ATMOSPHERIC AIR

Gases in the atmosphere are held in an envelope around the earth by gravity. The atmosphere is 80 km thick and is divided into four layers, namely troposphere, stratosphere, mesosphere, and thermosphere. About 75% of mass is found in the layer nearest to earth, the troposphere.

Atmospheric air definition

Atmospheric air is the type of air that can be observed under atmospheric conditions. Air is a mixture. When a sample of dry, unpolluted air is taken from the atmosphere and analysed, the composition by volume is often found to be:

 

Component	%	Boiling Point °C	Melting Point °C
Nitrogen	78.08	-196	-210
Oxygen	20.95	-183	-218.4
Argon	0.93	-186	-189.3
Carbon dioxide	0.03
Neon	0.002	-246.1	-249
Helium	0.0005	-269	-272
Krypton	0.0001	-153	-157.36
Xenon and minute other gases	0.0001	-108	-111.9

Carbon dioxide is present in the air in proportion of 0.03% by volume. It is formed during the combustion of all common fuels (wood, coal, coke, natural water gas, petrol, paraffin) all of which contain carbon.

C_(S) + O_2 (g)  →CO_2 (g)

The presence of carbon dioxide in the air is detected through aspirating air through a boiling tube containing calcium hydroxide solution. After some time, the solution goes milky showing the presence of carbon dioxide.

All animals breathe out carbon dioxide as waste product. The proportion of carbon dioxide remains constant although it is produced in large quantities through these means.

FRACTIONAL DISTILLATION OF LIQUID AIR

• The air is passed through fine filters to remove dust.
• The air is cooled to about -80 degrees Celsius to remove water vapour because it can result into serious blockage of pipes.
• The cold air is compressed to about 100 atmospheres of pressure. This warms up the air-so it is passed into a heat exchanger to cool it down again.
• The cold, compressed air is allowed to expand rapidly and that cools it still further.
• Process of compression followed by expansion is repeated until the air reaches a temperature below -200 degrees Celsius. At this temperature most of the air liquefies.

The gases are collected at their different boiling points. Refer to the table above.

OXYGEN

MEASURING THE PERCENTANGE OF OXYGEN IN THE AIR

Consider 100 cm³ in a syringe passed back and forth over heated copper granules as shown on the diagram below.

2KClO₃ (_s) → 2KC1(_S) + 3O₂(g)

Since oxygen is almost the same density as air, it cannot be collected by displacement of air. The gas is collected over water.

From Hydrogen Peroxide (H₂O₂)

Hydrogen Peroxide (H₂O₂) is added drop by drop to Manganese (IV) Oxide which catalyses decomposition.

2H₂O₂(aq)  → 2H₂O(_aq) + O₂(g)

Oxygen is collected over water. Passing the gas over anhydrous calcium chloride you III may collect dry oxygen.

From Hydrogen Peroxide (H₂O₂) and Potassium Manganate (VII)

Hydrogen Peroxide (H₂O₂) is added drop by drop to Potassium Manganate (VII) in the presence of dilute sulphuric acid. Oxygen is liberated until all the Manganate (VII) is decomposed. The mixture becomes colourless.

H₂O₂(aq)+2 KMnO_{4 (aq}) + 3H₂SO₄(_aq) →K₂SO₄(_aq) + 2MnSO₄ (_aq) + 8H₂O(l) + 5O₂ (g)

Oxygen is collected over water. Passing the gas over anhydrous calcium chloride you may collect dry oxygen.

PROPERTIES OF OXYGEN

• Colourless
• Odourless,
• Neutral
• Slightly soluble in water
• Has approximately the same density as air
• Reacts with metals and non-metals to form basic and acidic oxides.

USES OF OXYGEN

The uses of oxygen can be classified as medical, industrial, recreational and for research purposes. The uses of oxygen are as follows:

• Large quantities are used to convert pig iron into steel and for producing very hot flames for welding by mixing with gases such as ethylene (acetylene).
• It is used in hospitals to help patients with breathing difficulties.
• People such as mountaineers and divers use oxygen.
• It is carried in space rockets so that the hydrogen, and kerosene fuel can bum.
• Space shuttles use oxygen gas in fuel cells, which convert chemical energy into electrical energy.
• Astronauts must carry their own supply of oxygen as do fire fighters.
• It is used to restore life in polluted lakes and rivers and in the treatment of sewage.

ACTION OF OXYGEN ON METALS AND NON-METALS

Action of oxygen with Metals

Oxygen reacts with metals to form basic oxides. It reacts with some metals more readily than others. The oxides of K, Na and Ca readily react with water to form a hydroxide.

CaO(_S) +H₂O(i)→Ca (OH)₂(_aq)

The hydroxides of K and Na are very soluble in water while Ca is sparingly soluble. The solution turns litmus paper blue.

The oxides of K, Na, Ca, Mg, and A1 are not reduced by hydrogen. Oxides of Hg, Ag and Au are decomposed when heated.

Action of oxygen with Non-metals

Oxygen reacts with non-metals to form acidic oxides.

S(s)⁺ O₂ (g) → SO_2(g);

SO₂ (_g)+ H₂0(i) →H₂SO₃ (_aq)

(s)+ 5O₂ (_g) →P₄O₁₀ (_S);

2NO_(g) +O_{2 (g)} →2NO_{2 (g)};

CO₂ (_g)+ H₂O(i) →H₂ CO₂(aq)

P₄O_10 (S)+ 6H₂O(i)→4H₃PO₄ (aq)

2NO₂ (_g)+ H₂O (l)→HNO₃ (aq) + HNO₂ (aq)

Non-metal oxides dissolve in water to form acids. The acids turn the litmus paper red.

CARBON DIOXIDE

Laboratory Preparation of Carbon dioxide

Carbon dioxide is produced by reacting carbonate with dilute acid.

Example

CaCO_3(s) + 2HCl(aq) → CaCl₂ (aq) + H₂O(1) + CO₂(g)

The gas is collected through the wafer in a gas jar. If dry gas is required, it is passed through concentrated sulphuric acid and collected through upward displacement of air.

Industrial Preparation of Carbon dioxide

Carbon dioxide is prepared by passing air through a thick layer of white-hot coke in a producer. Strong heat is liberated in the process.

C(_s) + O₂(_g)→ CO_2(g)

As the gas rises, it reacts with white-hot coke. This absorbs a lot of heat.

CO₂ (g) CO₂(_g) + C(_S) →2CO(_g)

The resulting gas is a mixture of carbon monoxide and carbon dioxide. The gas is mixed with air further to produce carbon dioxide.

2CO_g + O₂(g) + Cg) → 2C0₂(g)

Properties of carbon dioxide

• Colourless
• Sparingly soluble in water to form weak carbonic acid

H₂O (l) + CO₂ (_g)→ H₂CO₃ (l)

• Denser than air
• Extinguishes a lighted splint
• Supports combustion for strong substances (e.g., Mg)

2 Mg(_S) + CO₂(_g)→ 2MgO_(s) + C(_S)

• Reacts with strong alkali to form carbonates and bicarbonates. For example, Calcium Hydroxide to produce a milky stuff. The milkish is due to a suspension of the insoluble substance, calcium carbonate.

CO₂ (_g) + Ca(OH)₂₍aq) → CaCO₃(s) + H₂O₍₁₎

When bubbles through the liquid continue, it will eventually become clear. This is because of the formation of the soluble calcium hydrogen carbonate.

CaCO₃(_S) + H₂O (l) + CO₂(_g)→ Ca(HCO₃)₂(aq)

It is significant to note that carbon dioxide reacts with strong alkalis to form carbonates. Excess carbon dioxide results in hydrogen carbonate being formed.

Na₂CO₃(_aq) + HO(l) + CO₂(_g) → 2Na(HCO₃)(_S)

Uses of Carbon dioxide

• Carbonated Drinks: Large quantities are used to make soda, mineral water as well as beer. The carbon dioxide gas is bubbled into the liquid under high pressure, which increases solubility.
• Fire extinguishers: It is used in fire extinguishers for use in electrical fires. Carbon dioxide is denser than air and forms a layer around the burning material. It covers the fire and starves of oxygen. Carbon dioxide does not burn and so the fire is put out.
• Refrigerant: Solid carbon dioxide (dry ice) is used for refrigerating ice cream, meat, soft fruits as it is colder than ice and it sublimes.
• Special effects: Carbon dioxide is used to create smoke effects in concerts and TVs. Dry ice is placed in boiling water and it forms thick cloud of white smoke. It stays close to the floor due to its density.

•           Heat transfer agent: carbon dioxide gas is used for transferring heat to some nuclear power stations.