1969-12-10 — Page 17

育教儘華頁四第張四第日二初月一十年酉己曆夏 WAH KIU YAT PO

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UFO英文中學會考試題預習專欄

堅道英文書院主編

化學科

CHEMISTRY (6)

SOLUTIONS FOR LAST WEEK

1. (a) Concentrated sulphuric sold has oxidising

properties, which increase in power as the concentration of sulphur trioxide increases. Thus hot concentrated acid will oxidee sulphur to sulphur dioxide," whilst the acid itself is also reduced to sulphur dioxide:

+ S

2H2SO4 3502 + 211,0

Sulphuric acid in the concentrated state has a great affinity for water. Organic compounds that contain the elements of water are easily dehydrated. For sugar, action of concentrated acid will cause the formation of a spongy mass of carbon:

$12-22 11. • 12C + 11H_0..(removed by conc.

報日橋.

The presence of copper ions can be confirmed by taking a fresh portion of the solution and add ammonia solution to it. A pale blue precipitate will be formed which diseolves in excess ammonia solution to a deep blue coloration. This is a procesa of complex ion formations

Cu + 2NH OF

Gu(OH), + 4NH O

Cu(OH)2+2NH4

[cu.4NHţ}(OH)2 + 4#20

tetrammine cuprio hydroxs

The third solution should then be the one containing an iron salt. This can be shown by adding ammonia solution. A gelatinous green or brown precipitate will be formed, and is insolubla in' excess ammonia solution.

2NH OH

Fa(OH)

2NH

+ 3NH OH = Fe(OH) 3 ▼ + 3NH

In order to test for whether the solution containe ferrous or ferric ions, potassium: ferricyänide: solution is added to a fresh portion of the solution. A ferrous salt will give a deep:/ blue precipitate called Turnbull's blue, whilst a ferric salt will give a brown coloration, without "any precipitation.

(a) The outstanding characteristic of: ethylene is its great reactivity. In particular, it participates in addition reactions. Thus a mixture! of ethylene and hydrogen passed over platinum black, produces ethane:

C2H6

三期星

日十月二十年九六九一圈公年八十五國民華中

IANE (SODAKIDE》

ACETYLENE

ETHYLENE DIBROMIDE

4. Normality is the number of equivalents of solute per liter of solution. Molarity is the number of moles of solute per liter of solution. Formality is the number of formula weights of solute per liter of solution. There is a certain illogio in the molar system. Thus we consider dissolving a nols of NaCl (58.44 gm.) in a liter of solution, which becomes 1.000-M. But mole relates to the word molecule, whilat there are only ions (Ma and Cl-) in solution. Henas the formality Bystem. has become of wider use recently, the 58:44 gm. of NaCl being palled one formula weight rather than) one mole.

Calcium carbonate reacts with excess hydrochlorio acid to calcium chlorider-

CaCO3

+ 2HC1 = CaCl2 + H2O + CO2 ↑

The excess acid is then neutralised by sodium hydroxider

H01 + NaOH - NaCl + H2O

Concentrated eulphuric acid like the. ilute acid also possesses acidic properties. 'he acid, being dibasic will react with basee in two stages, forming firstly an acid salt, then with excess base to a neutral saltı

ZnO + H2SO4 + ZuSO4 + #20

(b) Hydrogen chloride can he prepared by

displacement from chlorides by heating with sulphuric acid. The apparatus is an shown's

This is an example of catalytic nydrogenation.

(b) Addition reactions are also possible with halogen acids. Ethylene will react slowly with hydrogen chloride, with formation of ethyl chloride?

CH + HC1. CH_CI

Equivalent weight of sodium hydroxide

Normality of sodium hydroxide solution

30+40= 0.75

Normality of resulting solution from reaction of

hydrochloric acid with calcium carbonate

25.2 ± 0.75 0.756

Thus number of equivalenta remaining in 250 ml. of Bolution;

- 250 0.756 x 100 = 0.189 1000

Thus umber of equivalents that react with the carbonate

250 1000

-0.189 0.0617

Thistle funnel

Delivery tuba

Flask-

funnel

-beaker

Mixture of:

---- water

01 &

Heat

Normally the reaction will terminate at the first stages

NH

+ HC19

HCL + H2SO2 = NHHSO4 +

Hydrogen chloride being very soluble in water. a solution of it has to be made using an inverted funnel.

Nitrous oxide can be prepared safely by mixing and heating ammonium chloride and sodiu nitrate in the correct proportions in a flask,

NaNO + NH C1- NaCl + N201 + 2H2O Nitrous oxide which is a colourless gas, is collected over luke-warm water because it is soluble in cold water.

To a portion of each of the three solutions, dilute hydrochloric acid is added. The one which. gives a white precipitate is the solution of lead salt. This is because the chlorides of copper and iron are soluble.

FB + 2HC1-

Phc324 + 2H+

This is further. confirmed by boiling the precipitate in water. The precipitate would diesolve, Lead chloride being insoluble. in cold but soluble in hot water.

To the other two acidified solutions, hydrogen sulphide gas is passed. The one that gives a btack precipitate is the solution of copper salt. Copper sulphide. being precipitated in the presence:

of: acid. However, the sulphide of iron is not

form in presence of acid.

The type of reaction here may be termed, "hydrohalogenation.

(c) The formation of athynol from ethylene may be considered as addition of water, fie, bydration However, the reaction has to be catalysed by

of acid. If sulphuric acid is used, it presence

lay be looked upon as initial formation of ethyl ydrogen sulphate followed by hydrolysist

CQR4 + H280q = CH2OHHSO4

H

H2O = CHţCH2OH

(4) Formaldehyde, is obtainable from ethylene by a cleavage reaction. The ethylene first reacts with ozone to give an ozonide, which on hydrolysis by water in presense of zinc dust as a reducing agent will give formaldehyde. As this involves splitting of the carbon-carbon double bond, it may be called ozonolysis:

· FORMALDEHYDE

OZONIDE

(e) To convert ethylene into acetylene, reaction involving two steps has to be used. Firstly, the ethylene is allowed to react with bromine to ethylene dibromide (addition of bromine); this is followed by dehydrobromination,

eliminated per molecule of ethylene dibromides

whence two molecules of hydrogen bromide ara

this should be the same as the number of equivalents:

of calcium carbonate in 6.85 go. of the sample.

Equivalent weight of CaCO3. (40 + 12 + 48) + 2

-50

Therefore weight of 0.061 equivalent of calcium carbonate 0.061 x 50 gm. - 3.056 gm.

Thus percentage of calcium carbonate in original mixture

3.056x100

6.85

44.6

QUESTIONS FOR NEXT WEEK

1. Explain the following terms: (a) allotropy.

(b) reversible reaction, (c) catalysis, d) double: decomposition, (e) deliquescence, (f) precipitation.) Illustrate with examples.

2. What are acids? Give the general methods fow

preparation of acids. How would you determine the basicity of sulphuric acid?

What are the processes where hyurogeri is prepared on a large scale? What are the impurities likely to be present in such prodesses? Calculate the weight of iron that would be required to generate by reaction with sulphuric acid, enough hydrogen' to blow up a spherical rubber balloon of 10 metre diameter at 10°C and 750 mm, pressure?

4. A substance A (containing only a single compound) exists as white crystals. Flame test gives an apple green coloration. A on mixing with S manganese dioxide and concentrated sulphuric acid generates a yellowish green gas, which turns mois blue Létmus to red and then white. A solution of/ A when added to silver nitrate solution forms white precipitate, this is insoluble in dilute nitric acid but dissolves in ammonium hydroxide.]

·The solution of A also gives a white precipitate if dilute sulphuric acid is added. What is

Give details of all reactions involved.

【倫敦商會商科春季考試

定明年四五月舉行

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