CHAPTER XV.

CLASSIFICATION OF ELEMENTS (continued).

THE elements chromium, manganese, and iron, are placed in the same class.

These elements form hydrated oxides M2O. 3H2O where M = Cr, Mn, or Fe.

Exp. 1. To solutions of (i) ferric chloride (Fe,Cl ̧), and (ii) chromic chloride (CrCl), add a slight excess of ammonia solution. Collect the pps. which form; wash them repeatedly with hot water; and prove that a small quantity of each dissolves readily in hydrochloric acid; set these solutions evaporating in the draught cupboard, (see next Exp.). Now place the rest of each pp. in a small basin and heat these at 100o so long as water comes off. The solids have now the composition MO.3H0 where M = Fe or Cr. Heat these solids strongly over a Bunsen-lamp; water is removed and the oxides M,O, remain. Prove that the oxides thus prepared are nearly insoluble in concentrated hot hydrochloric acid.

From these reactions we would expect that a solution of a manganic salt (say manganic chloride) would interact with ammonia to produce a pp. of the composition Mn,O,.3H0. But the salts of manganese corresponding to Fe,Cl, and CrCl are extremely unstable and can hardly be obtained even approximately pure.

Exp. 2. Add some ammonium chloride and then a slight excess of ammonia to an aqueous solution of manganous chloride (MnCl2); an almost white pp. of manganous hydrate, MnO.H2O, forms. Pour the contents of the tube into a large stoppered bottle, and shake up the pp. repeatedly, removing the stopper at intervals and blowing in fresh supplies of air. The pp. slowly turns brown; the brown solid eventually pro

duced is hydrated manganic oxide, Mn,O,.H2O. (Compare the composition of this compound with that of the compounds produced from ferric and chromic chlorides; M2O.3H2O.) The change which has proceeded in the bottle may be thus shewn; 2MnO.H2O+O (from air) = Mn2O¿.H2O+ H2O. Collect the brown solid on a filter, wash it well with warm water, dissolve a portion in warm concentrated hydrochloric acid, and set this solution evaporating in the draught place; keep the rest of the Mn2O.H2O for the next Exp.

When the solutions in hydrochloric acid of the hydrated oxides M2O, have been evaporated nearly to dryness, each yields a solid on cooling; these solids are yellow-brown ferric chloride Fe,Cl, greenish-violet chromic chloride Cr2Cl, and slightly pink manganous chloride MnCl2.4H2O.

Exp. 3. To solutions of ferric chloride (Fe,Cl) and chromic chloride (CrCl) add a solution of caustic potash until a pp. is formed in each case. These pps. are Fe,O,.3H,O and Cr2O.3H0 respectively. Now add a considerable excess of potash to each pp., the Cr2O,.3H2O dissolves but the precipitated ferric hydrate remains unchanged. Now add a considerable quantity of potash to the pp. of Mn,O,.H2O remaining from Exp. 2; it remains unchanged. Hydrated chromic oxide therefore differs from the corresponding compounds of iron and manganese in being soluble in a solution of caustic potash (or soda). As solutions of oxides in caustic alkalis often contain salts the negative parts of which are composed of the oxides in question (e.g. P2O dissolves in KOHAq forming P,0.3K,OAq), it may be that the fact of the solubility of CrŮ.3H0 in potash shews that Cr2O,.3H2O is slightly acidic in its functions. (s. further Exps. 7, 10, and 11 in this Chap.).

The hydrated ferrous and chromous oxides MO.H2O where M= Cr or Fe are very unstable; they quickly combine with oxygen and become M2O.3H2O. But it is possible to prepare FeO.H.O nearly free from ferric hydrate.

Exp. 4. Arrange an apparatus as shewn in fig. 34. A is a flask containing zinc and dilute sulphuric acid; B and C are each short wide test tubes supported in stands; each of these tubes is about half filled with distilled water, and a very little sulphuric acid is also poured into C; a and b represent pieces of caoutchouc tubing. Hydrogen is passed through the whole apparatus for some minutes; the corks are then loosened in

B and C (the hydrogen is allowed to continue passing through the tubes); the contents of each tube are boiled for 5 or 10 minutes, to remove the air dissolved in the water. The corks

are now replaced in B and C and the passage of hydrogen is continued; when the water in B and C is cold, the corks are removed for an instant, a piece of solid potash is dropped into B, and a small piece of granulated zinc and a clear crystal of ferrous sulphate (FeSO4.7H2O) into C, and the corks are quickly replaced. After a minute or two the tube B is inverted; the hydrogen then forces the potash solution from B into C, and the potash and ferrous sulphate interact to produce white ferrous hydrate and potassium sulphate; thus 2KOHAq + FeSO2Aq = FeO.H2O + K2SOAq. Now remove the cork from C, and pour the contents into a vessel where they are freely exposed to air; the colour of the pp. soon changes to greenish, then to greenish-brown, and then to brown (FeO.3H2O is eventually produced).

The elements we are considering form oxides of the composition M2O1.

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Exp. 5. Dissolve about 20 grams of ferrous sulphate in cold water; convert about of this into a solution of ferric

sulphate by adding some sulphuric acid, heating to boiling, and adding concentrated nitric acid, drop by drop, so long as a change of colour is produced. Allow the solution of ferric sulphate to cool; then mix it with the ferrous sulphate solution; add a slight excess of ammonia solution; boil for some time; allow the pp. of Fe, which is produced to settle, wash it thoroughly with hot water by decantation, transfer it to a porcelain dish, and dry it at about 60°.

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Prove that the Fe,O, you have prepared dissolves in HCIAq with production of both ferrous and ferric chloride; making use of the facts (1) that potassium sulphocyanide solution (KCNSAq) gives a deep red colour [Fe(CNS), Aq] with ferric salts, (2) that potassium ferricyanide solution (K FeCy,Aq) gives a pp. of prussian blue (Fe,Cy,) with ferrous salts. Compare this reaction with that which occurs when HClAq and MnO4 interact; prove that manganous chloride (MnCl ̧) is produced, and chlorine is evolved, in this case.

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Contrast the reactions between (1) dilute nitric acid and Fe,O,, (2) dilute nitric acid and Mn,O,; prove that in the former case the Fe,O, dissolves and that the liquid contains both ferrous and ferric nitrate, and that in the latter a brown solid remains undissolved this solid is MnO,—and the solution contains manganous nitrate.

Prove the presence of a manganous salt in the liquid filtered from the insoluble MnO, by adding NH,Aq in slight excess and then NH,HSAq; a buff coloured pp. of MnS is produced.

Besides the oxides examined, manganese forms a peroxide MnO, and chromium a peroxide Cro

Exp. 6. To an aqueous solution of a manganous salt, say MnSO, add a saturated solution of sodium hypochlorite (NaClO), and a considerable quantity of concentrated soda solution; heat until the pp. which forms is very dark brown and apparently homogeneous; collect and wash this pp. It is manganese dioxide or peroxide (MnO).

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Recall the results of Exp. 2 wherein hydrated Mn,O, was obtained by the action of air upon precipitated MnO; in the present Exp. the NaOHAq precipitates hydrated MnO, and at the same time this is oxidised to MnO, by the oxygen produced by the decomposition of the NaČIO; (NaClOAq heated in an alkaline solution gives NaCIAq+0).

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Exp. 7. To about 200 c.c. of a cold saturated aqueous solution of potassium dichromate (K,Cr,O,) add, slowly and

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with constant stirring, about 300 c.c. concentrated sulphuric acid, and allow the mixture to cool. Red crystals of CrO, separate out. (K,Cr2O,Aq + H2SO,K,SO,Aq + H2O + 2CrO2). Place a little glass wool in the neck of a funnel, and pour the liquid through this funnel; when the liquid has drained away from the crystals, remove the latter, by the aid of a platinum or glass spatula, to a clean dry porous tile, and allow them to drain there for a short time; then move the crystals to a fresh part of the porous tile and pour a very little cold water over them; when the water has soaked into the tile, repeat this treatment with another very small quantity of cold water. Now dissolve the crystals in a beaker in as small a quantity of hot water as possible, evaporate the solution and allow it to crystallize. Again collect the crystals, and dry them on a dry porous tile. In this way you obtain chromium trioxide or peroxide (Cro) nearly freed from adhering sulphuric acid. The oxides MnO, and CrO, readily part with a portion of their oxygen, MnO, being reduced to MnO, and Cro, to Cr203

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Exp. 8. To a solution of oxalic acid (H,C,O) add some manganese dioxide (MnO2), and a little sulphuric acid, and heat nearly to boiling. The oxalic acid is oxidised to water and carbon dioxide; and the MnO produced is dissolved in the sulphuric acid forming MnSO,Aq. Prove that CO2 is evolved, and that the liquid gives the ordinary reactions of a manganous salt (s. Exp. 5).

Exp. 9. Place a little of the chromium trioxide (Cro) you prepared in Exp. 7 in a basin and allow a little alcohol (C,H ̧O) to slowly trickle on to it: the alcohol is at once oxidised to aldehyde, and green Cr2O, remains; (3C,HO+2CrO1 = Cr2O2+ 3C,HO+3H,O). Much heat is produced in this process and a portion of the alcohol is usually inflamed.

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Chromium trioxide (or chromium peroxide) CrO, is a markedly acidic oxide,

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Exp. 10. Dissolve the remainder of the chromium trioxide (Cro) prepared in Exp. 7 in water; neutralise the acid solution by potash; evaporate, and allow to crystallise. Yellow crystals of potassium chromate, K,CrO4, separate out. Dissolve these crystals in water; divide the solution into two parts; to one part add AgNO,Aq, and to the other Pb2NO,Aq. In the one