Page images
PDF
EPUB

nitric acid that is driven out by the hydrochloric acid. Nitric acid separates arsenious acid from an aqueous solution of arsenite of potash; but when nitrate of potash is ignited in contact with arsenious acid, nitrous acid is evolved and arseniate of potash formed. In this case, the stronger nitric acid is converted by loss of oxygen into the weaker nitrous acid, while on the contrary, the weaker arsenious acid is by access of oxygen converted into the stronger arsenic acid. The affinity of arsenious acid for oxygen + that of arsenic acid for potash overcomes the affinity of nitrous acid for oxygen + that of nitric acid for potash. (Sch. 99.)

The existence of reciprocal affinity has also been inferred from the fact that certain salts, which when dissolved in a small quantity of water decompose one another by double affinity, producing a precipitate of a difficultly soluble salt, give no precipitate in more dilute solutions although the quantity of water present would not be sufficient to hold in solution the less soluble salt which may be produced by the decomposition, if it existed in the separate state. For example, sulphate of lime requires about 400 parts of water to dissolve it; but chloride of calcium, dissolved in about 200 parts of water (which would produce more than an equal quantity of sulphate of lime) gives no precipitate with sulphate of potash. Many chemists conclude from this that when this large quantity of water is present, the chloride of calcium is not decomposed by the sulphate of potash, since if sulphate of lime were formed, more than half of it would be precipitated. But it is simpler to suppose that the formation of sulphate of lime takes place even in this case, but that the presence of the chloride of potassium, which is formed at the same time, renders it more soluble than it otherwise would be. Similarly, it was long since observed by Guyton-Morveau that lime-water rendered turbid by passing carbonic acid gas through it becomes clear again on the addition of sulphate of potash or chloride of potassium, as if the presence of these salis rendered the carbonate of lime more soluble: and Karsten (Schrift. d. Berl. Akad., 1841) has shown that many easily soluble salts are rendered more soluble by the addition of other salts. These mutual affinities of salts, and the greater solubility in water thereby produced, explain the occurrence of carbonate and sulphate of lime, carbonate of magnesia, &c., in mineral waters, in quantities greater than pure water could dissolve; so that this phenomenon by no means renders it necessary to suppose that such waters contain salts which are incompatible with each other, i. e., which at the given temperature would decompose one another and form a precipitate, if the quantity of water present were smaller. Comp. Berthollet (Statique Chim., 1, 103, and 129); Brandes (Schw. 43. 153, 46, 433.)

Many other facts relating to the theory of reciprocal affinity but requiring more accurate investigation may be found in Scheele (Opusc. 1, 223): Grotthuss (Scher. N. Bl. 275): N. Fischer (Pogg. 7, 263): Berthollet (Statique Chim. 1, 81, 99, 100, 401) and Dulong (Ann. Chim. 82, 273, also Schw. 5, 369). In the experiments of the last-named philosopher on the decomposition of insoluble by soluble salts, variation of temperature, which might have produced opposite results, does not seem to have been sufficiently attended to.

2. Circumstances and Results of Decomposition. a. Change of temperature. Since heat is generally set free in the combination of bodies, an equal quantity of heat must also become latent at their decomposition. Nevertheless most decompositions are accompanied by rise of temperature sometimes amounting to the most vivid combustion. With reference to this matter the following cases must be distinguished :

a. When peroxide of hydrogen is decomposed by heat or by the contact of pulverized bodies into water and oxygen gas, a great deal of heat and even light is developed, notwithstanding that a considerable quantity must be rendered latent in the formation of the gas. This remarkable instance leads us to suppose that in the formation of this very loosely united compound heat is not set free but rendered latent, that in fact oxygen gas + heat enters into combination with the water. -A similar constitution may exist in certain metallic bromates which when heated are resolved with sudden incandescence into a metallic bromide and oxygen.

B. When bodies separate in the solid form from their solution in a liquid or a gas, in consequence of their cohesion being increased by cooling, a development of heat takes place, the heat of fluidity which the bodies had previously absorbed in the act of solution being set free.

y. Most decompositions take place in consequence of weaker affinities being overcome by stronger ones. Now even if heat be rendered latent by the destruction of the compounds produced by the weaker quiescent affinities, a still greater quantity must nevertheless be set free in consequence of the neutralization of the stronger decomposing affinities, and the rise of temperature attending the decomposition marks the difference between these two quantities of heat.-If A develope a quantity of heat = 2 in combining with B, and = 3 in combining with C, then in the decomposition of A B and formation of A C, a quantity of heat must be developed = 3 - 2 = 1.

Lowering of T'emperature takes place when gaseous products of decomposition are evolved from solid or liquid compounds, and the absorption of leat thereby produced is not compensated by a development of heat resulting from the formation of new compounds.—The escape of carbonic acid gas from water charged with it, on diminishing the pressure, is accompanied by diminution of temperature; on the contrary, when this gas is disengaged from a solution of carbonate of soda by the addition of sulphuric acid, a slight rise of temperature is produced.

[On the development of Electricity by chemical decomposition, vid. Electricity.]

b. The time in which decomposition takes place depends chiefly on the circumstances uoticed on page 39. If one of the products of decomposition is gaseous and has to make its escape from a liquid, the decomposition is accelerated by the presence of angular bodies.

c. Qualitative alteration. Every decomposition results in the production of at least two heterogeneous substances or products of decomposition, which

may be either solid, liquid or gaseous, and—so long as they do not separate by virtue of their different specific gravities--produce a cloudy, opaque mixture.

If the decomposition is attended with the formation of gaseous products, effervescence and explosion may ensue.—Effervescence or frothing is produced when a gaseous body is continually developed and rises up in bubbles during the decomposition of a liquid;--Carbonate of potash and sulphuric acid.

In Explosion, Detonation (or Puffing, when the noise is fainter) a gaseous product of decomposition (or several) is separated almost instantly

from a solid (e.g. fulminating silver), a liquid (chloride of nitrogen) or a gas (oxide of chlorine)-and in endeavouring to expand itself into the much larger volume corresponding to its elasticity (often greatly increased by rise of temperature) forcibly presses back the air and other objects, prodacing detonation and breaking the solid bodies in its neighbourhood. Even gaseous compounds, such as oxide of chlorine, may be decomposed with explosion, if the separated elements, such as chlorine and oxygen gases, take up a larger volume than that occupied by the compound. The production of light wbich accompanies so many of these explosions often proceeds from this circumstance,-that great heat is developed in the decomposition in consequence of the formation of more intimate compounds, as in the explosion of gunpowder, detonating powder, &c. and this higher temperature imparts to the gases and vapours at the moment of their production a proportionately increased elasticity, and thereby strengthens the explosion. In the decomposition of oxide of chlorine, chloride of nitrogen, iodide of nitrogen, &c., which appears not to be attended with a rise of temperature amounting to ignition, the development of light may perhaps be explained by the violent compression of the air around the exploding body. — The theory of the detovation of an explosive mixture propounded by Brianchon (Bibl. univ. 28, 393) has been shown by Gay-Lussac (Ann. Chim. Phys. 29, 53) to be untenable. R. Böttger has also shown (Ànn. Pharm. 29, 75) that the explosive force of fulminating silver is equally strong in all directions.

When several liquid products result from a decomposition, they form a turbid mixture, until they have taken up the determinate relative positions corresponding to their different specific gravities:-Volatile oil dissolved in alcohol with water.

When in the decomposition of a liquid or gaseous body, solid products are formed and sink to the bottom in consequence of their greater specific gravity, they are called Precipitates, and a decomposition of this kind is called Precipitation, Spontaneous Precipitation, (Præcipitatio spontanea) when the separation of the solid body takes place merely from change of temperature (page 113);-Forced Precipitation, (Præcipitatio coacta) when it is brought about by the addition of another ponderable body, the Precipitant (Præcipitans). If the solid product of decomposition is specifically lighter than the liquid, it is separated as a Scum (Cremor):The precipitate and the scum may be either educts or products.—Lime precipitate from lime-water by alcohol is an educt; oxalate of lime precipitated from lime-water by oxalic acid is a product.

The atoms of the solid product of decomposition unite themselves at the moment of their separation or formation into larger masses, which however possess various magnitudes and forms depending chiefly on the nature of the bodies, so that to a certain extent the nature of the precipitate may be inferred from its outward appearance: considerable influence is howevei exerted by the time in which the precipitation takes place and the degree of dilution of the liquid. The following forms of precipitates may be particularly distinguished,—and of these the first two must be regarded as anarphous, the others as crystalline: Flocculent; aggregation in large, loose, threadlike masses: alumina, hydrated peroxide of iron, phosphate of lime.-Curdy; in this form the masses are still larger and more dense and solid, but still uncrystalline: chloride of silver as it is precipitated from solutions of silver-salts by hydrochloric acid; casein as precipitated from milk by an acid.—Pulverulent; the atoms are united in small, imperfectly crystalline masses: sulphate of baryta as precipitated

by sulphuric acid from solutions of baryta-salts; silver as precipitated by proto-sulphate of iron from nitrate of silver.Granular; coarsely pulverulent masses having a more distinct crystalline character: sulphate of potash as precipitated by sulphuric acid from aqueous solution of carbonate of potash.—Arborescent; the union of a great number of individual crystals into larger ramified masses: metallic trees.

V. MAGNITUDE OR STRENGT OF AFFINITY. There must exist a relation inexpressible in numbers between the magnitude of chemical affinities and those of other natural forces, such as gravitation, adhesion, or cohesion.

Absolute strength of affinity.--A solution of nitre saturated while warm deposits part of the nitre at 0° on account of increasing cohesion. Now supposing we were to determine what suspended weight would be required to break a crystal of nitre of a given thickness at 0°; then this weight would express the affinity of the water saturated with nitre at 09 for more nitre: for after the crystallization at 0° has ceased, this affinity is in equilibrio with the cohesion. A similar process might be adopted with other bodies soluble in water, the cohesion being always determined with reference to a crystal of given thickness. On the same principle Lavoisier and Laplace proposed to bring an acid of various degrees of strength into contact with ice at different temperatures below 0°, and to enquire at what degree of cold and at what dilution the acid ceased to exert any solvent power on the ice, and consequently its affinity for the ice became exactly equal in force to the cohesion of that substance—and thus to reduce the affinity of the acid for the ice, at different states of concentration, to degrees of the thermometer. A similar method might be adopted with various salts and ice: since for example common salt ceases to act on ice at —20° C., but chloride of calcium not till – 60°, the affinity of the latter for water must be much greater than that of the former. But these methods only enable us to determine the weights or degrees of temperature by which the weakest and least important affinities may be expressed. All affinities which have any considerable value exceed the force of cohesion to such a degree, that comparison between the two powers becomes impossible.

For the present we must content ourselves with an approximate determination of relative strength of afinity, i. e., of the proportion which individual magnitudes of affinity bear to one another without reference to other natural forces. Perhaps we shall some day be able to affix a certain relative number to each particular magnitude of affinity; at present, however, we are contented if we can determine, with some degree of certainty, in what order the affinities of different bodies for a given body succeed one another with regard to their strength.

Is the affinity between two bodies different at different temperatures ? Just as heat weakens cohesion by striving to increase the distance between homogeneous atoms, so likewise it may diminish the strength of affinity by increasing the difference between heterogeneous atoms. It appears, however, that as long as the action of heat does not go so far as to form a gaseous compound with one of the bodies, -in which case it would act like a third ponderable body in undoing the combination, it does not weaken chemical attraction, probably because in a combination of two ponderable bodies, it tends to increase the distance of the compound atoms only, not of the simple ones by whose union the compound atoms are formed. On the other hand, it might be inferred, from the phenomena mentioned on page 36 D, that elevation of temperature increases the strength of affinity. When, for example, sulphur combines with carbon at a red heat, we might suppose that the affinity between the two bodies is called into action by this temperature, or at least heightened in such a degree as to be able to overcome the cohesion of carbon; but in that case the sulphuret of carbon ought, on cooling, when the affinity is again diminished or annihilated and the cohesion of the carbon increased, to be again resolved into its elements. Such, however, is not the case either with this or with any other of the more intimate compounds, and therefore the affinity between such bodies exists even in the cold,—and heat does not develope affinity in the first instance, but favours the exertion of it in a manner not hitherto explained. At present, therefore, there is no ground for supposing that the affinity between two bodies is different at different temperatures. If, indeed, we would explain Berthollet's law of double affinity, not by the influence of cohesion, but on the supposition that out of a number of possible compounds those actually formed are always the most intimate, and have likewise the smallest relative solubility (page 125), it might, perhaps, be necessary to assume, with reference to the reciprocal affinity between common salt and sulphate of magnesia, for example (page 127), that the magnitudes of the affinities are different at different temperatures.

The following are the principal methods which have been adopted for the determination of relative magnitudes of affinity.

A. Difference of magnitude or strength of affinity is determined by the results of conflict or opposition of affinities, on the principle that the decomposing must be stronger than the existing or quiescent affinities. a. Decompositions in which the Affinity of IIeat contributes to the Result.

Many combinations of ponderable substances are decomposed by elevation of temperature, one of the ponderable elements combining with heat and forming a gaseous compound. The affinity of heat for ponderable bodies must be supposed to increase with the quantity in which it is accumulated, and therefore with the temperature; consequently, the temperature required to decompose a componnd of a less volatile with a more volatile body will increase with the affinity between the two. According to this, the strength or magnitude of the affinity may perhaps be found from the temperature required to effect the decomposition ;-the boiling point of the more volatile element must, however, be likewise taken into consideration.

Iron pyrites, Fe S?, when raised to a moderate red heat, which may be estimated at about 500° C., gives off vapour of sulphur, and is converted into Fe’S”; at a stronger red heat (perhaps 800°), a still greater quantity of sulphur sublimes, and FeS remains behind. Taking Dumas' determination of the boiling point of sulphur, viz., 440°, and supposing the higher degrees of temperature (a more accurate determination of which would, however, be desirable) to be correct, the affinity of Fe S for the quantity of sulphur required to produce FeS", may be expressed by 800 · 440 =

= 360, and that of Fe' So for as much sulphur as will produce Fe S?, by 500 - 440 = 60. Sulphuret of gold, Au So, parts with all its sulphur, perhaps at about 450°; if so, the affinity of gold for sulphur will be expressed by 450 – 440 = 10. But few of the other metallic salphurets are decomposed by beat; whence we may conclude that the number which would express the affinity of these metals for sulphur is

« PreviousContinue »