of the change, and what to the physical change that simultaneously takes place. As an illustration of the complex nature of chemical reactions when considered from a thermal standpoint, and of the disturbing effect of the accompanying physical changes, we may take the case of the action of aqueous hydrochloric acid upon crystallised sodium sulphate, Na2SO410H2O— Na2SO,,10H2O+2HCl=2NaCl + H2SO1+10H2O. The chemical action here consists of (1) the decomposition of sodium sulphate, (2) the decomposition of hydrochloric acid, (3) the formation of sodium chloride, (4) the formation of sulphuric acid. Heat is absorbed by the first two portions of the action, and heat is evolved by the other two. The physical changes include the passage of ten molecules of water of crystallisation (i.e. solid water) into liquid water, and the solution of sodium chloride in water. These changes are attended with absorption of heat, and the net result of the entire change is the disappearance of a considerable amount of heat, that is to say, the thermal value of the reaction is a negative quantity. The methods adopted in order to express thermo-chemical reactions are quite simple. The ordinary chemical symbols and formulæ are used, and represent, in all cases, quantities in grammes corresponding to the formula-weights of the substances. Thus CI represents 35.5 grammes of chlorine; H2O stands for 18 grammes of water, and so on. The chemical equation is followed by a number representing the quantity of heat, expressed in heat units, which is either produced or which disappears as a result of the change. The unit of heat is the calorie, or the quantity of heat that is capable of raising the temperature of 1 gramme of water from o' to 1°. Sometimes the unit employed is the quantity of heat required to raise I gramme of water from o° to 100°, and this unit (which is 100 times greater than the calorie) is indicated usually by the letter K. When heat is produced by a chemical change, the sign is placed in front of the number of units, and when heat disappears, the fact is indicated by the sign -. means that when 2 grammes of hydrogen combine with 71 grammes of chlorine to form gaseous hydrochloric acid, heat is disengaged to the amount of 44,000 calories, or 440 of the larger units, K. Or, in other words, that when these quantities of these substances combine, an amount of energy is lost to the system, represented by 44,000 calories. Therefore the energy possessed by 2 grammes of hydrogen and 71 grammes of chlorine is greater than that possessed by 73 grammes of hydrochloric acid gas by an amount which is represented by 44,000 gramme-units of heat. Hence the equation may be written 2HCl H2+Cl2-44,000 cal. which signifies that when 73 grammes of gaseous hydrochloric acid are decomposed into chlorine and hydrogen, it is necessary to supply an amount of energy equal to 44,000 calories. In order to indicate the state of aggregation of the different substances, the method introduced by Ostwald consists in the use of different type, thick type being employed to denote solids, ordinary type indicating liquids, and italics signifying gases, thus C+0g CO2+97,000 cal. means that the total energies of 12 grammes of solid carbon and 32 grammes of gaseous oxygen is greater than the energy possessed by 44 grammes of gaseous carbon dioxide by an amount equivalent to 97,000 calories. Or, again, the equation SO2+H2O=H2SO1+21,320 cal. signifies that So grammes of solid sulphur trioxide unites with 18 grammes of liquid water and forms 98 grammes of liquid sulphuric acid, with the liberation of 21,300 gramme-units of heat. Similarly, the heat evolved by the passage of water into ice, and the heat that disappears when water passes into steam, may be expressed by the equations H2O=H2O+1440 cal. when water takes a direct part in the chemical change, as, for example, in the action of sulphur trioxide and water already quoted, the formula represents a gramme-molecule just as in all other cases; but where the presence of a large quantity of water affects the thermal result of the chemical change, by exerting, for example, a solvent action, the symbol Aq is employed to signify that the presence of the water is considered in the thermal expression. Thus the expression HBr+Aq=HBrAq+19,900 cal. signifies that when 81 grammes of gaseous hydrobromic acid are dissolved in a large excess of water, 19,900 calories are evolved. Again, the equation— H2+Br2+Aq=2HBrAq+64,000 cal. means that when 160 grammes of gaseous bromine combine with 2 grammes of hydrogen, and the product is dissolved in an excess of water (i.e. such a quantity of water that no thermal change is produced by the addition of any further quantity), 64,000 calories are disengaged. Of this 64,000 calories, 19,900 × 2 = 39,800 are due to the solution of the twice 81 grammes of hydrobromic acid, and the difference, viz., 24,000 calories, represent the heat produced by the combination of 2 grammes of hydrogen with 160 grammes of bromine. If water is formed as one of the products of the chemical reaction taking place in the case of substances in aqueous solution, such as when a solution of hydrochloric acid is added to a solution of sodium hydroxide, HCl + NaHO= NaCl + H2O, as the water so produced simply mixes with the water in which the materials are dissolved, without producing any thermal effects by so doing, it is usually neglected in energy equations; although, as already stated (page 109), when explained from the standpoint of the ionic theory, the heat of neutralisation is here due to the formation of molecules of water by the union of Hions with HO' ions. Thus the above action may be expressed HCIAq+NaHOAq= NaClAq+13,736 cal. The heat that is produced, or that disappears, in a chemical change which results in the formation of a particular compound is termed the heat of formation of that compound. Thus in the equation— H2+Cl=2HCl +44,000 cal. the heat of formation of 73 grammes of hydrochloric acid is 44,000 thermal units. This number, however, is in reality the algebraic sum of three quantities. It does not express merely the heat developed by the simple union of chlorine and hydrogen. The chemical change expressed by the equation consists in reality of three operations (1.) H, H+H. (2.) Cl=Cl+Cl. (3.) CI+CI+H+H=2HCl. Each of these operations represents a distinct thermal effect; in Nos. (1) and (2) heat is absorbed, in No. (3) heat is evolved, and calling these values h1, h2, and h, we have as the net result h3−(h2+h2)=44,000 cal. The number of heat-units, therefore, which expresses the heat of formation of hydrochloric acid is the heat produced by the union of two atoms of hydrogen with two atoms of chlorine, minus the heat absorbed in the decomposition of one hydrogen and one chlorine molecule. Compounds such as hydrochloric acid, in the formation of which heat is developed, are termed exothermic compounds, the reaction by which they are produced being an exothermic change; compounds, on the other hand, whose heats of formation are expressed by a negative sign, that is, in whose formation heat disappears, are distinguished as endothermic compounds, and the reactions by which they are formed are endothermic reactions. signifies that in the formation of carbon disuiphide heat is absorbed, and the compound is therefore an endothermic compound. Thermo-chemical determinations are made by means of instruments termed calorimeters. These are of great variety, although the principle involved is the same. The chemical reaction is caused to take place under such circumstances, that the whole of the heat that is liberated shall be communicated to a known volume of water, at a known temperature.* Direct determinations of the thermal value of chemical changes have hitherto been made in only a limited number of comparatively simple cases; it is possible, however, from a few known data, to calculate the thermal values of a number of changes which cannot be directly measured. This depends upon the fundamental principle * For descriptions of the various calorimeters, see treatises on Physics. of thermo-chemistry, which is itself the corollary of the law of the conservation of energy, and which was first experimentally proved by Hess (1840). This principle, which is sometimes termed the law of constant heat consummation, or the law of equivalence of heat and chemical change, may be thus stated: The amount of heat that is liberated or absorbed, during a chemical process, is dependent solely upon the initial and final states of the system, and is independent of the intermediate stages. The following examples will serve to explain the application of the principle :— 1. Let us suppose it is desired to find the heat of formation of carbon monoxide, the data at our disposal being (1) the heat produced when carbon unites with oxygen to form carbon dioxide; and (2) the heat formed by the combustion of carbon monoxide to carbon dioxide. The thermal equations are (1) C+O2=CO2+97,000 cal. (2) 2CO+0=2CO2+136,000 cal. Halving the second equation, in order to get the heat produced in the formation of 44 grammes of carbon dioxide (i.e. the same weight as in the first), we may represent the equation as― CO+O=CO2+68,000 cal.* The difference between the two values 97,000 and 68,000 will be the heat of formation of carbon monoxide, therefore we get the equation C+0=CO+29,000 cal. 2. The compound, methane (marsh gas), CH,, cannot be formed by the direct union of its elements, but its heat of formation can be calculated by the application of this principle. The data in this case are the ascertained heats of formation of carbon dioxide * It must be remembered that this equation does not express the whole truth as it here stands it would imply that 68,000 calories represent the heat formed by the simple chemical union of 28 grammes of carbon monoxide with 16 grammes of oxygen. In reality this number is half the sum of the two values, namely, the heat of combination of 56 grammes of carbon monoxide with 32 grammes of oxygen, minus the heat absorbed by the decomposition of 32 grammes of oxygen molecules into their constituent atoms. The oxygen atom does not exist alone, and whenever free oxygen takes part in a chemical change the molecules of the element are first separated into their atoms. |