1973-01-07 — Page 18

Oxygen

Preparation:

From hydrogen peroxide

Hydrogen peroxide, when heat-

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b). of liquid air, the nitrogen may be separated from the oxygen,

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(+-)))

th. stry

Properties:

Physical --- colourless, odourless gas only slightly soluble in water Chemical-

Oxidation or burning of some of the elements in oxygen

(i) carbon burns with a white

flame

0402

(11) Sulphur burns with a bright

blue flame

S. +

phosphorus burno with brilliant flame

502

2Р205

(phosphorus. pentoxide)

Bao

Zu 50

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But it will also react with con.

centrated d sodium zincate, thus

behaving as an acidic oxide wher un alkaline solution.:

ZnO

(v)

+2NaOh

Na 200

other examples are aluminium oxide, stannous oxide and plüzb- ous oxide.

peroxides and dioxides.

All peroxides contain the 02 iron and have a

re a negative

valency of -1, These are oxides which give nydrogen peroxide dilute acids in the cold. For example:

Na2O2 + H2SO ̧

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ea, deconysoses violently into water and oxygen. Decomposition also occurs with catalysts such as platinum, manganese dioxide. copper, silver, etc. But manganese dioxide, when used as catalyst may cause an explosion.

2H2O2

2H

A catalyst is a substance which changes the speed of a chemical reaction but remains unchanged at the end of the reaction both in weight and com- Posi

(2) By heating potassium chlorate

The pure potassium chlorate melts when heated and gives off oxygen at about 38000, but when add some manganese dioxide as a catalyst the reaction takes place at a lower temperature (20000) Many other oxides, eg. Copper oxide, ferrie oxide, nickel oxide can also function as catolysts in this reaction.

2KC10

2KC1 302

The oxygen obtain in this way contains carbon dioxide, mois -ture and chlorine as impurities, These are removed by passing the gas through a tube containing. soda lime and then through con- centrated sulphuric acid. Finally the gas is collected over water if wet and over mecury if dry. Potassnim

chlorate

·Aloxide

охудел

• Fig 4 a queparation of oxygen

Absorbent for the gas: pyrogallol and comstic potash (3) By heating a normal oxide of

a metal

Oxides of metals below in the Electro chemical series can be decomposed by heating-

2HgC

2H2 + 021

(4) By heating the higher- of metals

oppe

xides

Many higher oxides, on heat- ing, yield oxygen and a lower oxide, e«g«

2002

iv) Calcium burns with a red

flame

20a +

20a0

(V) Magnesium burns with a brilli- ant white flame

2Mg +02

2Mgo

(vt) Iron burns with a beautiful spark in oxygen to form triferric tetroxide.

37€ + 20

Uses:

Fe,04

(i) Respiration--- oxygen 19 given to some people who are ill or in- jured.

(11) Flames and metal cutting-- (a) oxy-acetylene flame with a temperature about 300000. This is used to cut through steel. (b) oxy-hydrogen flame melt iron easi ly, and can be used to join pieces of the metal together..

(111) Oxygen in steel production- Oxygen is sometimes used instead of air in making steel from pig- iron in the Bessemer Comerter. A blast of oxygen is blown through the molten pig-iron and oxidizes the impurities, e.g, carbon. sul- phur, phosphorus.

Oxides:

Compounds formed by conbination of elements with oxygen are called oxides.

(1) Acidic oxide

Acidic, oxides are oxides or the non-metals, which react with bases to form salts.

For examples:

NaOH

Ca(OH).

acidre

Na2CO3

also contine with

They are calleg anhydrides. For exacules.

H2O

able in water.

it to form acids

(11) Basic oxide

30

Basic oxides are always oxi of metals and never of non-dets] which react with acids to form salt and water only for example

Cuo

2Pb0

470°C

021

Pbo

2HNO

400°C

(5) By heating oxy-salte,

KNO

KNO

The commercial preparation or oxygen from liquid air.

Oxygen is man factured by the fractional distillation of liquid

air.

Air is compressed to about 200 atmospheres and then washed with caustic soda solution to re- move carbon dioxide. It is cooled to -2500. by passing through a heat exchanger when the moisture is frozen out of the air and is removed.

The cooled compressed air is passed through a small spiral pipe to cool it again and agin, the air is finally cold enough to become a liquid.

The liquid nitrogen is more volatile than the liquid oxygen. By fractional distillation (b.pt. of liquid nitrogen is -196°C., and that of liquid oxygen is -18ju

CUSO

Pb(NO

HO

Oxides of sodium and potass- lum are highly soluble, the calci -um oxide are only sparingly soluble in water to form soluble hydroxides. they are called alkalis.

Na 0+ H20

CaO

(iii) Neutral oxides

2N50H

Ca(OH)2

These are oxides which do not exhibit any tendency to form salt- and have neither acidic nor bosi properties. Examples are nitric oxide, monoxide

(iv)

/xide. carpon.........

water.

oxides

Oxides which can benave as. acidic as well as basic oxides. for example, zinc oxide will reac with sulphuric acid, forming & solution of zinc sulphate--- thu behaving as a basic oxide when i acid solutions:

or.

BaSO

Dioxides are oxides convain ing two atoms of oxygen kud for responding to a high valency of the element. They giving chlori with hot concentrated hydroc acid, and often lose oxyete heating, but they never give

hydrogen peroxide win dilure. acids. For example:

MnO

4HC1

v) Compound arides

oxides which react like a nixture of two simpler oxides. For example:

Red lead, Pb 04, reacts like mixture of 2Pb0 + PbQ2

2PbO 4HC1 2PC12

FbC1,

4HC1

2H20

41,0

C12↓ other examples are magnetic oxide of iron, Fe, and Tri tetroxide, Mn.

Exercise Part I

imag

140 cc of air was mixed wi 15 cc of oxygen. Excess of

phosphorus was burnt in it. After cooling, what was the volume of gas left (All volumes were mea- sured under same temperature and pressure).

A.. 25 c.co.

B. 30. c.c.

C. 32 c.c.

D. 44 c.c. E. 55 0.0

The oxygen content of air obtained by boiling water in A. less than in normal air

B. greater than in normal air C. the same as in normal air:

D. non existence.

E widely variable

cannot be,

Obtained by

the

heating of

A. Hgo B. Pho

D. M1102 E. Ag20

4." In the diagram, the space above the water level at the beginning 18 2000 ml, and contain air, car- bon dioxide and oxygen in the ratio::1:1. When the phosphorus has finished reacting, the volume above the water level in the jar

A. 1800 ml 81400 al.

C. .D.

200 ml

800 ml.

дам

AD 500 ml

A

pupe oxygen-

the effect

Pyrogallo and Caustic notask

water

be produced by song heat on

souium nitrate

8. som trite

C. ammonium nit rate.

0. botn sollum nitrate and Lead

nitrate

ooth sodium nitrate and sodium nitritë

5. Air 19 passed through water until the water is saturated.. The apparatus is arranged and the flask boiled until the dissolved air comes out of solution and is collected. The dissloved air 19 found to be the same as the ap in the atmosphere except that

P. It has more oxygen in

Q. it has more nitrogen in it Rit has more carbon dioxide in 11 6. it has more nitric oxide in it

it is identical with atmospheric air

Heat

boled out.

air

7. Which of the following oxides

reacts with water?

P. magnesium oxide

2. Calcium oxide

R. Zinc oxide

5. Copper oxide

1. mercuric oxide

3. Oxygen can be prepared by heat- ing potassium chlorate and mangan- pse dioxide. This is an example of

P. oxidation

Reduction /

thermal dissociation double decomposition thermal decomposition

p. If the percentage or oxygen in air is determined by repeatedly passing a measured volume of air over some copper heated in a Bilica tube, one can. be sure that all the oxygen has been used úp,

by the presence of unchanged copper in the tube?

R. when the copper ceases to glow

when no more copper is seen to.. become dull

when the reduction in total : volume is one-tenth

when all the gas has been used

ub

It 19 reasonable to assume that a substance contains oxvæen when

P. it disappears go strong heating

It produces carbon dioxide if heated with charcoal

le burns when heated with copper tuming

It leaves a black residue on

strong heating.

itute on heating.