CHAPTER XII. 227 228 229 CONDITIONS WHICH MODIFY CHEMICAL CHANGE. BODIES interact chemically only when brought into very close contact. The contact may be effected by dissolving, gasefying, or melting, one or more of the reacting substances; or, in some cases, by submitting the substances to very great pressure. Barium chloride and sodium sulphate, for example, remain unchanged when mixed; but when aqueous solutions of these compounds are mixed, barium sulphate and sodium chloride are at once produced. Ammonia and hydrogen chloride gases combine to produce ammonium chloride. A mixture of iron and sulphur remains unchanged for an indefinite time; but when iron filings are added to molten sulphur chemical change occurs and iron sulphide is formed. The same product is formed by exposing the mixture of finely divided iron and sulphur to a pressure of several thousand tons on the square inch. As a general rule chemical change proceeds more rapidly between a solid and a liquid, or a solid and a gas, the more finely divided the solid is, that is, the greater the surface exposed by a given quantity of the solid. Thus a piece of iron oxidises slowly in ordinary air; but very finely divided ironobtained by strongly heating iron tartrate in a glass tube and closing the tube while hot-oxidises so rapidly in ordinary air that the particles of the oxidising iron glow and emit light. Granulated zinc, that is zinc in thin irregular-shaped pieces, dissolves in dilute sulphuric acid much more quickly than a compact mass of zinc. If one or more of the possible products of a chemical interaction is gaseous under the experimental conditions, that chemical interaction usually occurs readily. Thus calcium carbonate (CaCO3) rapidly interacts with dilute hydrochloric acid solution, to produce calcium chloride (CaCl ̧) which remains in solution, and carbon dioxide (CO) which escapes as a gas; CaCO3 + 2HClAq= CaCl, Aq + H2O + CO2. Calcium carbonate is decomposed by heat to solid calcium oxide and gaseous carbon dioxide: CaCO, (heated) = CaO + CO,. Zinc and dilute sulphuric acid interact at ordinary temperatures to produce zinc sulphate and gaseous hydrogen : Zn + H2SO, Aq= ZnSO, Aq + 2H. If one or more of the possible products of a chemical inter- 230 action between solutions is a solid under the experimental conditions, that chemical change usually occurs readily. Thus aqueous solutions of barium chloride and sodium sulphate interact when mixed to produce solid barium sulphate, and sodium chloride which remains in solution : BaCl,Aq + Na2SO2Aq = BaSO ̧ + 2NaClAq. A solution of antimony chloride in hydrochloric acid interacts with water to produce solid antimony oxychloride, and hydrochloric acid which remains in solution: SbCl, (in HClAq) + H ̧O + xH ̧O = SbOCl + 2HCl Aq + xH ̧O. 2 Potassium acetate is soluble in alcohol, potassium carbonate is insoluble in alcohol; if carbon dioxide is passed into an alcoholic solution of potassium acetate, the formation of an insoluble compound becomes possible; this compound, potassium carbonate, is formed and precipitated. On the other hand, if acetic acid is added to an aqueous solution of potassium carbonate, the formation of a gas becomes possible; this gas, carbon dioxide, is formed and the carbonate is decomposed. These reactions furnish an instance of the reversal of a chemical change by alterations in the experimental conditions such that in one case the formation of a solid, and in the other the formation of a gas, becomes possible. The conditions which chiefly modify chemical changes, besides those already mentioned, are temperature, and the relative masses of the interacting substances. Influence of temperature on chemical change. A 231 chemical change occurs within a definite range of temperature. Some changes take place readily and completely at ordinary temperatures; other changes begin only at higher temperatures. 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. 232 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: 233 (2) 2NaCl + H2SO1 = Na2SO ̧ + 2HCl. A solution of bismuth iodide in hydriodic acid interacts with cold water to precipitate bismuth iodide; BiI, (in HIAq) + xH2O (cold) = BiI+HIAq+xH2O. But the same solution interacts with hot water to precipitate bismuth oxyiodide ; 2 2 BiI, (in HIAq) + H2O + xH2O (hot) = BiOI + 2HIAq + жH2O. A cold aqueous solution of copper sulphate reacts with cold caustic potash solution to precipitate copper hydroxide; a hot aqueous solution of copper sulphate reacts with hot caustic potash solution to precipitate copper oxide: 2 2 (1) CuSOAq (cold) + 2KOHAq (cold) = CuOH + K2SO̟Aq. (2) CaCO, (heated strongly) = CaO + CO2. Ammonia and hydrogen chloride gases combine when mixed to form ammonium chloride; but ammonium chloride is completely resolved into ammonia and hydrogen chloride at a moderately high temperature: (1) NH2+ HCl = NH Cl; (2) NH Cl (heated) = NH ̧ + HCl. Sulphur trioxide and water combine to form sulphuric acid; but sulphuric acid is decomposed by heating into sulphur trioxide and water-vapour: (1) SO, + H2O = H2SO1; (2) H2SO, (heated) = SO, + H2O. 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: 3 4 (1) H+I=HI; (2) HI (heated) = H + I. 3 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 + CO2 = 2NaHCO,. 3 2 (2) 2NaHCO, (heated) = Na2CO ̧ + H ̧O + CO2. 2 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, PC, is heated__the vapour produced is a mixture of phosphorus trichloride, PCI, ვა and chlorine; when this mixture of gases is allowed to cool solid phosphorus pentachloride is re-formed. 2 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, NO,, 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 236 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 |