育義華 頁三第張六第 B-TAIFA WAH KIUAY YAT PO
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B70英文中學會考試題預習專欄
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CHEMISTRY (17)
SOLUTIONS FOR LAST WEEK
(十七)
1. When oxalie sold crystals are heated with
concentrated sulphuric acid, they are dehydrated to yield a mixture of carbon monoxide and carbon dioride as indicated by the equation:
H200 280 - 380 - CO ƒ + CO2 ↑
Thus 1 g molecule of oralio soid providem 1 gm. molecule of carbon monoxide and 1 gm. molecule of carbon dioxide,
According to Avogadro's Hypothesis, equal number of different molecules of gas cooupy the same volume under the same physical conditions. Henoa 1. molecule of oxalic mold provides z molecules of gas.."
Since molecular weight of oxalic acid crystals (2 x 1 + 2 x 12 + 4 ≈ 16) + 2(2 x1 + 16) 136
Therefore number of gm. molecules of oxalio sold uned in the experiment.
Therefore number of gm. molecules of gas renerated
Since Gram Molecular Volume for conditions is 22,414 litres. Therefore volume of gan at NTP
22.414:11tres
at HTP
According to the gas equation -
Therefore volume of gas at
and 570 mm. pressure
constant
760 x
22.414
273
0.9205 litre
920.5
570
litres
When the gas nizture is allowed to stand over potash, the carbon dioxide is absorbed, leaving carbon monoxide. As indicated previously, the mixture is equinolecular, and hence contains equal volumes of the two gases.
Therefore volume of remaining gas at NTP
122.414 litre
0.3295 Litre 329.5.0.0.
Elements may be classified into three groups gotale, somi-metals, and non-metals socording to their physical properties (*.g, electrical, structural)..
A metal may be defined as an element which readily lesss electrons, and a non-metal as ar element which readily accepts electrons. Thus metala are electropositive elements whilst non- motala are electronegative elements.⠀⠀
The differences in physical properties may bt Visualised from a consideration ofs
Atomicity of vapours — Non-metals are capable of electron-sharing, thus the elements often form diatomic molecules, such as H, Cl, F, in which all the valencies of both atoms are used up in binding themselves together by single, double, or triple bonds. Such molecules are hande "saturated" and have little attracti upon one another. Hence many non-metals are ganes, volatile liquids or volatile solidaj they are examplified by the halogen familys fluorins and chlorine are gases, bromine is y liquid and iodine a volatile solid. On the other hand, metals have very high melting and boiling pointe, because in the orystalline solide there existe strong metallic bonding between atoms. Furthermore, metallio vapour generally consist of single atomo, as metals not readily share alsotrons.
Thermal and electrical conductivities-In the solid state, metals appear to be closely-packed cations held together by mobile electrons, Thus the metallic state is characterised by (1) metallie conductivity, since the mobile. electrons can carry current without decomposition of the metal; (ii) thermal conductivity, since the mabile electrons can also carry heat energy; (iii) metallic lustre, which depends on opacity and reflecting power.
Structural properties--Xetals consists of layers of atoms which can easily slip over one another. They are therefore malleable and : duotile and often of great tensile strength, Such properties are not found in non-metals, Also most metals hava high density but non- metals are comparatively light. This is because of the magnitude of the positive charge by which electrons are bound to the nucleus---obviously
郭日橋塞
greater in the case of metals.
The differences in chemical properties can bo generalized by the fact that metale form itable sations by electron loss and non-metais form stable anions through electron gainı yi
Oxides ——Metals form basis oxides, which, if soluble in water, produce alkalis.
102
+80 201
Non-metals never form baslo orideas, Their obarasteristic oxides are covalent and is an acidio anhydride, which in combination with water, produces an acidi
- $03 + 1,0 - H2SO4
In addition, thay may forn other covalent oxides that are nentral (e.g., 00, 820)
P. Reastion with acids--Netals replace hydrogen in acide, directly or indirectly, forsing salts, Netals more electropositive than hydrogen replace it directly from acides
++
$21
less electropositive than hydrogen replass it indirectly, nog... via the oxide.. or hydroxide of the metali
*** ; 201 ̄ + 2H 30
Cu
Non-metals never replace hydrogen in acids to form #alts, bocause they are electron-acceptors and to yield hydrogen would mean that they have to`yield up electrons for the process.
b. Chlorides---Netals form oblerides that are typical electrovalent compounds, namely solids, non-volatile, electrolytes when solten or in solution and not hydrolysed by water i
Ga + G12 =
Na + ich2 Nat
201
Some chlorides from predominantly metallio elemate do, however, hydrolyse in water, e.g. ZaC1, and are hence atypical. Hon-metals fora chlofides by covalent combination, the chlorides are typiselly liquids, volatile, non-electrolytes and completely hydrolysed b by watarı
РС13 + 3520 - КРОЗ 4 ЗАСТ
WOL3 + 3820 • NEz ↑ + 3H0C1
d. Redox reactions Metals are typically reducing agents. They supply electrons ozidising agents:
(roducing agent)
C1 (oxidising agent)
Non-metals are oxidising agents. They accept electrons from associated reducing agente during redox actions, 9.8.
2e (reducing agust)
(oxidizing agent)
•,, Bydrides --Metals do not often produce stable hydridea. This is because they do not favour sovalent combination by electron-zharing, However, the most electronegative metals. (Xa, K, Ca) san cause hydrogen to accept electrons and produce solid hydrides, electrolytes when
moltex and attacked by water to geid, hydrogen and the metallic hydroxide s
44.0
20
2
2M + 21 0 ➡ 201
51
"
Fon-ustale form sany and stable hydrides by covalent combination. The simple hydrides are always ganeous (1.6, vory volatile), non- electrolytes when anhydrous, and they are not attasked by unter to liberate hydrogen. A few examples are CH,
HOL, FH
NH:
The above differences may be indios.
iron as a typical metal and sulphur, as typisal non-metalı
PHYSICAL PROPERTIES
1.Characteristic
#ppearanGo' 2.Gonductor of heat 3.Coaduoton or
eleotriod ty 4.Mechanical. Kroperties
„Nolting unins
Density
GRUENEGAL PROPERSIJES 7.0x43
8. With diluckan
by taking
TRON
SULPHUR
metallic lustra no metallic
Dinstre
good good
bad.
insulator
malleabl
ductile
tennile
strength high 1500 c high
brittle.
sannot stand
stress or strain low 119
·low.
bazio Feo,
23
seidio 50, 50,
react to yiela no reaction
alert
sovalens
3018
10.Eydride
11.Ton formatien
no
eleo
stable EgS
opositive electronegative.
anion 3
Fo
(a) Load mitrate
When lead nitrate is heated strongly, firnt molts to a liquid and then desomposes, producing a brown gas. The reaction involves the formation of nitrogen dioxide and oxygens:
2Pb(XO3)2 2PbQ+4M0.
+
四湖星日六月二年〇七九一曆公年九十五國民華中
(b). Crystalline ferrous sulphate”
This in ferrous sulphate with seven molecules. of water of crystallization Fo30.78,0 and is pale
green in salour. When heated, it is completely
dehydrated by about 300 G yiolding stean, the ronidum, in the absence of air, is white. The anhydrous solid decomposes on furtier heating to yield sulphur dioxide, sulphur triexide, and ferzia oxide which is a reddish-brown powder.
Ta50 TE ̧0 • F650 + 78,0
*203
(0) Agmomium nitrate
Ammomžen nåtrain is a crystalline solid, hoatné it melts and offervadoes, with tha -generation of nitrous oxide.
+ 2820
Thus fáanlly, there will be no residuo remaining in the tube.
(d) Warning soda
This is the decskydzate In C.108 0. When
hsated, firstly the ndaohydrate EagCo,.,0.
is formed and on farther heating akh
EC is obtained. It is a very stable compound
3
and molts at high temperatures without dacsaposition,
(e) Crystalline cospor sulphate
Copper sulphate is usually encánnterad as the santakyázzio, da blue ozzotale. Cuão....5,0. Nhân
bested be about 110°C, the pentahydrate is scavaried to the monohydrate C500, which is only slightly bluish. At a higher temperature (about 250°C) the salt becomen white and anhydrous At about 700°C, the sulphate is aanverted to oxide. by loss of sulphur trioxide
Hethyl aleshol is manufactured on a very large- scale bypassing a mixture of carbon monoxide and hydrogen under-a prossure of 200-300 atmospheres, over a catalyst heated to about 350 Cs
20 + 212 = CH2OH + 34 kosi,
The catalyst consists of sino oxids and another -oxide such as chromio ozide, as the reaction
proosaðs from left to right with a diminution in- võlums, it follows from Le Chatelier's principle. / that the yield of methanol in increased by raising. the pressure; and because the reaction is exothermi raising the temperature will drive the squilibri, tamm to the leth and decrease the yield of methanol, However, relatively high temperatures of about 350°C are employed, because the rate of the react- ion is atharules too slow.
The mixture of hydrogen and sarbon monoxide is obtained mainly by the action of steam on zed hot: cokes
- CO + E
Additional hydrogen is obtained by treating some of the water gas with more atesa at 500°C in the presence of an iron oxide 'catalysts
- CO2 + #2
the carbon dioxide being removed by waver unasi
Hobbyl alcohol may be identified by itk zordinous to ba ozidlesd to formís noid of E characteristic odour. Thus shen methyl alcohol 1 heated with a solution of a mixture of sodium diabronata and dilute sulphuric asid under a feflux gondenser, fornio acid is formed and the orange selour dichromate is changed to the green colour, ohromic salt
■ H.COOH + H_0
0 + 2(0) = 0
Ethyl alsshol can be distinguished from methyl alcohol by Bodoform test. This is carried out by treating the alcohol in a tent tube with a little lodine and sodium carbonate solution. Ethyl alcohol forms yellcu-coloured crystals of iodoform which have a characteristic anally if it is pure methy】 alcohol, iedoform cannot be obtained. The actual sourse of the reactions san [DE] interprete am followns
наод ог
Cooln
QUESTIONS FOR TEXT WEEK
1. 48.5 g of ash was obtained at € 788)
combustion of a large quantity of plant material, The ash was dissolved in water and the solution made up to 250 mL, in a volumetzie flask. 25 ml. of the prepared solution was neutralised by 24.5. ml, of hydrochloric acid containing 5 gm. of aoid. per litrs,
Calculate the percentage or potassium carbonate in the anh, sasuming that no other substance in present which would neutralisa tie aold.
How would you prepare a specimen of pure quicklime What experiments would you make to show that (1) it is a chemical individual (ii) that it the oxide of a metal (iii) that 11 sontaine 28.53% of oxygen?
Why is potassium nitrate preferred to sodium nitrate for use in gunpowder, and sodium nitrato generally med rather than potassium nitrate in
· fertilisers?
By what sotkods would you distinguish between para - specimens of those two salte?
4. Describe the notion of hånɓ on (a) ammonium-
aktoride, (b) anmonium nitrite, (o) potassium nitrate, (4) copper nitrate. By what tests would. you distinguish the solid products left in (*) and (4) from the original substances?