頁二第張士站日十初月一十年售巴愿夏
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四期日八十月二十年九六九一圈公年八十五國民 售中 育文僑華
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10英文中學會考試題預習專欄
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化學科
CHEMISTRY (7)
SOLUTIONS FOR LAST WEEK
(七)
1.(a) Allotropy. When an element appears in more than one form, the phenomenon is known as allotropy. The various forms being known as allotropes. Thus molecular oxygen 0, and ozone 0, are allotropes of oxygen. More often, it is the solid state which may exist in more than one crystalline form. Solid phosphorus, for example may exist in more than one form. The two most common are red phosphorus, atable under ordinary conditiona; and white phosphorus, which tenda to change into the red form, Allotropes have different physical properties. Thus red
Pas por ser than the white and possesses less energy. 4P(white) 4F(red
anergy
When red and white phosphorus enter into similar chemical reactions, the red form is less vigrous. Other examples are the different allotropic forms of carbon: diamond and graphite.
reversible reaction. This is a chemical process where a forward change from reactants to products, and a backward change from products to reactants can occur. ï‚0. A + B→ 0 + Das Fell as + D→ A + B
Common examples are 24g + Do
JFe + 4H, 0 - Cach, Cao + DO
28g0
Any physical or chemical process, if left in a given condition, will approach a final state of rest or equilibrium. Thus a mixture of H gas and vapour in a container of fixed volume at a definite temperature reacts to ro the gad HI. As soon as some HI is formed, the reverse reaction--the decomposition of HI to
-- also begins to take place. As
Hande have a mixture of the gases HI
ESENT,
and HI. In effect, two reactions are taking place at the same time, and is customarily represented by a single equation with arrows pointing in both directio
2HI
If the reaction proceeds long enough, it will establish an equilibrium, and the concentration of I, and I will each reach a definite value for the particular oxe
experimental condition,
(o) oatalysis. I
In certain chemical reactions, the rate at which products are formed can be altered appreciably by the addition of a small amount of another substance. The substance which remains chemically unchanged at the end of the reaction is called a catalyst, and the phenomenon is known as catalysis.
Many industrial processes are dependent
on catalysts that accelerate the establishment of equilibrium. Thus the Haber process for the synthesis of ammonia from nitrogen and hydrogen, and the manufacture of hydrogen from steam and carbon monoxixe required finely divided iron as catalyst: The oxidation of ammonia to nitric acid requires platinum zauze.
The manufacture of sulphuric acid heeds vanadium pentoxide or platinized asbestos (ih Contact Process), or nitrogen dioxixe (in Lead Chamber Process). Hydrogenation to products such as margarine from whale oil, also involves the use of the use of catalysts.
(a) double decomposition. This is a most common type of reaction, involving a double replacement between the reacting substances, They often boour with gas evolution, precipitation, or release of energy.
FoS + 1,504 - Fešo¡ + XLS /
[M83No2 + 3H20
Cuso
3120 + 2NH ↑
+ Na, Sio, - Cusio, Wa003 + 2HCL
2a01 + H2O + CO2 ↑
(e) deliquescence. The process whereby a substance takes up water vapour until it actually enters into aqueous solution is known as deliquescence This is exhibited by some anhydrous salts, they firstly absorb sufficient water vapour to form a hydrate, and then more to form a solution of the hydrate. Thus anhydrous CaCl, is firstly hydrated to CaC1,61,0 and then to droplets or solution. The explanation is that water vapour pressure of the hydrate is lower than that of
the water vapour in ordinary air, and to
increase the vapour pressure of the former,
Water is absorbed.
precipitation. When a product formed in a reaction is a solid and does not dissolve under the conditions chosen, it will be separated from the other substances in solution. This is known as precipitation.
Precipitation reactions are orten utilized in qualitative analysis of cations and anions. Thus silver, mercurous mercury and lead are precipitated as chlorides, tin, copper,. cadmium as Bulphides in acid medium, aluminium chromium and iron as hydroxides, and nickel, cobalt, zinc and manganese as sulphides in alkaline medium.
3CH - A1(OH) 3 ▼
Zas
The halide ions can be precipitated as silve
to as barium salt
- AgBry
Ba
BASO
The term "acide" may be defined in many diffǝre! ways. The name is given to a class of compounds that have the following properties in aqueous solution:
(a) a characteristic sour taste. (b) the ability to change the colour of litmus from blue to red, (c) the ability to react with certain metala to liberate H, gas (d) the ability to react with tr hydroxides and oxides (bases) of metals to form in each case a salt and water.
If the discussion of acid behaviour is limited to aqueous solutions, then an old may be conveniently be defined as a substance that yields hydrogen ions H when in aqueous solution. The general methods for preparing acids fall into three categories: the reaction of water with an acid anhydride (usually an acidic oxide from a non-metal); the direct combination of hydrogen with a non-metal; the displacement of a more volatile acid from its salt by a less volatile acid (usually a stronger acid).
(a) Combination of a non-metallic, oxide with water
+
CO
B20 = H,C03
3H 0-
2H PO
Direct combination of a non-metallic alement with hydrogent
A2 + Cl2 = 2HC3
Double displacement of salt of a vo acid by another acids...
NaCl + 204
NaCl + NaHSO
4
NaNO3 + H2SO4
NaHSO+HGL
+ HCL
- NaHSO, + MO, ↑
asually sulphuric acid is employed because has a very low volatility.
The basicity of an acid may be defined as the number of replaceable hydrogen atoms in a molecule of the acid. Determination of the basicity of an acid by preparing as great a number of salt as possible with a base. Since the minimum number or hydrogen atoms per molecule acid replaceable by a univalent metal is one, the total number of different salts obtained is equal to the basicity of the acid.
If sulphuric acid is titrated against sodium hydroxide and the resultant, neutral solution concentrated and cooled, crystals of sodium sulphate are obtained. If after neutrali sation, a volume of sulphuric acid equal to that. already used is now added, then a different salt is obtained, this being sodium hydrogen sulphate, As it has not be possible to obtain any other salt by different combination of acid and alkali, it is concluded that sulphuric acid must be dibasio,
Hydrogen may be prepared by action of acide on metals, action of alkalis on metals, action of water on metals, action of steam on coka methane, and electrolysis of water.den (i) Action of acids on metals. Hydrogen as
usually prepared in the laboratory by action o of moderately dilute sulphuric acid or hydrochloric acid on granulated zinc in Kipp's apparatus. Za + 2HC1-
EnCl2 + H2
The hydrogen obtained by this method is rather impure due to traces of moisture, arsine ABH, hydrocarbons, and acidio impurities such as hydroman chlorida / laudpochlors
acid, or sulphur dioxide and hydrogen sulphide (from sulphuric acid)
1)Action of alkalis on metals. Certain metals
that form anphoteric oxides, such as aluminium, zinc, or tin are dissolved by sodium or potassium hydroxide with evolution of hydrogen. However, the action is slow and requires heat. The hydrogen is relatively pure, moisture being thej only major impurity.
241 + 2NaOH + 2H_0 = 2NaA10 + 3H1⁄2 7 (iii)By the action of water on metal. The reaction of an amalgam of sodium and mercury with cold water give a gentle effervescence of hydrogens
+
2820
→ 2NaOH + 4μg + H2↑
In Lane's Process, hydrogen is manufactured by passing steam over finely divided iron heated to 650-800 cu
3Fe+ 420 - F¤304 + 4H ↑
is reduced back to izon by passing water
FO3 CO + H) over it, so that the iron can be
gas
used over agains
·F6304 +442 = 356 +420
·Fe3+ 400 3re + 400
By the action of steam on coke. Enormous quantitives of hydrogen may be manufactured by passing steam over white-hot coke forming water gas, this can be converted almost completely into carbon dioxide by adding more steam and heating at a higher temperature,
CO+
By electrolysis. Very pure hydrogen and oxygen are prepared in the laboratory by electrolysis of water with the addition of an electrolytes
2120 21202
Cathode Anode
Caplicity of balloon - — x 3.14 x (500)3 c.a.
ཟ་
528,000 litres
Volume of hydrogen at N.T.F. required to fill the balloon at 100 and 750 mm.
528.000 x 273
750 (trak –
760
4.985 x 102 litras
Since 1 gm, molycule of hydrogen occupies
22.4 litres at V.T.F.
Number of gn. molecules or hydrógén required
4.935 x 105
22.4
Áð 21 - Y6 - Yo2+ + Hy
Thus weight of iron required - 56 x 2.225 x 104
gm. =1,246,000g
The substance A is barium chloride crystals The flame test gives an apple green coloration characteristic of prsence of barium ions.
On admixture with manganese dioxide and concentratec gulphuric acid, the barium chloride is oxidised to chlorine:
Jyo2 + 201 ̄ + 45′ » Mn++ + 2H20 + 012 1
When moist blue litmus is exposed to chlor
chlorine, the hydrochloric acid formed, changes it to a red colourt however the further oxidizing action of chlorine bleachad the dye to a white coloration.
When barium chloride is added to silver solution, it first dissolves to provide ions which in turn is precipitated chloride?
(1 + AR ABOL
nitrate chloride
the silver chloride is insoluble in acids bút dissolves in excess ammonium hydroxide to give apnino silver chloride;
2ARC1 + 3NE
зNE OF
(→ 2AgC1, SNH, + 38,6
bolution of barium chloride will give a white precipitate with dilute sulphuric acid, due to the insoluble barium sulphate:
QUESTIONS FOR NEXT WEEK
Baso
A. If you were given four samples of chemicals, being
either lead nitrate, lead carbonate, sodium. nitrate or sodium carbonate; what experiments would you perform to find out which was which?
2. Suggest four substances that are usually employed for drying gases, Explain why each substance is used for the purpose, and discuss which ones are suitable for drying of (i) ammonia, (ii) hydrogen chloride, (iii) carbon dioxide, and (iv) hydrogen sulphide.
3. Describe the preparation of (a) copper sulphate crystala starting from malachite, (b) ferrous sulphate crystals starting from iron wire, (c) nitre from potassium nitrate, (d) calciu sulphate starting from marble, (s) sal ammoniac from ammonium chloride.
4. How would you prepare a specimen of pure carbon monoxide? In what way and under what conditions will carbon monoxide react with (i) sodium hydroxide, (ii) nickel, (iii) chlorine, (iv) hydrogen? On what evidence is the molecule formula Co assigned to the gas?