CHAPTER XVII.

OXIDATIONS AND REDUCTIONS.

ANY chemical change which results in an increase in the ratio of the negative to the positive constituents of a compound is called an oxidation. Thus the change of FeO to Fe,O,, or of FeSO, to Fe,(SO,),, or of HgI to HgI,, is a process of oxidation. Any change which results in an increase of the ratio of the positive to the negative constituents of a compound is called a reduction. Processes of oxidation and reduction usually occur together; one part of the complete change is called an oxidation, and the other part is called a reduction. Oxidation is sometimes effected by the direct addition of

oxygen.

Exp. 1. Cut a piece of sodium from the inside of a stick of this metal, and notice that the bright surface is at once covered with a non-lustrous film; this film is sodium oxide (Na,O).

Exp. 2. Place a little very finely divided iron in a crucible, counterpoise the whole, then allow it to stand in the air for about an hour, and counterpoise again; there is no change of mass, nor is there any change in the appearance of the iron. Now heat the crucible over a Bunsen-lamp; the iron glows; allow to cool, and counterpoise again. There has been an increase in weight; the iron has been oxidised (to Fe,O) by the oxygen of the air.

Exp. 3. Place a very small piece of lead, and a very small piece of tin, on charcoal, and direct the outer flame, i.e. the oxidising flame, of a blowpipe on to each in succession; the lead is slowly oxidised to a yellowish film of lead oxide, and the tin to a nearly white film of tin oxide.

But oxidations are more frequently accomplished by decomposing some compound of oxygen in contact with the body to be oxidised.

Exp. 4. Heat a few pieces of charcoal on an iron tray placed over a large Bunsen-lamp; then allow concentrated nitric acid to drop on to the hot charcoal. The charcoal is burnt (to carbon dioxide) by the oxygen furnished by the decomposition of the nitric acid (s. Exp. 13 of Chap. XIV.).

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Recall Exps. 11 and 15 of Chap. XIV. wherein Sb,O, was oxidised to Sb., and Sb,O,, and phosphorus was oxidised to phosphoric acid, by interacting with concentrated nitric acid.

In all these experiments the nitric acid is reduced at the same time as the other body is oxidised.

Exp. 5. Heat a little potassium chlorate in a dry test tube until it melts, and apparently boils; then drop in a few small pieces of charcoal; notice the rapid burning of the charcoal, and prove that carbon dioxide is evolved. Allow to cool, dissolve the solid residue in water, and prove that the solution contains potassium chloride. The chlorate has been reduced, while the carbon has been simultaneously oxidised [2KCIO, (heated) + 3C = 2KCl + 3CO2].

Exp. 6. Heat a little potassium nitrate in a crucible until it melts, then throw in a few small pieces of sulphur; after cooling, dissolve in water and prove that the solution contains a sulphate.

The sulphur has been oxidised while the nitrate has been simultaneously reduced to nitrite [KNO, (heated) = KNO, + 0]. Recall Exp. 15 of Chap. XV., wherein a salt of manganese was oxidised by fusion with nitre in presence of potash.

Exp. 7. To a solution of potassium nitrite (KNO), acidulated with a little sulphuric acid, add a solution of potassium permanganate, drop by drop, until the permanence of a slight pink colour shews that a trace of permanganate remains unchanged. Now prove (1) the presence of a nitrate, (2) the absence of a nitrite, in this solution (s. tests in Exp. 4 Chap. VII. and Exp. 9, Chap. XIV.).

The potassium nitrite (KNO,) has been oxidised to nitrate (KNO,), and the permanganate (KMnO,) has been reduced to MnO and K,O which have reacted with the sulphuric acid to form K,SO, and MnSO, (compare Exps. 16 and 17 of Chap. XV.).

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Exp. 8. Prepare a little very finely divided sulphur by adding hydrochloric acid to ammonium sulphide solution; now add some more HClAq, heat nearly to boiling, drop in a crystal of potassium chlorate, continue to heat and to add KClO, in very small successive quantities, until the sulphur has completely disappeared. Now prove that the solution contains a sulphate.

By the interaction of HCIAq and KCIO, a mixture of chlorine and oxides of chlorine is formed; but chlorine oxides are very easily decomposed by heat to chlorine and oxygen; hence in the foregoing process oxygen was freely evolved in contact with finely divided sulphur; the sulphur was thus oxidised to sulphuric acid. At the same time the potassium chlorate was reduced to chloride.

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Recall Exp. 12 of Chap. XIV. wherein Bi̟ O̟, was oxidised to Bi O, by forming and decomposing KCIÓÅq in contact with the Bi O, in presence of much potash.

2 5

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Exp. 9. To a solution of potassium chlorate add a few pieces of zinc and a little dilute sulphuric acid; hydrogen is evolved; after a little filter some of the liquid and prove the presence of a chloride in the filtrate. The chlorate (KCIO) has been reduced to chloride (KCl) by interacting with the hydrogen evolved in contact with the chlorate, and simultaneously the hydrogen has been oxidised to water.

Exp. 10. To a solution of mercuric chloride (HgCl2) add a little stannous chloride solution (SnCl2); a white pp. of mercurous chloride (HgCl) is formed; now add more stannous chloride and heat; the colour of the pp. is changed from white

to grey. This grey solid is very finely divided mercury; by collecting it on a filter, drying, and rubbing in a mortar, globules of mercury are obtained. The reactions may be thus represented ;

(1) 2HgCl, Aq + SnCl2Aq = SnCl ̧Aq + 2HgCl,

(2) 2HgCl + SnCl2Aq = SnCl ̧Aq + 2Hg.

While the HgCl, is reduced to HgCl, and this is further reduced to Hg, the SnCl, is oxidised to SnCl.

Exp. 11. Add some sulphuric acid to a solution of sodium sulphite (Na,SO) and prove that sulphur dioxide (SO) is evolved, by leading the gas into a dilute solution of potassium

dichromate and noticing the green colour (due to the formation of chromic sulphate) which the liquid assumes (comp. Exp. 14, Chap. XV.).

Now place a few crystals of sodium sulphite in a little ferric sulphate solution, and add some hydrochloric acid; heat to boiling, and continue to boil until only a very little SO, is coming off (SO, is easily detected by its smell). Then cool, and prove that the liquid contains ferrous sulphate.

In this reaction ferric sulphate (Fe,(SO)) has been reduced to ferrous sulphate (FeSO,), and sulphur dioxide (SO) has been simultaneously oxidised to sulphur trioxide (SO) which has interacted with the water present to produce sulphuric acid :

Fe¿(SO), Aq + SO2+ 2H2O = 2FeSOAq + 2H2SOAq.

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Exp. 12. Place a little mercury in a solution of silver nitrate; silver is slowly precipitated and some of the mercury passes into solution.

Place a piece of copper in a solution of mercurous nitrate; mercury is slowly precipitated and some of the copper passes into solution.

Place a piece of iron in a solution of copper nitrate, or sulphate; copper is slowly precipitated and some of the iron passes into solution.

These changes may be thus represented :

(1)

2AgNO,Aq + Hg = Hg(NO„),Aq + 2Ag, (2) Hg(NO), Aq + Cu = Cu(NO), Aq + Hg,

(3) Cu(NO), Aq + Fe = Fe(NO), Aq + Cu.

In each case the metal placed in the solution reduces the salt in solution, the metal itself being simultaneously oxidised.

Compounds which readily part with oxygen, or with a part or the whole of their negative constituents, are often called oxidising agents; compounds, or elements, which readily remove oxygen, or negative elements such as chlorine &c., or negative groups of elements such as SO, &c., and combine with the oxygen &c. so removed, are usually called reducing agents.

Reference to "ELEMENTARY CHEMISTRY." Chap. XI. pars.

183 to 186.

CHAPTER XVIII.

STRONG AND WEAK ACIDS.

VARIOUS meanings have been given to the terms strong and weak as applied to acids and bases. The sense in which the terms are now generally used is shewn by considering the ordinarily occurring interaction between equivalent quantities of an alkali MOH and two acids HX and HY in dilute aqueous solution, to form two salts MX and MY in solution. In most cases portions of the alkali interact with each acid, so that when the interacting bodies have settled down into equilibrium the solution contains the two salts MX and MY, and also the two acids HX and HY. If the acid HX is a stronger acid than HY a greater quantity of the salt MX than of the salt MY will be formed when the change is completed. If for instance equivalent quantities of potash (KOH), nitric acid (HNO,), and sulphuric acid (H2SO1), are mixed in dilute aqueous solution, about of the potash interacts with the nitric acid to form the salt KNO,, and about of the potash interacts with the sulphuric acid to form the salt K2SO; nitric acid is therefore said to be a stronger acid than sulphuric, and the relative strengths, or affinities, of the two acids for potash are said to be approximately in the ratio 2 : 1.

At a later stage of the course quantitative Exps. will be performed in illustration of the meaning of the terms strong and weak acids (Part III. Chap. III.); meanwhile Exps. will be conducted to shew that one acid can sometimes partially or wholly replace another from combination with a base, and to illustrate the conditions under which such replacement is effected.

Exp. 1. To two quantities of water coloured light yellow by addition of methyl orange', add a little boric acid