-glass test ai tube until a constant weight is obtained. The re- sidue left in the tube la ferric oxide, which is a ‚red, infusible powder.

\ 2Fe50 • 720 ** Fe203* 50 * +90 3′ • L4_H2O;

ferric oxide

(red solid)

'QUESTIONS FOR NEXT WERK

1. State the assumptions made in Dalton's "Atomic" Theory)

1. State the Law of constant composition.

Show

tátel

how the truth of this statement may be explained by means of Dalton's. Atomic Theory, taking black cappens oxide as an example-

• equations showing three different ways of pre- eparing black copper oxide.

In a series of experiments, 2 gus,' of black copper oä“ ide were found to contain 1:6 gms, of copper, and 5 ga of oxygen ware found to combine with 20 gme, of copper State the law governing these results and show how they illustrate it,

5. State the Law of Multiple Proportions, and describe | how the truth of this law could be illustrated in the laboratory in a particular case. Nitrous oxide was passed over heated copper until 2.24 litres of nitno- (measured at N.T.P.) were collected, and the weight of the copper was found to bave increased in\

·veight by 1.6 gms. The experiment was then repeated by using nitrogen dioxide in place of nitrous oxide,} When 1.12 11tres of nitrogen (m asured as N.T P.) were collected, there was a corresponding increase of 3.2 gma in the weight of copper, are these figures in agreement with

R

the Law of Multiple Proportions?

華僑

化

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至正午起在大會堂正座展出 但該標主人强到任

時布桂續和獎

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*榮承用:竹石

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金多心 - 傳懋主:川水。

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穎粱 - 山水

王小案-花卉

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| 中褘學學節目!定九晚八時

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舉行音樂晚會

熊味辣神 另坊,高服才

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人賦詩贈別

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葉跡八十

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