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10英文中學會考試題預習專欄
堅道英文書院主編
化學科
CHEMISTRY (11)
SOLUTIONS FOR LAST WEEK
(+-)
1. Nitrogen exhibits oxidation states from +1, +2, +3,
+4 to +5 in its compounds with oxygen. These oxides are nitrous oxide, nitric oxide, dinitrogen trioxide, nitrogen dioxide and dinitrogen pentaxide respectively.
(a) Nitrous oxide, N2
Nitrous oxide is prepared by cautions heating
of ammonium nitrate i
++
4
NH NG N207 + 2820 ↑
Nitrous oxide is a colourless gam. It is slightly soluble in water, giving a neutral solution. It is chemically unreactive and is not easily oxidized or reduced. However, it decomposes into the elements on heating or when, passed over heated metals, e.g. Cu, Pb, Fe, it is reduced to nitrogen. The decomposition is exothermic, so that onee the reaction is started it supports combustion of many substances that can burn in the liberated oxygen.
N20
=
N2 ++02
+heat
(b) Nitric oxide, NO
Nitric oxide in the most stable of all the oxides of nitrogen. It is prepared by the action of 50% concentrated nitric acid on copper turnings:
3Cu + 8HNO3
3Cu(NO3)2 + 4320 + 200 ↑
Nitric oxide is a colourless gas, only alightly soluble in water, with which it does not react, It is less easily dissociated into the elemente than is nitrous oxide. Thus it will only support the combustion of vigorously burning substances such as phosphorus, but not of sulphur and charcoal.
Chemically, nitric oxide 18 more reactive than nitrous oxide, the reactions being classified in terms of electrons I
Sharing of electrone: nitric oxide combinee with oxygan spontaneously to give nitrogen dioxide
2N0 + 02 - 2N02.
It combines with chlorine to give nitrosyl chloride
2NO + Cl2
A
2N0C1
Loss of an electron: this gives the nitrosonium ion, example of such a salt in
nitrosyl hydrogen sulphate, (NO)(HS04) ̄,
which is formed in the lead chamber process for sulphuric acid manufacture.
Domation of electrons ? a brown compound ia formed when nitric oxide is absorbed by cold ferrous sulphate solution; in this compound the bydrated ferrous ion, Fe(H20) has one water
•
nolecule replaced by nitric oxide to give
2+
Fo(NO)(H2O)5
Gaiu of an electront nitric oxide reacts with sodium in ammonia to sodium hyponitrite
2+
Na2 (NO)2
•
Furthermore, nitric oxide is readily oxidized or reduced. Thus it is oxidized to nitric acid by acid permanganate and reduced to nitrous oxide by sulphurous acid:
6KMnO
4
✦ 10NO + 9H2504 - 10HNO3 + 3K504
+ 4H20
2NO + E503 - 207 + H2SD4
(0) vinitrogen trioxide. 1203
+ 6MnSO
4
This is the anhydride of nitrous acid, and is very unstable. At low temperatures it
dissociates thus: 2N203- - 2NO + N2°4
(d) Nitrogen dioxide, NO2
This is usually prepared by heating the nitrate of a heavy metal:
́2Fb(NO3)2
2pb0+
ANO 27 + 021
as the mixture of gases is passed througa » U-tube immersed in a freezing mixture, the dioxide condenses and the oxygen passes on.
Nitrogen dioxide is a red brown gạ", condensing to a brown liquid around 22°C, further cooling changes it to a paler colour and finally solidifies as colourless crystals. It has been shown that the latter forms are association of two molecules together, namely dinitrogen tetroxida, N2-
Nitrogen dioxide is a powerful oxidizing agent. Thus phosphorous, charcoal and sulphur will burn in it to yield the oxides and nitrogen and it will react similarly with
heated iron and copper. Sometimes, however, it is only reduced to nitric oxide, as when the reducing agent is hydrogen sulphide, or potassium ideide.
25 + 2N02
2502 + N2
NOZ - NO+ H20 + Sy
H2S
↓
NO2
++
H20 +21
- NO + I
12
+ 20HTM
Nitrogen dioxide may be oxidized by aoid permanganate to the nitrate:
2+
2Mn0
4
+ LONO2+ 2H20 - 2n
+ 4K* + IONO2
On dissolving in water, a mixture of nitrous and nitric acids are formed:
2NO2 + H2O INO 2
+ HNO
(e) Dinitrogen pentoxide, N205
This in the anhydride of nitrio sold and can, be prepared by removing water from pure nitria acid by means of phosphorus pentoxide. It is
a crystalline solid having an ionic structure (NO2) (NO), nitronium nitrate, but is anetable above 0°c. 2N205 - 2N2°4 * °2
2. The possible structural isomers for CHC12 are:
(a)
-C-C-C-E
!
1,2-dichloro-propana
H C1 01
K H N
(b)
1.3-dichloro-propane
C1 H ci
(c)
H— — С — O — 01
--
и
H C1
(d)
H ----
M
61 H
1,1-dichloro-propane
2,2-dionioro-propane
The reaction of an alhyl chloride with aqueoue sodium hydroxide is affectively a substitution of the chlorine atom by a hydroxyl group:
- R.O. NaCl
R.C1 + NaOH (aq.) Thus for isoners (a) and (b) dihydroxyl alcohols are formed:
CH3-CHC1.Ch201 + 2WM
here there 18 hydroxyl group.
инзос
.CHOM.CH OH + 2C1 CH2OH Propan-1,2-diol
one primary and one secondary
CH2C1.CH2.CH2C1 + 20H ̃ ̄
CH2OH.CH.CH2OK + 201TM
Propan-1,3-diol
nere both hydroxyl groups are primary.
For isomers (o) and (d), they are go- dichloro-compounds; however, gem-dihydroxyl groupe are unstable readily losing a molecule of water, whilst the oxygen becomes doubly bonded to carbon.
CH3-CH2-CHC12 + Biết - Cly.CH, CHO + 2C1
propionaldehyde
+
H20
this belongs to the class of compounds kno WII aa aldehydea.
CH3-CC12 CH3
咖 201*
E
CH•CO.CH3
+ 201 + ₤20
acetone
this belongs to the class of compounds known as ketones.
3. For an aliphatic hydrocarbon completely saturated with hydrogen, a skeleton of four carbon atoms should have the formula C4H10. Since the given compound has a molecular formula of C
C4H8 having
two hydrogen atoms less, the compound must either have the presence of a ring structure or a double bond. Thus the possible structures are i
Kingi
H
H-C
H
H
H
cyclo-butane
H
Double bond:
-0-0-0-0
CH3-CH2-CH=CH2 Out-1-8ne
CH-CH-CH-CH but-2-ene
©-3-c
CH. CH2
CH3
<e Tay 1-pro-leng
(isobutene)
Since all the reactions quoted for the compounds decolourization of bromine solution, acid permanganate solution, reaction with conc. Bulphuric acid, hydrogenation by hydrogen over platinum; are all characteristic reactions of unsaturation, the compound cannot be cyclo-butan, Furthermore, sinne on hydrogenation, i... ramoval
isobutane is obtained--therefore the
compound
of the double bond without skeletal changes,
Og mentioned bere must be isobutene,
The reactions are as ZOLLOWE I
CH3-C
CH3
-
CF2
+ Br 2
CH3-CB-CHB
сиз
Br
1,2-dibromo- 2-mathyl- propane
+ H20 + 0 (from permanganataj
CH3-CH-CH2OH 2-metny1-propan-1,
cf3
2-diol
y-...
(can be further oxidized to
aldehyde and then acid)
+ H2SOA -CH-C(HSD)-CH3
CH3
Isobutyl hydrogen sulphate
+
#2 = CH3-CH2-CH3
CH3
isobutane
4. The reaction of A with hot concentrated sulphurio
acid yielding a gas that supports combustion indicates that it must be an oxidizing agent. An oxidizing agant that is not readily soluble in. water or acids predicts that it cannot be those that are salts or easily yeilding salta. Furthermore its black colour, coupled with the characteristic reaction with concentrated hydrochloric acid confirms that it must be manganese dioxide.
Since B on admixture with manganese dioxide and concentrated sulphuric acid on heating yielded a greenish yellow gas which is chlorine, B must be a chloride. B is a white deliques cent solid. The solution of B upon passage of hydroger. sulphide in acid medium yielde no precipitate, but a white precipitate is formed if the medium is alkaline. Thus the sulphide must be acid soluble, and since it is white, aan only be zing sulphide, Hence B is zino ohloride.
The reaction of A, B, and conc. B SO, Follows :
MnO2 + 22n012 + 4H*
18 as
4
+ 2Zn 2+
MnCl2 + 0127+ 2H20
The reaction of a with conc.
-
B2SOA
281
2MnO2 + 2H2S04 2MnSO4 +02+ 2H20
The reaction of A with hot conc. EC1 is like the first reaction 3 is deliquescent, and on heating readily yields a basic chlorider.
ZnCl2 + H2
Zri(OH)Cl + HCL
A solution of B firstly reacts with ammonium hydroxide to give zinc hydroxide, which is readily soluble in excess ammonium hydroxide by complex formation to the tetraammine:
ZnCl2 + 20H - Zn(OH)2 + 201 ̃
Zn(OH)2 + 4NHOK = (Zn. 4NH3)(OH)2 + 4H40
The Zn
2+.
aon in solution can be precipitated by hydrogen aulphide in alkaline mediumt
2+
Zn + B2S ZnS + 2H+
QUESTIONS FOR NEXT WEEK
1. 75 c.c. of a mixture containing 30% methane and
70% hydrogen by volume were mixed at room temperature with 200 c.c. of oxygen and exploded. What is the residual volume on cooling again to room temperature?
2. The solubilities of potassium chloride and nitrate
in g. of the salts per 100 gm. of wate
KC1
10° 20° 400 500 500 70°0
27.6 31.0 14.0 17.0 40.0 42.6 45.5 48.3gm./100gme
H20
KNO3 13.3 20.9 31.6 45.8 63.9 85.5 110 138gm./100gm.
(a) What is the temperature where both KC1 and
NO possess the same solubility?
(b) What is the range of temperature where the
chloride is the more solubla?
(c) What is the solubility of the nitrate at 25°C
and 55°C?
(d) What weight or crystals would be deposited by
cooling a saturated solution of the nitrated containing 30 g. of water, from 70°C, to 10 C? What percentage is this of the weight of walt originally dissolved? (c) If a similar experiment as in (a) 18 carried
out on the chloride what are the corresponding figures? Which is the more suitable salt for this type of crystallisation?
(f) At what temperatures do these salta differ an
solubility by 5 gm.?
(8) Utilizing Le Chatelier's Principle what can
you may about the heats of solution of the two salts?
passed
3. what do you observe when carbon aloxide 18
for a long time through lime water? What happens when the final mixture obtained is treated with soap solution, (b) boiled, treated with Na2CO3 solution?
8
4. Calculate the weight of pyrolwite, containing
60% by weight of manganese dioxide, which would ba required to liborața sufficient chlorine frea hydrochloric acid to make 10 g. of anhydroVE ferrio ahleride from the metal. Ascue that 10% of the available ohlorina is war.