REFERENCE LIBRARY
育教僑華頁三第張六第
26 NOV 1969 日七十月十年酉己曆夏 WAHLIU YAT PO
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B70英文中學會考試題預習專欄|
堅道英文書院主編
化學科
(四)
CHEMISTRY (4)
SOLUTIONS FOR LAST WEEK.
Sodium sulphate and sodium sulphite may be distinguished by their behaviour on heating with dilute acida, Sulphates are stable whereas sulphites decompose with evolution of sulphur dioxide. The sulphur dioxide is identified by its pungent smell as well as reducing action en moist dichromate paper, ohanging the colour fron orange to green.
Na 304 + H2SO4
no onange
Na 30 + HS04 - Na2301 + SO2/ + H2O
Ammonium chloride is distinguished from potassION. chloride by the action on heating with an alkali or base. In ammonium compounds, ammonia gas is evolved and can be dected by ita suall and its turning damp red litmus to blus,
NH
+ KOH = KCL + NH3
KCL + KOH no change
Anhydrous sodium carbonate, unlike the carbonates of "heavy metals" does not evolve carbon dioxixe on heatings whereas sodium bicarbonate, like all other bicarbonates, is decomposed by heat. The carbon dioxide evolved turns lime water milky.
Na2CD ̧ • no 2NaHCO3-
change
3. Na co
+ HU
d. Red mercuric oxide is stable at ordinary
temperatures but on heating it becomes almost black and at 500°C decomposes into mercury and oxygen. The oxygen gas pan re-ignite a glowing splint. Red lead is very stable at bigh temperatures, it is actually prepared from lead monoxide by heating in air to above 400 C.
28g0-2g+2
Fb304
* no change
Black copper oxide is a babio oxire, reacting with aoids to give cuprio salts. Manganese dioxide is a compound of high valency, and cam act as an oxidising agent; thus it reacts with hot conc. EG1 to give manganous chloride and. chlorine. The chlorine gas is detected by it Bmell and bleaching action on moist litmus,
duo + 2HC1
HaO2+ 4HCL - Knčią
(c). Amphoteric oxides. An amphoteric oxide is one
which have both acidic and basic properties. Thus aluminium oxide reacts with acids and bases to give salts 1
Al2O3 + 3H250 - A1, (504)3 + 35,0 2Ná‡R + A12O2 = 2N■410, + H2O
Other examples are zinc oxide Zau, stannous oxide Sno, plumbous oxide Pho.
Ferorides. A peroxide is one which yields hydrogen peroxide on acidification with a dilute acid:
BaSO+
१६० १५
Na2O2+ 150 Na2
*
22
(a) Compound oxides. A compound oxide may be
regarded as formed by the combination of twa oxides of the same element. Thua red lead Pb:0
appears to be a compound of litharge Pbo with lead dioxide Pho,, since it reacts with hat dilute nitric acid to give a solution of lead nitrate and a precipitate of lead Jioxide:
Pbz0 + 4HNO3 = 2Pb(NO3)2 + Pb02 | + 2H 0
Similarly, ferroso-ferric oxide Fe, can be regarded as a compound of ferrous and ferrio cxides.
(f) Neutral oxides. A neutral oxide is one which
do not react with an acid or a base to give salt. Thus carbon monoxide CO and nitrous oxide N,0 are examples,
2
In addition to the above types, Bome- elements combines with oxygen to a greater or shaller amount than is required by their normal
valencias, The former may be termed as polyoxides and are examplified by lead dioxide Pt0 and manganese dioxide Mno, they are capable of acting as oxidizing agents in reactions. The latter may be termed as suboxides, and are usually unstable; carbon suboxide being the best known example.
Dalton's Atomic Theory is an attempt to elucidate the concept of atoms having fixed properties in xplaining, the laws of chemical combination. It nay be summarised as follows i
L.. Elemente are composed of indivisible partioles
called atoms. Atoms retain their identity through all chemical changes,
Atoms of a particular element have the same average mass, and their other properties are also the same. Atoms of different elements. generally have different average masses and different properties
Compounds are formed by combinations of the atoms of different elements.
Atoms of two or more elements may combinein more than one ratio to form more than one compound;
三期星日六廿月一十年九六九一曆公年八十五國民華中
12-7 8-31.75
The reactions involved ares
Cu: +
4HNO3
-
Cu(NO3)2 + 2NO2 ↑ + 2820
2Cu(NO3), • 2CuQ + 4NO2 1+ O2 }
Ethane is a saturated hydrocarbon because LE
contains carbon and hydrogen atoms linked together only by single covalent bonds where the proportion of hydrogen is a maximum. Ethylene although contains carbon and hydrogen, is unsaturated because it has less than the maximum
possible percentage of hydrogan due to the presence of a multiple covalent bond (a double bona) linking two carbon atome,
Saturated compounds give substition reactions with reagents, whereas addition
unsaturated compounds? - reactions characterizs (a) with chlorine
C2H6
C2H501 + H01
athyl chloride
ethylene
dichloride
(b) with hydrogen
:Substitution
(further subati-
tution with
exosas chlorine possible).
addition (product is saturated, thus no further addition with excess chlorine)
Saturated compounds do not react with hydrogen, whereas unsaturated compounds are bydrogenated to a saturated compound.
eddition
A$ 24 ml. or the mixture or gasee requirea 114 ml. of oxygen for complete oxidation, and produced 72 ml. of carbon dioxide.
Therefore 1 ml, of the mixture required.
15 ml oxygen, and produced
3 ml. carbon dioxide
Let mole fraction of propane in the mixture be x, thús mole fraction of the olefin
in (1x).
According to Avogadro's. Hypothesis, the ratio between the mole fractions of the two gases also represent the relative proportions by volume.
For complete combustion
C2a + 502 - 3002
1000+ a20
Thus for amount of oxygen requireds
5x + lần 1 - x) x 175
For amount of carbon dioxide produceds
3x + (1 - x) 3
Solving equatione (1) and (2) for the two unknown n and x we get:
23.3
Unlike nitric acid, nitrous acid is a very weak acid and can be liberated from its salts even by acetic acid. Furthermore, nitroua asid is very unstable, The addition of a dilute acid to a nitrite in the cold therefore leas to effervescence, with liberation of brown fumes of nitrogen dioxide and nitric oxide.
2KNO2 + 2HC1 - 2K01 + H2O + NO ƒ +
KNO + HCL no observable change
Urides of metals and non-metals constitute an important class of compounds. They can be best classified on the basis os their reactions with acids and/or bases. The general types are: basit oxides, acidic oxides, amphoteric oxides, paroxides, compound oxides, neutral oxides, (a) Basic oxiges, ▲ basio oxide is one which
interacts with an aoid to give a salt and water only. Basic oxides are generally formed" by combination of oxygen with a metal. 8.£.
2Cu+
02-2010
Cuo + H2SO = CuSO4 + B20
Based oxides of alkali metala and alkaline earths unite with water to form soluble. hydroxides. which are also basic1
CAU + H2O – Ca(OH)2
Ca(OH)2 + 2HC1 = CaCl2 + 2820
(b) Acidic oxides. An acidic oxide is one which
reacts with a base to give a salt. Oxides of non-metals are udually acidic, and may unite with water to form an acidi
CO2 + H2O » W Công H2CO2+ Ca(OH)
CaCO3
The higher uxiges or some metals are also. acidic. Thus although chromic oxide Cr20, is basic, chromium trioxide Cro, 38
Cro
+ 2NaOH.
Naz
Cro
Explanation or the Law of Conservation öf Kass. The indestructibility of atoms explain why all matter obeys this Law. If chemical changes praduce only a rearrangement of atose in various combinations, then, of course, the total mass of the material involved in those transformations must remain unchanged.
Explanation of the Law of Constant Compositzan. If compounds are formed by the combination of definite numbers of atoms of different elements, and if aach atom has its own characteristic weight the relative weight of each component in the compound must always be the same.
Explanation of the Law of Multiple Proportions If in one compound, one aton of an element A is combined with one atom of the element B, and in another compound, one stom of a is combined with two atoms of B, the weights of B that are combinad with a fixed weight of A in the two compounds would be in the ratio of 1 to 2. The ability of one ston of an element to combine with one atom of another element to form a compound, or with two or more atoms to form other compounds, explains the observed Law of Multiple Proportions.
Explanation of the Law of Equivalent (Reoiprocal) Proportione From the Atonio Theory it leads directly to the idea that each alement has a definite combining weight or equivalent, and that when elements combine they do so in simple multiples of these quantities. This is in support to the fact that when two elements
combine with a given weight of a third
the relative proportion by weight is in a ratio to those which they combine with one another.
From 12.7 gm, or copper, 15.9 gn of oxide is obtained.
Thus weight of oxygen that reacts with 12.7 mm. copper (15.9 12.7) g. 3.2 gm.4.
since equivalent weight of an element may be defined as the parts by weight of it which combines with eight part by weight of oxygen, therefore equivalent weight of coDDET
Hence the formula of the olefin le 0H¿
The composition of the mixture by volume is 50%
of Call with 50% of C2H2
QUESTIONS FOR NEXT WEEK,
1. Describe briefly the principles underlying the
extraction of metals from their ores. Illustrate your answer by reference to the manufacture of iron and steel,
A is a light grey powder which reacts vigorously. with dilute sulphuric acid to produce a gan B which extinguishes, a glowing splint. Gas B burns in air but has no effect on lead acetate solution.
If the mixture remaining from the reaction with dilute sulphuric acid is filtered, a yellow residue, Cremains. The filtrate will produce s white precipitate; D, with sodium hydroxide. solution. This precipitate is insoluble in excOBS. audium hydroxide solution.
When a small portion or A is heated in a test tube proctected by a safety screen, a violent reaction occurs. The product of this reaction, E, reacts readily with dilute sulphuric acid to produce a gas, F, which burns. in air. Freacts with lead acetate solution te give a black precipitate, G.
What are A, B, C, D, E, F and C? Discual the chemistry of the reactions described.
3. State Faraday's laws of electrolysis.
A steady current of electricity was passed through. two voltameters in series containing respectively copper sulphate solution and water acidified with sulphuric acid. After a period of time, 0.062 gm. of increase in weight was found in the cathode of the former, whilst the latter generates 23.5 0.0. of hydrogen gas at 15 C and 740 mm. pressure. What is the equivalent of ceaper?
What convenient physical or chemical mathoda qan you apply to the following mixtures in order to obtain a pure sample of the first named substances in every case..?
(a) Copper and zinc
Oxygen and dinitrogen tetroxide
(o) Nitrogen and carbon dioxide
(d) Concentrated nitric acid and concentrated
sulphuric acid
is
(e) Sodium carbonate and calcium carbonate
Sodium bromide and ammonium bromide