1973-03-15 — Page 24

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日五十月三年三七九一曆公年二十六國民華中 育教僑華

1973英文中學會考試題預習專欄:

BY IN A MAPA 1 Skú

化學科

(#−)

Chemistry (21)

Solution for last week Part

1. (a)

B

MADA◄

POWHO

acid

Toluene solution:

Electricall It does

bonductiv-not con-

Dilute

hydrochloric

It conducts electricity

ity

duct

electrici

-ty:

Action of No effect

Litmus

Action on

Mg, Zo, Fe It does

Action on

carbonate

Action on ammonia

not form

hydrogen with metals

It does not react with

carbonate

It turns. litmus red

Hydrogen liberated

Carbon dioxide produced

A white Ammonia precipitate dissolves

(b) In toluene, hydrogen chloride does not ionise, exists as a Covalent compound, and is there- fore fairly inactive. In water, hydrogen chloride ionises complete -ly to H and C1 ions, and these Zons are responsible for its acidity and activity.

(c) Cold concentrated hydrochloric acid is oxidised by potassium permanganate

16HCT 2KC1 + 2MC12

+ 8H20 + 5012

Solid potassium permanganate is placed in a flask and concent- rated hydrochloric acid is dropped on to it from a tap-funnel. Since potassium permanganate is a very strong oxidising agent, the react- ion takes takes place in the cold, chlorine is liberated immediately and is first passed through a containing water, which is used to remove hydrogen ahloride. The gas is then dried by passing through the second bottle contain- ing concentrated sulphuric acid... It is collected by downward deli- very, the gas being denser than air

2. (a) Hydrogen chloride can be prepared by displacement from chlo -rides by heating with sulphuric acid.

Normally the reaction will termin- ate at the first stage

NH C1+ H2SO4

ANH HSO4

HCL

Hydrogen chloride being very soluble in water a solution of it has to be made using an invert- ed funnel.

(b) 1) A white precipitate is geen when dilute hydrochloric acid is. added. Then the precipitate dis- appears when the mixture is heated It is because lead chloride which is formed at first is insoluble in water, but dissolves in hot water.

Pb(NO3)2 + 2HC1-

+2HNO

11) A jar of hydrogen chloride is placed mouth to mouth with a jar of ammonia, dense white fume. of solid particles of ammoniun chloride funnel.

NH3 +H01 -ÀNH C

Nitrogen and its compounds(1).

Occurrence.

Nitrogen is familiar to us all as the most plentiful constit- uent of the atmosphere (about 79 per cent. by volume, 77 per cent by weight). It is also present in various minerals, as sodium nitrate (Chile saltpetre), as · ammonium salts in small quantities in soil; and as a constituent of

living tissue,

Preparation:

(a) From air, by removing carbon

dioxide with caustic soda, water with concentrated sul phuric acid, and oxygen with heated copper,

2NaOH + CO.

2Cu + 02-

Na2CO

$20u0

The resulting gas, which is main- ly nitrogen, is collected over water.

(b) By heating a solution of anmonium nitrite.

Ammonium nitrite readily decomposes to give water and nitrogen. In practice, a mixture of sodium nitrite and ammonium chloride is used

NaNO

NH CL

NaCl + NH NO

NH NO2

2H20

Froperties:.

Physical

It is colourless, odourless, almost insoluble in water and is liquefied only with difficulty. Chemical

Nitrogen is noted for its chemical inactivity. It does not burn or support combustion, but 1t combines with some of the more active metals when heated, e.g. calcium and magnesium, to form nitrides, which react with water. forming ammonia:

3Mg N

M6372

OH O 2NHA + 3Mg(OH)

This is a good test for nitrogen.

When sparked with oxygen, nit- rogen forms small quantities of nitric oxide, and when sparked with hydrogen it yields small quantities of ammonia, both react- ions are reversible.

N2 + O2

N2+ 3H2

Ammonia:

2N0

2NH

Preparation in Laboratory

Heat by any ammonium selt with any alkali, e.g. ammonium chloride and slaked lime:

2NH CA

Theat

Ca(OH)2-

2NH ↑ CaCl2

2H 0

day the NH3

Collect: by upward displacement

(as it is lighter than air); or over mercury.

Dry :

by quicklime. Neither con- centrated sulphuric acid nor calcium chloride is suitable, as both react with ammonia. The sulphuric acid combines igorously to give ammonium Bulphate.

2NH3 H2S04

(NH4)2SO

Calcium chloride reacts to give an addition compound in much the same way as it does with water.

CaCl2 + 8NH,

Fest:

CaCl2.8NH3

1) Detected by its small

and by its turning moist litmus blue.

ii) Formation of a dense white fume with HO1.

iii) Delicate test. Format-

ion of a yellow precipit -ate with Nessler's reagent.

Manufacture:

+

3H2

Conditions:

Haber process

2NH H31 + Heat

Pressure--the equation shows that 1 molecule of nitrogen combines with 3 molecules of hydrogen to give 2

reactions like this, there is a decrease in volume, high pressures give the better vield" (200 atmos. to 1000 atmos,), Temperature---the equation

also indicates that the reaction is exothermic, there -fore, low temperatures give the better yield. (In pract ice, therefore, a moderate

is temperature about 500°C. used)

of ammonia. In 21

iii) Catalyst---At low temperature

-s, however, chemical react- ions proceed slowly, in practice, with a catalyst, which also increases the rate of the reaction.

The Catalyst for the synthesis or ammonia is iron; this is mixed with aluminium oxide which helps to maintain a large active surface of iron.

Properties

hysical

red litmus

Blue lid

-Red Titano

Solution

It is a colourless gas with a “ungent smell. Much lighter than

fir. Most soluble gas known. This broperty is most convincingly shown by the fountain experiment. A round bottle flask is fitted rubber bung, through which a glass tube drawn out to 1 jet as shown. This flesk is completely filled with ammonia. The bung and jar are placed in position and the flask is placed

ith its mouth under water colour- ed with red limus. Water rises

lowly up the glass tube and event ually trickles over the top of he jar. A large surface area of he liquid is then suddenly ex- posed to the ammonia, which rapid-

y dissolves. The resultant de- rease of pressure inside the lask is so great that air press- are forces liquid through, the jar in the form of a fountain. At the

Jame time the litmus tums blue in colour, showing that when ɔmmonia dissolves in water an ilkali, actually armoniaum hydro- cide, is formed:

NHHO

hemical

NH OH

oxidation of ammonia:

Conbustion: of ammon19.

2N ̧ + 6H2O

NH3 + 302

ii) Catalystic oxidation of

ammonia.

When ammonia. burs in air with the aid of platinum catalyst, oxides of nitroger being formed:

4NH

502

4NO + 6H20

Uxidation of ammonia by copper oxide.

Ammonia gas passed over red-hot copper oxide reduces the oxide to metal:

30uO+ 2NH3 →→3Cu + 3H20 + Nz

iv) Uxidation of ammonia by

chlorine.

Ammonia reauces. chlorine to hydrogen chloride, when a cooled solution of ammonia is treated with chlorine gas The hydrochloric acid first formed unites with excess

or ammonia to form ammonium chloride.

2NH3C12

HOT NH

N2+ 6HC1

•NH 4

C1

2. Alkalinity and salt formation

Ammonia reacts with water

producing a substantial concentra

-ion of hydroxide ion in solution,

1.e. the solution is alkaline. The degree of dissociation of the ammonia is small, so ammonia is

Woak alkali,

NH + H2O− NH CHINH

3

+ OH

Ammonia solution is alkaline enough to precipitate many inso→ Tuble hydroxides from solutions. of the corresponding salts, e.g.

3NH C1 + Fe(OH)3.

FeCl3 + 3NH4OH

4

brown ppt.

3NH OH

3NH4C1 + A1(OH)3,

(colourless Ppti

With acids ammonia forms ammonium salt, which can be ory stallised.

NH OH + HCL

INH OH + H2

NHẠC +

(NH4) 250

With carbon, dioxidé..

Ammonia, can react with carbon dioxide in the presence of water,

forming a salt, ammonium carbonate

2NH

Uses

CO

H2O

NH

20

1. Manufacture of nitric acid

(Ostwald process.)

This has three steps, as follows:

1) On the surface

at about 50000,Catalyst,

ammonia is

oxidized by the oxygen in

air to nitric oxide:

NH3+502

The catalyst

6H2O + 4NO

etallic gauzę

mode of platinum or a rhodium-

· platinum alloy.

11) The

nitric oxide, after.

cooling, is oxidized by the Oxygen in air to nitrogen dioxide:

2NO+02

2NO,

The nitrogen dioxide is dissolved in water with which it reacts to form nitric acid:

3NO + H2O

2HNO + NO

reparation or ammonium salts. Ammonium sulphate, ammonium nitrate and ammonium dihydrogen phosphate, are sold mainly a9. fertilizers. They are soluble in Water.

CO

Caco

+ NH3

+ H20 + 2NH

HPO

NH3

+ (NH4)250

Kerrigeration

THÁNG

>NH HO

Liquefied ammonia Is the refrigerant in industrial refrig- erators and in ice-making plants. 4. Cleaning purposes.

Ammonia solution is used Im Laundry work. It removes tempor ary hardness by precipitating the colcium ion of calcium bicarbonate. as chalk,"

Catt + 2Hco2+ 20H

Caco +21120+ co

It also dissolves our acids left by evaporation of perspirat- ion from under-clothing.

Ammonium salts

• Ammonium chloride ( Sal ́ammoniac NH CL +4

It is a white crystal, used in dry-battereries and in sold- ering. It sublimes. Ammonium sulphate (NH4)2504

White crystalline solid uses as artifical manure: principal source of other ammonin compounds.

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