Chemistry Form Two Notes – Oxygen

These Chemistry Form Two notes cover Oxygen. The material is organised in clear sections for easier reading, understanding and revision.

Oxygen

Oxygen exists in air to an extent of 21% by volume (or 23% by weight). It is the most abundant element on earth, accounting for 4 the total mass of the earth’s crust. Oxygen is mainly found in combined states as oxides, hydroxides, silicates, sulphates, carbonates, water, etc. Its ease of combination with other elements to form compounds shows that oxygen is a very reactive

element.

Preparation and Properties of Oxygen Oxygen can be prepared in the laboratory from either hydrogen peroxide solution or potas

chlorate salt.

A Sample of Oxygen Gas in the Laboratory

Prepare a sample of oxygen gas in the laboratory

(i) Laboratory preparation of oxygen from hydrogen peroxide solution

The most common method for the preparation of oxygen in the laboratory is by decomposition of hydrogen peroxide solution. The gas is prepared by catalysing the decomposition of hydrogen peroxide with manganese (IV) oxide. At room temperature hydrogen peroxide decomposes

(breaks down) very slowly. It decomposes to water and oxygen

wi) 24,0) + Os. A slow reaction

To speed up the decomposition process, and hence collect substantial amount of oxygen gas

within a short time, black manganese (IV) oxide is added as a catalyst.

> 44) “> 2H,,0 y+ O;,,: A faster reaction

A catalyst is a substance that, although present in small quantities, will alter the rate of a

chemical reaction but will remain chemically unchanged at the end of the reaction.

Preparation method

Hydrogen peroxide (20 vol.) is added drop by drop to manganese (IV) oxide, which catalyses the decomposition of the peroxide. Oxygen is collected over water as shown in figure bellow. The

gas is collected by downward displacement of water because it is only slightly soluble in water.

20 vol. hydrogen

peroxide yp

solution | E;

{

• j
• is
• manganese(IV) {i ; 2
• oxide Sa water

Apparatus for labor y preparation of oxygen from hydrogen peroxide solution

(ii) Laboratory preparation of oxygen from potassium chlorate

Oxygen can also be prepared by thermal decomposition of potassium chlorate. When this compound is heated, it decomposes slowly into potassium chloride and oxygen:

2Kcio,,,, > 2Kc1,,, +30,

The reaction can be speeded up by adding a little manganese (IV) oxide as a catalyst. If KC/O, is heated alone, it gives off O,, but only ata fairly high temperature of about 400°C. If mixed with manganese (IV) oxide, KC/O, gives off oxygen at a much lower temperature and much more readily.

Preparation method

A grinded mixture of potassium chlorate and manganese (IV) oxide, at a ratio of 4:1, is placed in

hard glass tube and fitted up as shown in figure bellow. The mixture is then heated and oxygen gas is readily given off. The gas is collected over water. Oxygen has almost the same density

air, so it cannot be collected by the upward displacement of air. It is possible to collect it by

downward displacement of water as shown in the figure because it is only slightly soluble in

water.

oxygen

potassium chlorate (4 parts)

manganese (IV) & oxide (1 part)

Test for oxygen

Oxygen rekindles a glowing splint of wood. No gases behave like this except dinitrogen oxide,

NOz, from which oxygen can be distinguished by the following properties 1. Oxygen has no smell but dinitrogen oxide has a sweet, sickly smell

2. When heated with nitrogen monoxide, oxygen produces brown fumes of nitrogen dioxide

2NO\,) +Ox,) > 2NO2,,

Dinitrogen oxide has no effect on nitrogen monoxide.

Simple Experiments to Demonstrate Properties of Oxygen Gas Perform simple experiments to demonstrate properties of oxygen gas

1. Action of oxygen on metals

The manner in which oxygen reacts with metals is summarized in the list below.

— ———_—s——

OO ——————

~~ When heated in air, these metals oxidize with a ~ readiness indicated by the order shown; that is,

— potassium most easily, copper least readily.

OS

— oo 7

~ These metals do not oxidize easily; their oxides _ yield oxygen on heating.

Reaction with specific metals Sodium When burt in excess of oxygen, sodium burns with an intense yellow flame to give sodium

peroxide

2Na,,, +Oy,, > Naz0,,

The product is a yellow solid which dissolves in water to give an alkaline solution. Calcium

The metal burns in air with a red flame giving a white solid of calcium oxide

' 2Ca,,, +Oz,,, > 2CaO, Magnesium

Magnesium burns with a brilliant white flame, leaving a white ash of magnesium oxide.

2Mg,, + O.

Iron

Iron burns in air with a shower of sparks leaving a brown-black solid of triiron tetraoxide

3Fe,,, + 203 i) > FesOx,

Copper

Copper burns in a stream of oxygen to give a black solid of copper (II) oxide

2Cu,,) + Os.) 2 2CuO,,

In general, metals react with oxygen to form basic oxides. Action of oxygen on non-metals Carbon

Red-hot carbon combines vigorously with oxygen to form carbon dioxide, giving no residue:

G40, 300,

Sulphur

Sulphur burns with a blue flame giving misty white fumes of sulphur dioxide:

S,, +03, 250,

Phosphorus

Phosphorus bursts into flame in air or oxygen, without being heated (that is why it is stored

under water). A white solid, phosphorus pentoxide is formed

i) Py +503.) > P.O o.,, OF POs

Properties of Oxygen

Physical properties It is a clear, colourless gas with no smell It is a neutral gas (it is neither basic nor acidic in character)

It is slightly soluble in water (100 cm’ of water at room temperature dissolves about 4 cm’ of

oxygen).

It has almost the same density as water although slightly denser than air. 5. It boils at -183°C and freezes at -218°C.

Chemical properties

Oxygen supports combustion It is a very strong oxidizing a;

Oxygen is very reactive. It reacts vigorously with a great many metals and non-metals to form basic and acidic oxides respectively. Metal + Oxygen gives metallic oxide (most of these are basic in character). Non-metals + Oxygen gives non-metallic oxide (most of these are acidic in

character).

Uses of Oxygen

Uses of Oxygen in Daily Life List uses of oxygen in daily life

includes

The oxygen in the air and that dissolved in water and soil is used by all respiring organisms. Also

all types of burning need oxygen. It is used in the oxyacetylene (oxygen-ethyne) flame for welding and cutting steel.

It is extensively used for removing impurities from pig iron in order to produce steel. Oxygen is blown into molten iron to remove impurities such as carbon or phosphorus, which are expelled in

the form of gases, i.e. their oxides.

Oxygen is used as an aid to breathing in hospitals, high altitude climbing or flying, and in deep

sea diving

Liquid oxygen is used in the burning of fuels such as kerosene, hydrogen and hydrazine used in

various types of ro

It is used in the L-D process for making steel

Relationship between Some Uses of Oxygen to its Properties

There is relationship between uses of hydrogen and its properties. For example, oxygen is used as an aid to breathing in hospitals and at extreme altitudes because it supports life, and for combustion because it supports burning. Likewise, due to its highly reactive nature, oxygen is used for removal of impurities, welding, in the L-D process for making steel, and in burning of

fuels in rockets