育教僑華·頁三第張六第日一廿月二十年己愛 WAH KIU YAT PC 報日橋
170英文中學會考試題預習專欄
堅道英文書院主編
化學科(十三)
SOLUTIONS FOR LAST WEEK
CHEMISTRY (131
1. Weight of 50 0.0, vinegar used in preparing the
Bolution - 50 x 1.02 gm. - 51 gm.
Since 50 c.o. of the solution was neutralized by 16.9 0.0mg of 0.5N NaOH,
#
*#
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Normality of the solution –
16.9 x 0.5 50
-0.169
Assuming that the acidity, was only due to acetic acid of equivalent weight
- 12 x 2 + 16 x 2 + 1 x 4 - 60.
Therefore weight of acetic acid in 250 c.c. of the solution
250
- 0.169 x 60 x 100 m. 2.535 gm.
As this is contained in' 51 gm. of vinegar, percentage by weight of acetic acid in vinegar
2.535. x 100 - 4.97%
.53
In order to obtain a correot reading for this titration, phenolphthalein should be used as the indicator. In a volumetric titration, the aim is to get the 'end point' where an aquivalent amount of acid is mixed with alkali. However, this Tend point' is not necessarily "neutral' in the sense that water is neutral, it is dependent upon the type of acid and base used in the titration. Thus strong acid with strong base has the end point' identical with the neutral point' (e.g. HCl with NaOH); strong acid with weak base has an acidio
'ead point' (e.g. HCI with NH 0); weak base with strong acid has an alkaline and point' (e.g. CH,COOK with NaOH); weak base with weak acid is roughly neutral. The action of an acid-base indicator is to give a colour response at the
'end point' of a titration, however, since change of colour is dependent on the acidity (or alkalinity, of the solution and different for different indicators, no indicator can be suitable for al cases. Phenolphthalein is particularly suitable for titration of weak acids with strong bases, because its colour response. is within the range of alkalinity of end points of such titrations.
2. Synthetic Preparation:
i from alcohols
The most generally useful method of synthesis of alkenes utilizes an alcohol ae a starting material. The hydroxyl group presents a point of vulnerability Conversion into an alkone requires simultaneous elimination of the hydroxyl group with a hydrogen from an adjacent carbon atom/
H
#
H
OH
Η
C -
By loss of -OH from one carbon and of - from another, ethyl alochol yields ethylene. Practical methods of dehydration is either catalytically or chemically ı
For catalytic dehydration, vapourised alcohol is passed through a tube packed with coarse ́granules of alumina and maintained at a temperature of 350-400° by electrical heating
alumina 350-400°
OH, GE, ON
CH -
CH2
Alcohols can be dehydrated at lower temperatures by chemical dehydrating agenta, one of the commonest being sulphuric acid. Ethylene can be produced from a mixture of ethyl alcohol and sulphuric acid heated at a temperature of 170 The reaction can be visualized as the formation of ethylsulphuric acid and then its decomposition to ethylene.
CH2 CH2OH + HOSO OH
H 冠
E
CHCH2O50OH+
B2O
1706
→CB2 - CA2 ✦ HOSO2O
ii. from alkyl halidon
The introduction of a double bond can be accomplished by treating, an alkyl halide with a basic reagent capable of splitting out the elements of hydrogen halide from adjacent carbon atoms?
文
+ Baso
--- + Salt + H20
In acid-binding reagent commonly employed is an alcoholic solution of potassium hydroxide theoretically ethylene may be prepared from sthyl bromide !
HCHBr + KOH CH2 - CH2 + KBF + X20
However, the side reaction to give diethyl ether, ©255-D-Calça la so predominant, that this method for ethylene is impraótions.
iii. from vicinal-dihalides
A compound in which, wo kislogan SVUM - BES situated on, adjacent positions in the carbon chata, and are hence in the same vicinity, im described as a vicinal-dihslide. The halogen atoms in such a compound can be eliminated simultaneously by treatment with a bivalent ustal, particularly sino.
Br
The reaction is not of much use in motual synthesis of alkenes because the required. vio-dihalides are not sufficiently availele, but it is frequently employed in purification of ethylenic compounds.
CK,Br.CH B
Zin
СЕ
The structure for ethylene suggested by the Kokule theory is that, in which two valences of #155 aasian pton #zo utilized to Izen a OGELS abima double bond, or ethylenio Linkage
LAN molécular formula has
been shown by
*
slamental analysis and molecular weight determination to bẹ CH,.. It therefore containe
*4* two atoms of hydrogen less than that of ethans, and the only way in which the tetracovalency of the carbon atoms can be satisfied is by a double bond between, then ast
H H
#
H
It might be supposed that the two carbon atoms were joined by a single bond” and that
>ach had a single unpaired valency electron, that the molecule was unsymmetrical with va free electrons on one carbon atoms
H
H
#
19
ΟΣ
t
н 11
# : C: C
•
H
Both these formulae are unacceptable because free radicals are extremely reactive and can only have a transient existence. In terms of the electronic theory, the double bond consinta of two pairs of shared eleotrons. Sinoé z single bond is sufficient to provide a firm link between the carbon atoms, the presence of an extra bond❤ represente latent combining capacity which determines the characteristic addition reactions. Thus the structure provides considerable information concerning remotivity, ability to enter into additions, addition of atoms or groups in pairs, and attachment of groups to adjacent carbon atoms.
3.(a) Lead monoxide is different from cuprio oxidə
in that it is amphoteric readily dissolving in moide and alkalis. Thus it may be separated from the latter by wing excess of caustio alkali, whence it dissolves as the plumbite, leaving ourpic oxide behind. The solution is
P
filterad, excess of dilute nitric acid added tc convert into lead nitrate solution. The nitrate is easily crystallized without water. of crystallization, which upon heating decomposes to pure dry lead monoxide.
Pbo + 2NaOH = 2Na* +
602
+20
2Pb(NO3)2 = 2PbQ + 4NO, ƒ + 0, ↑
(b) Magnesium ions in solution can be precipitate. aa gelatinous mass of magnesium hydroxide on adding sodium hydroxide to it. Thus this in the method employed in separating the magnesium salta from sodium salts in solution. The gelatinous magnesium hydroxide is filtered ang washed thoroughly with water, then, dilute sulphuric acid is used to prepare a solution of magnesium sulphate. Upon crystallization, the heptahydrate separates out. The anhydrou salt is obtained by igniting the hydrate.
++.
Kg
+ 2NьOH -
Mg(OH)2 + 2Na*
- X*90,
+ 78204
'o) Fitrous
oxide may be separated from nitrio oxide by taking advantage of the compound
formation of NO with Fe50, solution to nitroso-j
ferrous sulphate. Thus thể mixture is firstly
purified by passing through a solution of
ferrous sulphate and then concentrated éulphuric sold to remove moisture, the purified gas is them collected over mercury,
FeSO4*
FSOA
+ NO -
NO
4.(a) The elementa constituting the compound ammonium
chloride are chlorine, nitrogen and hydrogen. 20 a sample of ammonium chloride, concentrated sulphuric acid and manganese dioxide are added. | a greenish yellow gas is evolved which may be collected, this gas is chlorine.
4NX C1 + Nn©2 + 4E* - MnCl2 + 0121 + 2H20 +4NB ̧TM
In order to prepare nitrogen and hydrogen from the ammonium chloride, ammonia gas must be prepared first. This is done by action of sodium hydroxide upon the malt: 4
O Naci
NH ̧C1 + NaOH = XX, f + 20 + N
The ammonia is collected and allow to burn in oxygen. Under this condition nitrogen and water vapour are formed i
·4853 + 22 ↑ + 6820 ↑
302-
The gases thus evolved are passed through a cooling mixture, the nitrogen gas' does not condense and may be collected. To the liquid water condensate, pieces of sodium metal are added, and hydrogen gas is generated steadil3
(b) The elemente constituting concentrated
sulphuric acid are bydrogen, sulphur and oxygen A portion of concentrated sulphuric acid is diluted, and iron filings added, bydrogen gå. is evolved and can be collected.
8,80 • F. - Fo50,
→
Bat
To a portion of the concentrated sulphurio acid, very reactive metals such as zino, 18 added, in this case the aoid behaves as an oxidizing agent and 18 reduced hydrogen sulphide :
47m+
H2S04
4ZnO →
when the hydrogen sulphids evolved is bubbled through dilute nitric acid, a yellow prapipitate of sulphur is obtainadı
יד
2HNO3 + HS 20,0 +
2N02
When the vapour of sulphuric acid is passed through a red-hot silica tube, it undergoes · decomposition to water vapour, sulphur dioxide and oxygen. The eulphur dioxide is removed by passing through water, and the water vapour is removed by passing through a wash bottle of concentrated sulphuric acid. The oxygen gas can thus be obtained in a fairly pare atate. (note: actually some of the oxygen reacts with sulphur dioxide in water
back to sulphuric acid, but the action is slow)
2H2SO4 = 2H2O + 250, + 0,
QUESTIONS FOR NEXT WEEK
1. How would you distinguish between aqueous solutic of hydrochloric, hydrobromic and hydriodio aoideí Suggest two tests for each case.
It was experimentally determined that 2.0517 gm. of potassium bromide precipitated from solution 3.2375 gm. of silver bromide. What is the atomic weight of potassium, if it be assumed that the element is monovalent?
2. Suggest experiments by which you would perform to
distinguish between:
a. manganese dioxide and carbon;'
b. hydrogen and nitrogen;
c. sulphuric acid and hydrochloric acid,
d. red phosphorus and mercuric oxides
e. potassium nitrate and potassium sulphate
3. State Le Chatelier's principle. Show how this
principle can be used to predict the effect of changas of temperature and pressure on the following systems :
་
POL PC13 G£2
D. CO. #20002+ B2
N2
*
02 2NC
+
→ 2903
1 2502
8.
GaGog—– GaO + 002
2
4. Describe four reactions of industrial importance in which the presence of a catalyst is required. What axplanations have been offered of the action of catalyste?