SS2 Chemistry Lesson Note on Industrial Preparation of Oxygen

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INDUSTRIAL PREPARATION

Oxygen is produced, in large quantities from the following sources.

1. From liquefied air: Oxygen is obtained industrially from atmospheric air by liquefaction, followed by fractional distillation of the liquefied air.

• Atmospheric air is purified by the removal of Carbon(IV) oxide, water vapour and dust.
• The purified air is compressed at a pressure of about 200 atmospheres, heated and then cooled. It is then allowed to expand suddenly, for further cooling.
• By successive heating and cooling, the pure air becomes liquefied at about -200 [73k]. This process is called LIQUEFACTION.
• The liquefied air is then fractionated by boiling. Nitrogen, being more volatile, boils first at – 196 [77k], leaving oxygen, which is about 99.5% pure. This separation technique is called FRACTIONAL DISTILLATION.

Liquefied oxygen is usually stored in cylinders for medical and industrial use.

2. By electrolysis: Oxygen, like hydrogen is obtained during the electrolysis of dilute H₂SO_4.

EVALUATION

1. State two main processes involved in the manufacture of oxygen from air.
2. Outline industrial production of oxygen from air.

PERIOD 3: PHYSICAL AND CHEMICAL PROPERTIES OF OXYGEN

PHYSICAL PROPERTIES

1. Pure oxygen is colourless, odourless and tasteless.
2. It is slightly denser than air.
3. It is slightly soluble in water.
4. It is a diatomic (O₂) gas that is neutral to litmus paper.
5. It boils (turn to gas) at -183⁰C and solidifies at -225⁰C

CHEMICAL PROPERTIES

• Reaction with metals: Metals such as Na, K, Ca, Mg, Al, Zn burn brightly in oxygen. Metals such as silver, Gold and platinum do not react with oxygen. Oxides of metals are generally basic. e.g.

4Na_(s)+  O_2(g)    →    2Na₂O_(g)

C_(s)   +    O_{2(g)        →}       CO_2(g)

• Reaction with non-metals: Some burning non-metals when lowered into a jar of oxygen, burn to form acidic oxides or acid anhydrides which when dissolved in water form acidic solutions. E.g.

CO_2(g) + H₂O_(l)  → H₂CO_3(aq)

• Test for oxygen: Oxygen is identified by its ability to rekindle a glowing splint. Nitrogen (I) oxide does this as well but is distinguished from oxygen by its pleasant sickly smell while oxygen is odourless. Oxygen also reacts with nitrogen (II) oxide to give brown fumes of nitrogen(IV)oxide which nitrogen(I)oxide does not do.

NO _(g) + O_2(g)    →        NO_2(g)

Reaction of oxygen:

Oxidation and combustion:

Many compounds are oxidized when heated in oxygen (combustion). E.g. all hydrocarbons burn in oxygen to form carbon (iv) oxide and water, sulphides burn in oxygen to give sulphur (iv) oxides.

1. With carbon

C_(s) + O_{2(g)          →}    CO_2(g)

2. With sulphur

S_(s) + O_{2(g)   →}        SO_2(g)

3. With phosphorus

4P_(s) + 5O_2(g)        →    P₄O_10(s)

4. Formation of water: Hydrogen burns violently in oxygen with a pale –blue flame, to produce water.

2H_2(g) + O_{2(g)       →}  2H₂O_(l)

5. Formation of Ozone: When a silent electric discharge is passed through oxygen, gaseous ozone, O₃, (trioxygen) is formed. The reaction is reversible:

3O₂(g)           ⇌          2O_3(g)

Ozone is an allotrope of oxygen. Allotropes are two or more different forms of the same element, in the same physical state. The phenomenon is called allotropy. Other elements that exhibit allotropy are sulphur, carbon and phosphorous.

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