Hydrogen : Hydrogen From Acids

Hydrogen is evolved by the action of zinc, magnesium and iron on dilute hydrochloric and sulphuric acids and by the action of concentrated hydrochloric acid on aluminium and tin. Nitric acid does not give hydrogen unless cold 2 per cent, acid and magnesium are used: all other metals give various oxides of nitrogen, ammonia, etc., but no hydrogen. The rate of liberation of hydrogen with a particular metal depends on the strength of the acid, a property which must not be confused with the concentration.

Expt. 5. - Arrange three flasks with delivery tubes under graduated tubes in a pneumatic trough

Fig: Hydrogen evolution by zinc

Experiment to compare rates of evolution of hydrogen by zinc from different acids.

. In each place 5 gm. of zinc, and pour in 50 c.c. of solutions of hydrochloric (36-5 gm. per litre), sulphuric (49 gm. per litre), and acetic (60 gm. per litre) acids. All these solutions contain i gin. of acidic hydrogen per litre. Add i c.c. of dilute copper sulphate to each, and after a minute has elapsed fit on the corks and observe the rate of collection of gas. The "strong" acids (hydrochloric and sulphuric) react much more rapidly than the "weak" (acetic), and hydrochloric acid more rapidly than sulphuric.

The usual laboratory method for the preparation of hydrogen is to act on granulated zinc with dilute sulphuric or hydrochloric acids:

Zn + H₂SO₄ = ZnSO₄ + H₂

Zn + 2HCl = ZnCl₂ + H₂

Expt. 6. - A bottle is one-third filled with granulated zinc and fitted with a tap-funnel and delivery tube

Fig: Preparation of hydrogen

. Diluted hydrochloric acid (1 vol. of concentrated acid to 4 volumes of water), or dilute sulphuric acid (1 vol. of concentrated acid carefully poured into 5 vols. of water, with stirring), is dropped in. If very pure zinc is used, the reaction may be slow, but may be accelerated by adding a few drops of copper sulphate solution. The gas is collected over water in jars, care being taken to allow all the air to be displaced from the apparatus: a little gas collected in a test-tube should burn quietly, and not explode, as when air is present.

If the solution of zinc in sulphuric acid is filtered from black particles of lead or copper which were contained in the metal, slightly evaporated and set aside, colourless prismatic crystals of zinc sulphate ("white vitriol"), ZnSO₄,7H₂O, separate.

Iron turnings may be used instead of zinc: Fe + H₂SO₄ = FeSO₄ + H₂, but the gas then has an unpleasant smell, due to hydrocarbons derived from iron carbide, Fe₃C, in the metal, and burns with a greenish flame. The solution in the flask, after filtration and slight evaporation, deposits on cooling green crystals of ferrous sulphate ("green vitriol"), FeSO₄,7H₂O.

Purer gas may be obtained from dilute sulphuric or hydrochloric acid and magnesium: Mg + H₂SO₄ = MgSO₄ + H₂; or by the action of a solution of mercuric chloride, slightly acidified with hydrochloric acid, on aluminium: 2Al + 6H₂O = 2Al(OH)₃ + 3H₂. This gas is odourless. The liquid mercury deposited on the aluminium prevents the formation of a protective film of aluminium hydroxide.

Instead of a flask, a Kipp's apparatus

Fig: Kipp's apparatus

may be used, the metal being placed in the central globe and acid poured in the top funnel until the lower bulb is full, and the metal covered with acid. When the tap is closed evolution of gas continues until the liquid is forced by pressure partly into the upper globe, and the metal is brought out of contact with the liquid, when the action ceases.

The gas from zinc and sulphuric acid may contain some hydrogen sulphide, especially if the acid is hot, or the zinc not quite pure. The gas from commercial zinc and acid may be purified by passing over red-hot copper turnings, or through wash-bottles containing a saturated solution of potassium permanganate, followed by bottles containing a 5-10 per cent, solution of silver nitrate. Impurities such as hydrogen sulphide, phosphide and arsenide, and oxides of nitrogen, sulphur dioxide, and volatile hydrocarbons, are removed. Unless hot copper is used, a little oxygen remains, but may be removed by a solution of chromous chloride or by passing over red-hot copper. On a large scale, bleaching powder or a solution of bromine is used to remove arsenic compounds from the gas.

Hydrogen is dried by passing over granular calcium chloride, or broken sticks of caustic potash, in a tower

Fig: Drying tower

final drying may be effected by phosphorus pentoxide dusted over plugs of glass-wool in a U-tube

Fig: Phosphorus pentoxide drying tube

Sulphuric acid should not be used, as sulphur dioxide is formed: H₂SO₄ + H₂ = SO₂ + 2H₂O, unless the acid is cooled in a freezing mixture.

Protein crystallography
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Hydrogen
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