要二第張五第二日五十月 四年丑癸歷
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1973英文中學會考試題預習專欄
化學科 (三十)
Chemistry (30)
Solution for last week
Part I
B
2. E
4.0 5.
Part II
65829
1. (a) In 13.2 gm. of carbon
dioxide, it contains
13.2 gm x
carbon.
3.6 gm.
er, it
In 8.1 gm. of
contains 8.1 gm. x 13 = 0.9 gm
of hydrogen.
The compound x should contain carbon, hydrogen and oxygen.
WAH KIU YAT PO
a) Flame test
日七十月五年三七九一懸公年二十六國民華中育教儒華
報日僑華
四期星
Observation
Inference
White ppt. soluble in excess
*Zine salt
White ppt. insoluble in exces$
A minute portion of the solid is taken up on a clean platinum wire which has been moistened with concentrated hydrochloric acid, and heated in a colourless Bunsen flame. Many metals give characteristic colours:
Yellow indicates Sodium
Violet
Light green
Brick-red
Deep green
Potassium.
Barium
Calcium
Copper
b) Carbon block test
By fusing the unknown metall -ic compound with sodium carbon -ate, the metallic compound is first converted to a carbonate of the metal, e.g.,
PbCl2 + Na2CO3 2NaCl + PbCO
Which on further heating, de- composes into the oxide and carbon dioxide;
PbCo
FOO CO
This metallic oxide is finally reduced by the hot carbon to a metal.
Pb04 C
Pb+00
-ic particles;
pale-blue ppt. soluble in excess
Lead or aluminium salt
b) Action with dilute hydrochlor
ic acid
Place some of the substance. in a test-tube and add dilute
If there hydrochloric acid.
is no action, warm gently. Test any gas given off by smell, lighted splint and lime water..
*Copper salt
Greet
ppt. insoluble
Ferrous salt,
Gas evolved
Ferric salt
in excess
Reddish-brown ppt. insoluble in excess
* Zinc and copper hydroxides dissolve in excess ammonium hydroxide to form complex compounds: Zn(OH), +4N 3
Cu(OH)2 4NH.
Carbon dioxide
Zn(NH
NH3 ) 4 (OH) 2
soluble, colour Less
Sulphur dioxide
Cu(NH3) 4 (OH)2 deep blue solution
f) Test for ammonium radical
Add sodium hydroxide to thes substance in solution and warm. If the gas given off moist red litmus paper blue or produces a white fumes with concentrated hydrochloric acid, this is a definite proof of the presence of an ammonium radical.
g) Confirmatory tests for lead
add dilute sulphuric acid a white ppt. form.
2-
Metal
Silver
Appearance Indication shining metall
The weight of oxygen 69 gm. 3.6 gm -
malleable
0.9 gm
Copper
red-scales
sa
24 gm.
Lead
0.9
CHO
greyish-white. red when globules; soft hot; enough to mark yellow paper
when cold.
Zine
yellow
when hot; white when cold.
0.30.90.15
Therefore the empirical formula of x is CH
CHO
The x is ethyl alcohol
H H
(b) a) When ethyl alcohol is heated with acidified potassium dichormate, it is oxidized to acetic acid.
H H
H&H
H H
acetaldyde
H
acetic acid
ii) When methyl alcohol is heated with acetic acid in the presence of concentrated sul- phuric acid, methyl acetate is obtained.
I-C-OH-H-
H
OCH3
methyl acetate
+ H20
c) Heat the solid in a small dry tube!
Many compounds decompose on heating and leave a residue of oxide or metal, which may some- times be identified from is appearance.
Observatión
A white sublimate: is formed and
ammonia is detected.
A white sublimate, but no ammonia
A residue which liquefies and turris yellow on cooling
A solid residue which is yellow when hot but white when cold:
Pb + SO
PbSo
ii) add concentrated hydro-
chloric acid, a white
ppt. when cold but soluble in hot water.
2+
Pb +201 > PbC12
h) Confirmatory tests for iron salts
To separate portions of the original solution add
Ferric salt
Light blue Dark blue precipi
Hydrogen sulphide
Indication
Carbonates or Bi- carbonates
e.g
Na Co.
NaHCO
+ 2HC1
2NaCl +
+ Hol
H2O
NaCl + H2O + CO2
Sulphite e.g..
Maz
SO
For
bisulphite
+ 2HC1
2NaCl + H2O + 502
нсі
NaCl + H2O
Sulphide
e.g.
SO
FeS + 2HC1-FeCl2 + H2S
Nitrite
Nitrogen dioxide
e.g.
Chlorine
+HC1
NaCl + HNO
2HNO2→ H2O + NO +NO2
Hypochlorite
e.g.
NaOCl + 2HCI
Naci+ H2O
Action with concentra sulphuric acid:
C12
Adding a little concentrat ed sulphuric acid into the compound, and warming gently, observing all effects.
Gas or Indication Vapour
Hydrogen Chloride. chloride e.g.
ZnCl2
Reagent
Ferrous salt
Inference
Ammonium
Potassium ferrocyan- ide K Fe(CN)6
・ate⋅
precipit- ate
Dark blue
precipit-
Dark solution
ate
K-Fe(CH)
acid fume
e.g.
Potassium No react
thiocyanate ion
Deep-red solution
Mg (NO3)2
MgSO
2HNO3
KSON
Mercury,
Arsenic or Antimony
Lead
Zinc
d) Action of sodium hydroxide solution
Potassium ferrocyan-
ide
2. Detection of anions (acid
radicals
a) Heat a small amount of the
dry substance in a hard-glass tube:
Gas or Vapour
To the solution add sodium hydroxide solution in small quantities at a time until it is present in excess. If there water is no precipitate, warm gently Vapour and test for ammonia.
Observation
Inference
White ppt. insoluble Calcium or
in excess
magnesium
Indication
Water of crystallisat- ion.
e.g. CuSO
>Cuso +5H2O
2. Hydroxides of heavy
metals.
Zns04
2H01
Nitric Nitrate
Carbon Formate monoxidee.g.
2HCOONa
SO
Na 50
2HCOOH
Pb(OH)2
(c) PC1
OH
White ppt. soluble
C2H5Cl +
+ HC1
in excess
Blue ppt. insoluble in excess
*Lead, Zinc or Aluminium Copper salt
Pbo + H2O
Oxygen
1. Oxides of silver or
mercury
2Hg +02
PC15
CH COOH
CH C001 + POC1 - HC1
The similarities are that both ethanol and acetic acid give off fuming hydrogen chloride gas, and phosphorus oxychloride.
The difference is that while ethyl chloride is obtain ed from ethanol, acetyl chlo- ride is obtained from acetic acid.
19. Qualitative Analysis
The aim of qualitative analysis is to determine the identity of the elements or radicals present in any compound
In general, the formula of a compound AB, consists of two
A is called the parts, A and B. cation or basic radical which may consists of a metal or ammonium radical. B is called the anion which usually consists of an acid radical.
1. Detection of cations
Following are different methods used for detecting the cations:
Green ppt. insoluble Ferrous salt in excess
Reddish-brown ppt. insoluble in excess
Pb2+ 20H
eig. 2Hgo
2. Nitrates of Potassium
or Sodium
Ferric salt
+ 30H
Pb(OH)2
Zn(OH) V →→ Al(OH)3 +
e.g. 2KNO
2KNO 2
3. peroxides
e.g
2Ba02
2Ba0+
02
Zn + 20H A13+
Hydroxides of lead, zinc and aluminium are amphoteric and dis- solve in excess sodium hydroxide. solution:
2
Pb(OH) + 2NaOH
Zn(OH)2+2NaOH A1(OH)3 + NaOH
Na PbO2+ 2H20
→→→Na2 ZnO2 + 2H20
NaA10+2H20
e) Action of ammonium hydroxide golution
To the solution add ammoniur hydroxide in small quantities at a time until it is present in excess.
Carbon dioxide
Nitro- gen dioxide
permanganates and chlorates
eig. 2Kmn0
4.
no
2KC10, 2KC1 + 02
Carbonates or bicarbonates
e.g.
3
Caco,
Cao + 00,
Ca (HCO3)Ca0+2002+H20
Nitrates or nitrites of heavy metals
e.g.
2Pb(NO3)2 →2FbQ+4N02+02
Carbon Oxalate
monoxide e.g. and
carbon dioxide
2HCOOH
2H2O + 200
Ca + H2SO
Caso C202H2
·H2O + CO + CO.
a) Confirmatory tests for
thiosulphate
All thiosulphates decompose when heated alone, forming a yellow deposit of sulphur and giving off SO2 and HS. ii) When the solid or its
aqueous solution is treated with any dilute acid, SO, is evolved and the solution tums milky or yellow owing to the liberation of sulph-
ur.
iii) A solution of iodine in
aqueous KI is immediately decolorised.
Exercise Part I
1. Concentrated hydrochloric acid
is used in flame tests of metallic ions for the following purpose:
A. to assists combustion. B. to clean the platinum wire C. to make the sample more
volatile
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