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Thus sodium or potassium oxidises rapidly in ordinary air ; iron filings are oxidised rapidly only by heating in oxygen. Iron and dilute sulphuric acid readily react to produce iron sulphate and hydrogen; copper and sulphuric acid do not interact until the temperature is raised to 100° or more. Hydrogen and oxygen do not combine until the temperature is raised very considerably.

The products of a chemical interaction sometimes vary according to the temperature at which the substances are caused to interact. Thus sodium chloride and sulphuric acid react at ordinary temperatures to produce sodium-hydrogen sulphate and hydrogen chloride, but at higher temperatures the chief product, besides hydrogen chloride, is sodium sulphate; the two reactions may be represented thus :

(1) 2NaCl + 2H SO, = 2NaHSO4 + 2HCI ;

(2) 2NaCl + H SO, = Na SO, + 2HCl. A solution of bismuth iodide in hydriodic acid interacts with cold water to precipitate bismuth iodide; Bil, (in HIAq) + xH,0 (cold) = Bil, + HI Aq + xH,0. But the same solution interacts with hot water to precipitate bismuth oxyiodide; Bil, (in HIAq) + H2O + 2H,0 (hot) = BIOI + 2HIAq + 2H,O. A cold aqueous solution of copper sulphate reacts with cold caustic potash solution to precipitate copper hydroxide; a hot aqueous solution of copper sulphate with hot caustic potash solution to precipitate copper oxide : (1) CuSO, Aq (cold) + 2KOHAq (cold) = CuO,H, + K SO Aq. (2) CuSO, Aq (hot) + 2KOHAq (hot) = CuO+K_SO, Aq +H,0.

Sometimes a certain chemical change occurs within a defined range of temperature, and the reverse change takes place within another defined range of temperature. Thus lime and carbon dioxide combine at ordinary temperatures to form calcium carbonate; but calcium carbonate can be wholly changed to lime and carbon dioxide by raising the temperature;

(1) CaO + CO = CaCO.

(2) CaCO, (heated strongly) = CaO + CO Ammonia and hydrogen chloride gases combine when mixed to form ammonium chloride; but ammonium chloride is completely resolved into ammonia and hydrogen chloride at

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a moderately high temperature: (1) NH, + HCl = NH CI ; (2) NH CI (heated) = NH + HCI. Sulphur trioxide and water combine to form sulphuric acid ; but sulphuric acid is decomposed by heating into sulphur trioxide and water-vapour: (1) SO, + HO = H SO,; (2) H SO, (heated) = SO, + H.O. Hydrogen and iodine combine when heated 'to 400-500° to form hydrogen iodide; but hydrogen iodide is separated at a higher temperature into hydrogen and iodine:

(1) H+I=HI; (2) HI (heated) = H +I. When carbon dioxide is passed over moist sodium carbonate, sodium-hydrogen carbonate is formed; but this salt, when heated, is changed to water, carbon dioxide, and sodium carbonate :

(1) Na CO2 + H2O + CO,= 2NaHCO3

(2) 2NaHCO, (heated) = Na,C0, +H,O+CO,. If amylic bromide, CH, Br, is heated in an enclosed space to about 270° it is changed into amylene, C,H,,, and hydrogen bromide, HBr; if the temperature is now allowed to fall the amylene and hydrogen bromide recombine to form amylic bromide.

When phosphorus pentachloride, PCI,, is heated the vapour produced is a mixture of phosphorus trichloride, PCI, and chlorine; when this mixture of gases is allowed to cool solid phosphorus pentachloride is re-formed.

The amount of chemical change produced, in many of these 234 cases, is conditioned not only by the temperature but also by the pressure. Thus when nitrogen tetroxide, N, O, is heated, it is changed to nitrogen dioxide, NO,; the amount of change at 16° under a pressure of 229 mm, of mercury is the same (20 p. c.) as that at 27° under a pressure of 755 mm.

The change of calcium carbonate into carbon dioxide and 235 calcium oxide, brought about by the action of heat, and the reverse change of calcium oxide and carbon dioxide into calcium carbonate, may be regarded as a completed cycle of change. Changes such as this are classed together under the name dissociation.

The following changes, already referred to, which are brought about by altering conditions of temperature, are instances of dissociation ; (1) the change of hydrogen iodide into hydrogen and iodine, and of hydrogen and iodine into hydrogen iodide; (2) the change of ammonium chloride into ammonia and hydrogen chloride, and of a mixture of these

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gases into ammonium chloride, (3) the change of amylic bromide into amylene and hydrogen bromide, and of a mixture of these gases into amylic bromide, (4) the change of phosphorus pentachloride into phosphorus trichloride and chlorine, and of a mixture of these gases into phosphorus pentachloride, &c. &c.

Every dissociation is brought about by the action of heat alone ; in every case there is (1) the production of less complex from more complex substances, one at least of the less complex being a gas; (2) the possibility of reversing the process by cooling the products of the change in contact with each other.

When calcium carbonate is heated to a specified temperature in a closed vacuous vessel, to which is attached an apparatus for measuring the pressure inside the vessel, dissociation into solid calcium oxide and gaseous carbon dioxide proceeds until the pressure of the carbon dioxide reaches a certain amount, when the dissociation ceases, and the system, consisting of calcium carbonate, calcium oxide, and carbon dioxide, remains in equilibrium. If temperature is now raised, more carbonate is decomposed, more carbon dioxide accumulates, the pressure increases, and at last the dissociation stops. If temperature is lowered, some of the carbon dioxide and calcium oxide combine, and pressure falls until a new state of equilibrium is attained. If temperature is kept constant when a certain quantity of carbonate has been dissociated and the system is in equilibrium, and pressure is suddenly lowered by removing some of the carbon dioxide, dissociation proceeds until the accumulation of carbon dioxide brings the pressure to its former value; when this pressure is reached dissociation stops, and the system remains in equilibrium.

The relations of temperature and pressure to chemical change are important, not only in cases of dissociation, but also in cases of ordinary double decomposition in which gases are produced and the original bodies and the products of the change remain in contact and are capable of chemically interacting. Thus, when steam is passed over hot iron, iron oxide and hydrogen are produced ; if the hydrogen is removed as it is formed and more steam is supplied, the whole of the iron is changed to oxide; but if the hydrogen is caused to accumulate in contact with the other members of the system, pressure increases, the rate of change decreases, and at last a state of equilibrium is reached, the system is composed of definite relative masses of steam, hydrogen, iron, and iron oxide, and no more change occurs. If hydrogen is now removed, other conditions remaining constant, pressure falls, and change proceeds with formation of more iron oxide and more hydrogen; if hydrogen is forced into the apparatus, other conditions being constant, change occurs with production of more iron and steam. If all the members of the changing system are allowed to remain in contact with each other, then a state of equilibrium is sooner or later attained.

The conception of a chemical system as being in equili. 237 brium under certain conditions, and as having this equilibrium overthrown by altering these conditions, is of much importance. We shall have to return to this conception again, and more fully to examine the conditions which affect the equilibrium of such systems. (s. Chaps. XIII. and xiv.)

Influence of the relative masses of the interacting 238 substances on chemical change. We have already had many examples of the necessity of using an excess of one or other of two interacting substances in order to complete a chemical reaction. Thus, to change a specified mass of lead monoxide (PbO) into lead dioxide (Pb0,), the lead monoxide is treated with a very large excess of potassium hypochlorite (Chap. XI. par. 158); a given mass of manganese dioxide is changed to potassium manganate by fusion with caustic potash, but very much more potash must be used than is actually converted into the new salt (Chap. XI. par. 196).

Sulphuric acid and sodium nitrate interact as shewn in the equation 2NaNO, + H 80,= Na SO, + 2HNO,; but if a large quantity of nitric acid is heated with sodium sulphate, sodium nitrate and sulphuric acid are produced Na S0, + 2HNO, + «HNO, = 2NaNO, +H SO, + xHNO3.

If silver chloride is heated with a considerable quantity of an aqueous solution of hydriodic acid some silver iodide is produced; but if silver iodide is warmed with hydrochloric acid silver chloride is formed. The two changes may be represented thus; (1) AgCl + HIAT

= «AgI + xHCIAq + (1 – x) AgCl + (1 – 2) HIA; (2) AgI + HCIAT

= xĀgCl + XHIAq+ (1 – x) AgI + (1 – x) HCIAq. In the first reaction little AgI is formed unless a large excess of HI is used ; in the second reaction a considerable quantity of AgCl is produced even when a small excess of HCl is used.

If aqueous solutions of potassium sulphocyanide (KCNS) and ferric chloride are mixed, ferric sulphocyanide [Fe(CNS).] and potassium chloride are formed. In this case the four substances all remain in solution. The change may be represented thus; 6KCNSAq + «KCNSAq + Fe Cl. Aq=6KCIAq + Fe (CNS)Aq

+ XKCNSAq. If the salts are allowed to react in the ratio 6KCNS : Fe Cla very little of either is decomposed; in order to change the mass of ferric chloride represented by the formula Fe,cl. almost completely into ferric sulphocyanide the salts must be mixed in about the ratio 500KCNS : Fe Cle But at the close of the reaction only 6KCNS has been decomposed for every Fe,cl, changed; the rest of the potassium sulphocyanide remains chemically unchanged.

When hydrogen and iodine are mixed at 440° and under a pressure of 340 mm. a portion of the elements combine to form hydrogen iodide; the amount of hydrogen iodide formed is increased by increasing the mass of either hydrogen or iodine relatively to the other gas. The following numbers shew the influence of increasing the mass of hydrogen :Ratio of H to I. Ratio of HI formed to

total possible HI. I:H

74 I: 2H

.84 I: 3H

.87 I: 4H

.88 The influence of the relative masses of the interacting substances on the course of a chemical change is most marked when all the interacting substances and all the products of the change are in solution, or are liquids. Ethylic alcohol and acetic acid for instance react to produce ethylic acetate and water, thus C,H,OH + C,H,O.OH=C,H,O.OC.H. +H,0; but much more acetic acid than is represented in this equation must be used to complete the change. Such changes as this, or as that which occurs between solutions of potassium sulphocyanide and ferric chloride (v. ante), may be divided into two parts; the direct, and the reverse, change. The direct change in the case of alcohol and acetic acid is that shewn in the equation, the reverse change is that from ethylic acetate and water to ethylic alcohol and acetic acid (C,H,0.0C,H, +H,0=C,H,O.OH + C,H,OH).

The direct and reverse

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