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Oxygen, Its Properties, Preparations, And Uses

Oxygen is the most abundant element on earth. It is found almost in everything in nature and also in free state.

Laboratory Preparation of Oxygen

Oxygen can be prepared in a number of ways in the laboratory. These include:

(a). Heating a mixture of potassium trioxochlorate(V), KClO₃ and maganese(IV) oxide, MnO₂.

The MnO₂ acts as a catalyst. The reaction is actually the decomposition of KClO₃.

2KClO_3(s) → 2KCl_(s) + 3O_2(g)

(b). The decomposition of hydrogen peroxide, H₂O₂ using manganese(IV) oxide - this does not require heating.

Hydrogen peroxide is added drop wisely unto MnO₂.

2H₂O_2(aq) → 2H₂O_(l) + O_2(g)

Or by the drop wise addition of hydrogen peroxide on acidified KMnO₄

5H₂O_2(aq) + 2KMnO_4(aq) + 3H₂SO_4(aq)  → K₂SO_4(aq) + 2MnSO_4(aq) + 8H₂O_(l) + 5O_2(g)

(c). The reaction between water and sodium peroxide.

Hydrogen peroxide is formed, and it immediately decomposes by the catalytic effect of the OH^- ions in solution.

Na₂O_2(s) + 2H₂O → H₂O₂ + 2Na⁺ + 2OH^-

Then, 2H₂O₂ → 2H₂O + O₂

(d). Application of heat on trioxonitrate(V) salts of metals.

Trioxonitrate(V) salts of metals, e.g, sodium trioxonitrate(V), NaNO₃ give off a part of their oxygen upon being heated. NaNO₃ loses one third of its oxygen.

2NaNO_3(s) → 2NaNO_2(s) + O_2(g)

(e). Application of heat on certain oxides of the least active metals, e.g., mercury oxide, HgO and silver oxide, Ag₂O. These oxides undergo complete dissociation when heated.

2HgO_(s) → 2Hg_(s) + O_2(g)

2Ag₂O_(s) → 4Ag_(s) + O_2(g)

(f). Application of heat on oxides of certain metals with more than one oxidation state, e.g., lead(IV) oxide, PbO₂ and mangan- ese(IV) oxide, MnO₂.

These oxides give off only a part of their oxygen when they are heated. Such reactions usually require very high temperatures.

2PbO_2(s) → 2PbO_(s) + O_2(g)

3MnO_2(s) → Mn₃O_4(s) + O_2(g)

(g). The electrolysis of water.

Oxygen is produced at the anode - see the electrolysis of water for details.

Test for Oxygen

Oxygen can be distinguished from all other gases except dinitrogen oxide, N₂O by its rekindling of a glowing splint of wood.

It is however distinguished from N₂O by the following observations:

(1). Oxygen does not have smell, while N₂O has a sweet, sickly smell.

(2). Oxygen produces brown fumes of nitrogendioxide, NO₂ with nitrogen monoxide,

2NO_(g) + O_2(g) → 2NO_2(g) (brown fumes),

while N₂O does not.

Properties of Oxygen

Physical Properties:

(1). It is colourless, has no odour and is neutral.
(2). It is slightly soluble in water.
(3). It has almost the same density as air.
(4). It freezes at 54K.
(5). It has a boiling point of 90K at 1 atmosphere pressure (i.e. 760 mm Hg).
(6). It is very active and reacts with many metals and non-metals to form basic and acidic oxides respectively.

Basic oxide - examples include MgO, Na₂O and CaO;
Acidic oxide - examples are CO₂, SO₂ and P₄O₁₀ (acidic oxides of non-metal are also called acid anhydrides).

Summary of Reactivity of Oxygen with Metals

K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu - - show decreasing readiness to form oxides when heated in air, with Cu the least reactive with oxygen.

Hg, Ag, Au - - these metals show the least readiness to form oxides. Their oxides are easily decomposed to the
metal and oxygen.

K, Na, Ca, Mg, Al, Zn - - the oxides of these metals are not reduced to the metals by heating in a stream of hydrogen,
carbon or carbon(II) oxide.

Fe, Pb, Cu - - the oxides of these metals are reduced to the metals by heating in a stream of hydrogen, carbon or carbon(II) oxide.

Note: the more readily a metal combines with oxygen to form an oxide, the less readily it will be reduced to the metal by either heating in a stream of hydrogen or CO.

Therefore, the oxides of K to Zn are not reduced, while those of Fe and below are reduced.

Chemical Properties:

(1). Reaction with compounds - most hydrocarbons and compounds of carbon, hydrogen and oxygen burn in oxygen to form carbon (IV) oxide and water.

CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(g)

C₂H₅OH_(l) + 3O_2(g) → 2CO_2(g) + 3H₂O_(g)

4NH_3(g) + 5O_2(g) → 4NO_(g) + 6H₂O_(l)

Uses of Oxygen

(1). Used as an aid for breathing where problem of breathing arises. Example in high altitude flying or climbing, and when a patient is under anesthetics.

(2). In the oxyacetylene (i.e. oxygen-ethyne) flame - used in welding and cutting steel plate - due to the very high temperature of the flame (about 2200^oC).

(3). Used in the L-D process for making steel.

Commercial Production of Oxygen from Liquid Air

The process involves air firstly liquefied by compression, cooling, expansion and successive cooling. Then by fractional distillation of the liquid air - oxygen is separated.

Liquid air contains mainly oxygen and nitrogen. Nitrogen evolves first at 77 K while oxygen evolves later at 90 K, 760 mm Hg.