Oxygen : Combustion

The combination of substances with oxygen, when attended with the evolution of heat and light, is called combustion. Substances which burn in air do so with greatly enhanced brilliancy in pure oxygen, since the nitrogen in air acts as a diluent, absorbing part of the heat given off in the combustion.

The combustion of sulphur, phosphorus and carbon, giving acidic oxides, has already been described:

S + O₂ = SO₂; SO₂ + H₂O = H₂SO₃ (sulphurous acid).
2S + 3O₂ = 2SO₃; SO₃ + H₂O = H₂SO₄ (sulphuric acid).
4P + 5O₂ = 2P₂O₅; P₂O₅ + H₂O = 2HPO₃ (metaphosphoric acid).
C + O₂ = CO₂; CO₂ + H₂O < = > H₂CO₃ (carbonic acid).

Expt. 6. Magnesium ribbon, ignited in air and inserted into oxygen, burns with a blinding white light, forming white solid magnesium oxide, MgO, which is weakly basic and turns moist red litmus paper blue. Sodium and potassium, heated in iron deflagrating spoons until they begin to burn, and then lowered into dry jars of oxygen, burn with bright yellow and lilac flames, respectively, forming orange-yellow solid peroxides which dissolve in water with evolution of oxygen and formation of strongly alkaline hydroxides:

2Na + O₂ = Na₂O₂; 2Na₂O₂ + 2H₂O = 4NaOH + O₂;
2K + 2O₂ = K₂O₄; 2K₂O₄ + 2H₂O = 4KOH + 3O₂;

A spiral of iron wire, tipped with a bit of burning wood, burns brilliantly, giving off a shower of bright sparks, when lowered into a bottle of oxygen. Black oxide of iron, Fe₃O₄, is formed in fused globules, which crack the bottle when they fall on it, unless it contains a layer of sand.

A jet of hydrogen burns in a dry jar of oxygen, producing water, which condenses in drops on the cold sides of the jar: 2H₂ + O₂ = 2H₂O. If a jet of oxygen is thrust into an inverted jar of hydrogen, burning at the mouth, the oxygen takes fire, and continues to burn in the atmosphere of hydrogen.

Fig: Oxygen burning in hydrogen

The terms combustible, and supporter of combustion, are, therefore, purely relative.

Expt. 7. Dry barium or strontium chlorate is heated in a vertical spoon until it evolves oxygen freely. A globe of coal gas is then lowered over the spoon.

Fig: Apparatus for combustion

The oxygen from the chlorate, if the latter is sufficiently heated, takes fire, and burns violently in the coal gas, the flame being coloured intensely green or crimson by the volatile barium or strontium compounds, respectively.

Many combustible substances, in a finely divided condition, ignite spontaneously in air or oxygen.

Expt. 8. By means of a brush trace letters on a piece of paper with a solution of phosphorus in carbon disulphide. When the solvent evaporates, the finely divided phosphorus ignites, leaving charred letters on the paper.

Expt. 9. Precipitate a solution of lead acetate with a solution of Rochelle salt, KNaC₄H₄O₆. The white precipitate of lead tartrate, PbC₄H₄O₆, is filtered, washed, and dried in a steam-oven. Small portions are placed in narrow tubes, sealed at one end and drawn out at the other. The tartrate is heated until fumes are no longer evolved, and the tubes are sealed. If a tube, after, cooling, is cut with a file and the finely divided pyrophoric lead shaken out, the metal glows brightly, forming yellow fumes of lead oxide, PbO.

Many substances, such as phosphorus, oxidise slowly when exposed to air, without catching fire, because the heat produced is dissipated too rapidly to raise the mass to the ignition point. Oily cotton-waste, however, may become heated to the ignition point if stored in heaps exposed to air. This process of slow oxidation is known as autoxidation.

Oxygen is absorbed from gaseous mixtures by: (i) a solution of pyrogallol in caustic potash, which turns black (160 grams of KOH, 10 grams of pyrogallol, 130 c.c. of water); (ii) moist phosphorus (this does not glow in pure oxygen); (iii) an acid solution of chromous chloride, CrCl₂, which turns from blue to green, owing to the formation of chromic chloride: 4CrCl₂ + O₂ + 4HCl = 4CrCl₃ + 2H₂O; (iv) by mixing the gas with excess of hydrogen to form a non-explosive mixture, and passing over platinised asbestos at a dull red heat, or gently heated palladium, when water is formed; one-third of the contraction of the gas then represents the oxygen contained in it: 2H₂ + O₂ = 2H₂O (liquid).

Protein crystallography
Chemistry guide
Oxygen
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