1973-03-23 — Page 20

日三廿月三年三七九一层公年二十六回民華中育教僈業

2HNO3 H2SO4 2NO

四頁第張五第一日九十月二年丑癸歷夏

WAH

KIU YAT PO

報日僑華

五期星

200 + 92

2NO

1973英文中學會考試題預習專欄

SIMAM ABRE A

+

化學科

(#)

Pb 4HNO

Chemistry (22)

Solution for last week

Part I

2. B

3

4. D

5. E

Nitrogen (2)

12.

13.

10. T

The oxides of Nitrogen

■ Nitric oxide

Lab. preparation.

AABO

The copper filings are added moderately to concentrated

nitric acid

3Cu+ 8HNO

→→3Cu(NO3)2 + 4H2

2N0

The gas is collected over water.

properties:

It is a colourless gas, rather insoluble in water, neutral to litmus and other. indicators.

When the gas is mixed with air or oxygen, brown fumes are formed.

ZNO +0,

2N02

When bubbled through ferrous. sulphate solution, the sol

ion tums brown.

FeSO + NO

Nitrogen dioxide

Lab. preparation.

FeSO NO

The copper filings are added to concentrated nitric acid.

3.

Cu + 4HNO →→→ Cu (NO 3 ) 2 + 2H20

The gas is collected by ward displacement of air properties:

The nitrogen dioxide is dis- solved in water with which reacts to form nitric acid.

2HNO + NOA

The nitric oxide that is form- ed in this last reaction is used again in stage 2. roperties physical --- Heavy, colourless liquid, miscible with water in all proportions, pungent smell; corrosive action on: flesh, clothes, wood. etc. Boling point 8300, specific gravity 1.53. ordinary "concent

rated" nitric acid contains about 70 per cent. HNO

FINO 3•

Chemical --

Concentrated nitric acid decomposes slowly in light and on heating

· 4HNO

4NU + 2H2O + O2

and the nitrogen dioxide so formed accounts for the yellow or brown colour of the concent- rated acid.

Dilute nitric acid as an acid

It turns litmus red.

It will react with bases to give a salt and water:

NaOH + HNO3

NaNO

HO

iii) Magnesium is the only metal

which when treated with very dilute nitric acid, will give hydrogen.

Mg + 2HNO

NO ~~~> Mg (NO3)2 + F21

iv) It will react with carbonate

to give carbon dioxide Na Co 2HNO

2NaNO3 H2O + 60 1

Concentrated nitric acid is

a powerful oxidizing agent

Concentrated acid is an accepter of electrons, Concentrated

4HNO + 2e →2NO

2H20

2NO

2N0

Moderately conc.

8HNO

SNO 4HO

It is a brown gas, quite soluble in water, due to the formation of a mixture of nitrous-acid and nitric acid.

+ H20—> HNO + HNO3

It is acidic to litmus and other indicators, but has no reaction with air, oxygen or ferrous sulphate solution.

The other oxides are nitrous

oxide (N20), dinitrogen trioxide (N203) and nitrogen pentoxide. (P20)

Nitric acid:

Lab. preparation

Sodium or potassium nitrate is heated with concentrated sul- phuric acid in a glass retort. The nitric acid is given off as a gas and is condensed in a water cooled flask. With excess of potassium nitrate and higher temperature, potassium sulphate is formed, but this temperature may crack the glass and decompose some of the acid.

heat

and KNC

Industrial method

Much of the nitric acid for industry is made from ammonia by the Ostwald process. This has three steps, as follows:

1. On the surface of a catalyst,

at about 500°C., ammonia is oxidized by the oxygen in air to nitric oxide, NO. (The catalyst is metallic ganze. made of platinum or a rhodium- platinum alloy.

6H2O + 4NO

4NH3+

+502-

2. The nitric oxide, after cool-

ing, is oxidized by the oxygen in air to nitrogen dioxide, NO, which is also a gas;

on metals:

4HNO

3Cu 8HNO

4Zn

OHNO

4Zm + 1OHNO2

(dil)

+2NO!

(NO3)2 2H 0

2N024 3Cu(NO3)2

4H2O + NO 42n (NO3

3H,0 ANH

4Zn (NO3)2

5H0

on non-metals Carbon is oxidized to

hot conc

HNO

4HNO

NO

4HNO

1.)→ 2H 0 + NO

+ 300 2↑

Sulphur is oxidized to H2SO by hot HNO

S + 6HNO, (conc. )→→→ H2SO4

6NO2+ 2H20

4HNO, (dil. )→→→→→ 350-↑

4NO↑ + 2H2O

111) on compounds

Ferrous salts are oxidized to. ferric salt.

6FeSO4 + 2HNO 3 JHSO4

3Fe(SO4)3 + 4H20 + 2NO Hydrogen sulphide is oxidized to a yellow precipitate of sulphur. HS+2HNO

S+

2NO 2

2H20

Sulphur dioxide is oxidized

to sulphuric acid.

Nitratest.

oreparation.

By the action of nitric acid on most metals, e.g. copper, lead, silver,

etc.

+2H20

2N021

2. By the action of nitric acid

on bases, e.g. sodium, potass- ium,

ammonium hydroxides. NaOH + HNO

KOH + HNO3

NANO +

H20

→KNO3

OH + HNO →→→ NH

NH OH

H4NO3 + H2O

By the action of nitric acid on carbonates, e.g. calcium, Magnesium, Zinc, lead carbonate PÚCO + 2HNQ

+ H2O

3.

3

Properties + 002 A

+

All nitrates are soluble in water and react with hot concent- rated sulphuric acid to for nitric vapour. Many nitrates are also deliquescent.

Uses del

of potassium and ammonium nitrates.

Potassium nitrate is not deliquescent. So, it is used in the manufacture of explosives, such as Gun-powder, which contains roughly by weight:

1 part of charcoal; 1 rart of sulphur, 6 parts of potassium nitrate.

Ammonium nitrate used in many explosives, such as ammonal: (a mixture of ammonium nitrate and aluminium powder). When it is explodes,

2A1 + 3NH NO

A. exposing the gas to damp blue

litmus paper

B. mixing the gas with chlorine. C. mixing the gas with oxygen D. shaking the gas with aqueous

potassium hydroxide

E. passing the gas over heated

platium

5. A certain gaseous oxide of nitrogen produces its own volume of nitrogen when decomposed into its constituent elements. If one litre of the gas at N. I.P. weighs 2 gm, the molecular formula of the oxide is

A NO

B. NO

C. N203

D. NO2

EN

B. N205

When concentrated nitric acid reacts with sulphur the products of the reaction are

P. nitric oxide, sulphur dioxide

and water

**nitric oxide, sulphuric acid

Q.

and water

R. nitrogen dioxide, sulphur

dioxide and water

S.

nitrogen dioxide, sulphuric acid and water

1. nitrogen oxide, sulphur

dioxide and water

7. Some brown nitrogen dioxide gas is placed at the bottom of jar of air, very soon the whole jar is observed to be evenly. brown. This occurs because the nitrogen dioxide molecules are: able to

P. nove and mix with the mole-

cules in the air

Q. react with oxygen

R.

7A1203

6820 3N A

Again a large volume of hot gas is produced from a small volume of explosive. The other uses of ammonium nitrate asa fertilizer in nitrochalk (a mix- ture of ammonium nitrate and powder chalk).

Tests for nitrates:

1. Brown Ring test

Dissolve the suspected nitrate in water and add a fresh solution of ferrous sulphate, shake well. Incline the test-tube and slowly pour conc. sulphuric acid down the inside of the tube so that it forms a layer below the level of the aqueous solution, A brown ring (FeSO NO) forms at the junction of the two layers of liquid if a nitrate is present. 2. The copper test

Add a piece of copper to the solid suspected nitrate, and add concentrated sulphuric acid heat gently. If a nitrate is present, the brown fumes of nitrogen dioxie will be given off.

Exercise

Part I

1. Commercial nitric acid is often brown because it

A.

Contains dissolved sulphur contains dissolved of the cok

C. contains dissolved nitrous

acid

D. contains dissolved nitrogen

dioxide:

E. contains dissolved other

impurities

2. When nitrogen dioxide is passed through sodium hydroxide solution, which of the following is/are formed together with water?

1. Ammonia”

2. Sodium nitrite 3. Sodium nitrate.

A. 1 only

B. 1 and 2 only 0.1 and 3 only D. 2 and 3 only E. all

3. Which of the following state- ments is incorrect?. Nitrogen dioxide

A is formed when sodium nitrate

is heated..

B. is formed when sodium nitrite!

is heated.

C. is formed when concentrated

nitric acid reacts with `magnesium metal.

D. is formed by the thermal

decomposition of nitric acid

vapour.

E. dissolves in aqueous sodium

hydroxide.

Nitric oxide can be distinguish, -ed from ritrous oxide by

split up into smaller nitro- gen dioxide molecules

attain the equilibrium

2102

N204

T. decompose into nitrogen and

Oxygen

8. When mercurio

ly heated,

nitrate is strong products are

P. mercuric nitrite and oxygen Q. mercuric oxide and nitrogen

dioxide

mercuric oxide, nitric oxide and oxygen

S. mercury and nitrogen

1. mercury, nitrogen dioxide

and oxygen

9. Concentrated nitric acid is heated with benzene (CH) in the presence of concentrated sulphuric acid. The products are

P. carbon dioxide, nitrogen

dioxide and water carbon dioxide, nitrogen dioxide, nitric oxide and water

R. carbon, carbon dioxide,

nitric oxide

S. benezene nitrate, nitric

oxide and water

1. nitrobenzene and water 10. When a drop of concentrated nitric acid is added to ferrous. sulphate solution,

P.

a green precipitate is produced C. a brown precipitate is

produced

R the solution turns brown

solution turns deep blue

the solution turns brown, and a brown precipitate is produced.

Part II

1. Describe how you would show by experiment the presence of

a) oxygen in nitric acid b) copper in copper nitrate

2. Describe, giving all necessary experimental details, how you would prepare.

a) potassium nitrate from

potassium hydroxide solution b) lead sulphate starting from

lead nitrate on (a) Describe how would

ring" test

perform the "brodium nitrate.

on a sample of and explain the chemical react- ions involved.

(b) What is the action of nitric acid you have just prepared on dry sawdust, magnesium, solution of potass- ium iodide?

If two drops of this acid. added to a solution of sulphur dioxide in water and a solution of barium chloride is added, what do see? Explain this action.

NaNO3 + H2SO4

NaHSO4

+ HNO3

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