REFERENCE
24DEC 1969
育教僑華
真三筅货八第E六十月一十年巴展夏WAH KIUCHY HAL報曲橋
僑罩
罗僑榮
$#%$ #SX
*#$%$#$%$#$%$#$%$#3%82
#8%$#$%3#$%$#$%$#&%3##5
英文中學會考試題預習專欄
竪道英文書院主編
化學科
(八)
CHEMISTRY (8)
SOLUTIONS FOR LAST WEEK
1. Amongst the four substances, lead carbonate if the only one insoluble in water. Thus to the solid which is insoluble, dilute nitric acid is added. Effervescence will take place, and the evolved gas turna limewater milky. indicating it as carbon lioxide:
Росоз
+2HNO.
3 + 203 = P(NO 372 + CQ2 f + 8,0
To the remaining solution, hydrogen sulphide ga is bubbled, the formation of a shining black precipitate confirms the presence of laed
HAS 2H
Pbs
aqueous solutions of the three remaining. samples, dilute hydrochloric acid is added. Only the one which is sodium carbonate will give effervescence of carbon dioxide that turns Linewater milky:
*a2CO3 + 2HC1 = 2Na01
To confirm the presence of sodium, a flame tes performed with the resulting solution; this is characterised by an intense yellow coloration For the other two solutions that gave no efferves cence, one must be sodium nitrate, and one lead nitrate. However, lead is precipitated in the presence of chloride ions as lead chloride. On warming the suspension, lead chloride will dissolve, and is deposited as long needle-like crystals when cooled.
2E01 - FbClą
+2HNO
The
presence of nitrate can be confirmed by heating a small amount of the original solid gently. This will decompose, giving off copious brown fumes of nitrogen dioxides
2Pb(NO3)2 - 2PbQ + 4N02 T
The fourth sample must be sodium nitrate. Heating of the solid at low temperatures will cause no reaction. If the temperature is now raised so that the solid fuses, a pale yellow liquid will result. At the same time, a colourless gas that rekindles glowing splint, will be liberated?
2NaNO3
2NaNO2
The choice of substances as drying agente for gaseg depends on two factors. Firstly, the ability to remove water vapour; the substance either reacts with water vigrously but without liberation of any gas, or it is highly deliquescent and simply absorbe water. Secondly, the substance must be inert to the gas for which it is used to dry, however since it is almost impossible to have one substanc that do not react with all gases, there is no
universal drying agent' and different substances are particularly suitable for different cases, Tr commonest substances used include concentrated sulphuric acid, fused calcium chloride, calcius. oxide and phosphorus pentozide.
The affinity of concentrated sulphuric acid for water makes it useful as a drying agent for the. gases it does not attack. However, the drying is not complete, and it is only convenient only if intensive drying is not required. An it has bott acidio as well as oxidising properties, it is not suitable for basio gases nor reducing gases. Thus it cannot be used to dry either ammonia or hydrogen sulphide. Ammonia reacting to form ammonium sulphate
2NH + H,30
whilst hydrogen
(NH4)2SO4
Aphide is oxidised to sulphur
H2SO4 + 3H2S - 48,0 + 45-j
Carbon dioxide and bydrogen chloride do not react. with concentrated sulphuric acid and are thus conveniently dried.
Fused calcium chloride is exceedingly deliques cen This added to the easy handling and cheapness contributes to its commnom usage wherever a mild drying process is required. It is not possible to
dry a gas completely with calcium chloride, It cannot be used to dry ammonia, as the two substances react to form compounds such as CaCl,
ANH
CaCl2 4NH3
lalcium oxide reacts vigorously with wande, generating much heat, and calcium hydroxide being formed as a white powder i
CaO + HQ
Ca(OH)
Such a property makes quicklime useful as a drying" agent in oases where the more convenient calcium chloride or sulphuric acid cannot be used. Ammonia. is therefore dried with calcium oxide. As calciur oxide itself is basic, it cannot be used for drying acidic gases such as hydrogen chloride, carbon dioxide or hydrogen sulphide. For although there is no reaction in the cold, when heated the corresponding calcium salts will be formed.
Phosphorous pentoxide reacts vigorously with water. giving out much heati
F205
2HPO
PHP
The property of combining with water and forming non-volatile metaphosphoric acid renders it most useful for removing even traces of moisture from a gas already partially dried. However, as the meta-phosphoric acid forms as a gelatinous coatir”, it should not be used for removing large quantities of moisture, for the unused pentoxide may be soreened by the formation of metaphosphoric acid, Furthermore, phosphorous pentoxide is an acidio oxide, and it cannot be used in cases, where a basic gas, such as ammonia has to be dried,
Copper sulphate crystals from malachite Malachite is a naturally occurring basic coppar carbonate, CuCo. Cu(OH) ̧.
Copper sulphate may be obtained crystalline s the pentahydrate CuSO 5H, 0, commonly known as
blue vitriol. It may Be prepared by addition
of dilute sulphuric acid in slight excess, usi litmus paper as indicator. Copper carbonace will yield carbon dioxide, while coppar: hydroxide will be neutralized: CuCO.Cu(OH) + 2H
2CuSO + CD2 1 + 2H2O
Upon concentration of the resulting solution by partial evaporation of water, and cooling, copper sulphate pentahydrate will separate-
(b) Ferrous sulphate crystals fron iron wire
Ferrous sulphate is found as the stable heptahydrate, Fe50,71,0, commonly known as green vitriol. Although iron exists in compounds in both the oxidation states II and III, it is the ferrous state which persists when iron reacts with dilute sulphuric acid. Thus the iron wire is dipped into a sufficient quantity of dilute sulphuric acid until it completely dissolves:
FeSO;
This is followed by filtration, then evaporation, nerving both to concentrate the solution as well as to drive off any dissolved oxygen which might later partially oxidize the ferrous to ferrio. The concentrate is then allowed to crystallize.
(c) Nitre from sodium nitrate
Potassium nitrate is commonly known as nitre or saltpetre. It is usually used as a source for nitrate where a dried condition is necessary, as in making gun powder. Potassium nitrate may be prepared from sodium nitrate by mixing with an equimolecular proportion of potassium chloride, saturated solutions of both being used. Nearly all the sodium separates as chloride while the mixture is höt and concentrated, since sodium chloride is far lees soluble in the hot solution than any of the other salts. On cooling, the olear solution
dopeezvem nearly pure potassium nitrate, becauAIN AN potassium nitrate is less soluble in the gold than any of the other salts. The nitre may be further purified by recrystallisation.
KaCl√ + KX®,
KOL + NaNO3
(d) Calcium sulphate starting from marble
Karble is one of the massive forms where caloins occurs naturally as the carbonáte. Although the carbonate can directly react with dilute sulphuric acid, it is better to prepare the salphate via a soluble calcium salt. Pieces of marble are put into dilute bydrochloric acid and wait till reactions are completed: Caco + 2BCLL
· CaCl2 + CO2+
+ 120
CaSO CaCl, H2SO
2XCL
The solution is then filtered, dilute sulphuric acid added, and the mixture digested on a stead. bath for several hours. Very pure crystalline calcium sulphate will be formed, as it is only aparingly soluble in water,
Sal ammoniac from ammonium sulphate: Sal ammoniac, NH CI, can be p
by
distilling a mixture of sodium ohloride and ammonium sulphate. Because ammonium chloride 18 volatile, subliming when heated, it is easily separated as a pure sublimate from the other non-volatile saltsg
(NH) 50 + 2N■01 2NE C1 +
290
Carbon monox126 18 usually prepared in the laboratory by elimination of water from aither formic acid or oxalic acid by the action of concentrated sulphuric acid, Using formic abid is of advantage, because only carbon monoxixe but not carbon dioxide is generated;
FOODF
HO
三期日四十月二十年 九六九一菱公有了十五涸弓鱼种
CAUSTIC
„AJDIOPRETRR:
As very pure carbon monoxide is required, the gas generated is first washed with a solution of caustio potash to remove traces of carbon dioxide. and sulphur dioxixe (from chance reduction of ooncentrated eulphuric acid by carbon monoxide) than delad with shoạcherous pentoride before being
(1) with sodium hydrosida
Although from its preparetsen, carbon monozide appears to be the anhydride of formic acid, it does not dissolve in water to give the acid but it will react gith sodium hydroxide rolusion above 150 0 (under pressure) to give sodium forma təs CÓ NGOÀI HLOON)
(11) with nickel
Carbon sonoxide readily reacts with certaiı) metals to form compounde known as carbonyls The best known is nickal tetracarbonyl, volatile covalent liquid. This compound decomposes readily at high temperatures:
N1 (CO),
(iii) with chlorine
Carbon monoxide will give an addition reaction with chlorine, either in the presence of sunlight or charcoal in the dark as a catalyst, A poisonous gas carbonyl chloride or phosgene- is produced
CO
(iv) with hydrogen
COC1
Methanol is manufactured on a very large scale by passing a mixture of carbon monoxide and hydrogan under à pressure of 200-300 atrospheres over a catalyst heated to anouv 350 c
00 + 28
CH OF
The datalyst used is usually a mixture or sino and ohromic oxide. On burning in oxygen, carbor monozide yields only carbon dioxide, thus it must be a lower oxide of carbon. If carbon monoxide and oxygen are exploded in a sudióneter, two volumes carbon monoxide combines with one volume of oxygen to form twe volumes of carbon dioxide, Thus in accordance to Avogadro's Hypothesis, two moleculas of carbon monoxide combine with uns molecule of oxygen to two molecules of carbon dionide. However, it is known that oxygen is diatomic, whilst molecular formula for carbon droxide is CO2, hence: 2 mol. carbon monozide
2002
200
The formula deduced is in agreement with tu observed vapour density of the gas (14 timen as heavy sa hydrogen H,)
QUESTIONS POR NEXT WEEK
=
A 81.079 or 100 c.c. Dapacity was filled with vapour and sealed at 250 C and 740 mm. press UTS. As a result the globe increased in weight hy .805 gm. compared with its weight when full air at 17 C. and 740 un, pressure. Calculate (a) the normal density, and (b) the relative. density that the vapour would have if it were gas at N.T.F.
1 litre of hydrogen weigns 0.090 mm." and 1 litre air weighs 1.29 gu. at. N.T.P.
4 liquid compound of the formula C.H.O, when heated with an excess of concentrated sulphuric acid, gives a gam of the formula C, which is dissolved on shaking with cold sulphuric acid. On diluting and boiling the solution so formed, a liquid is recovered, but is isomeric with the original liquid. What explanation can you give of the course of the reactions?
The Haber process for the preparation of ammonia makes use of the reaction represented by the equation N2+ 3 2NH 22,000 calories being given out in the formation of 2 gram-molecules OL ammonia.
From this information deduce, giving reasons, the experimental conditions necessary for the. production of ammonia....
What additional conditions are necessary, in practice, to secure the best yield of ammonia? Describe how ammonia is converted into nitrio acid on a large scale.
Explain what is understood by oxidation and reduction. Give some examples of these processen with reference to any cases in which the reactions. are reversible.
Page 30Page 31