THERMOCIIEMICAL EQUATIONS. Every given quantity of matter carries with it a certain quan tity of energy. Heat being a form of energy, it is Clear that the hotter the body the greater its energy. To cool it. we must abstract sonic of its heat by bringing it into contact with some cooler body: and then, by determining the rise of tem perature in the latter, ‘se can learn how much energy the hot body has lost. But while we can thus readily find out how much more energy a body contains at one temperature than at an other, we have no way of telling how inuell energy it contains altogether. for we have no way of its energy entirely. Never theless, we know that different substances gen erally contain different amounts of energy, even if their temperatures are precisely the same. This is plainly shown by thu fact that different substances have different 'specific heats' that is to say, that different amounts of heat are re quired to cause an equal rise of temperature in equal masses of than.
Now, since during chemical reactions the given disappear as such and new Imes arise in their place, it is evident that chemical reac tions must, be acconqtanied by either evolution or absorption of heat. For, like the mass of matter, a quantity of energy can he neither de stroyed nor created out of nothing, by a chem ical or any other transformation. If the orig inal reacting substances contain more energy than the products of the reaction, the reaction will cause sonic energy to he given off: thus, hydrogen gas and oxygen gas contain mu•li more energy than the water vapor that may be formed from them, and hence their comliination (the 'burning') sets free much energy in the form of sensible heat. Precisely the same amount of energy would. on the contrary, be taken up if water were decomposed into its elements, hydro gen and oxygen. Reactions in which energy is given off are called exothermal reactions; those in which energy is taken up are called cm/other ma/ reactions. It must not be thought, however, that transfers of energy are involved in chemical reactions alone. Thus, the evaporation of water
is a purely physical transformation, and yet it involves absorption of much heat. For this reason, if a chemist wishes to ascertain how much energy has been given off or taken up in a given chemical reaction, lie must make a thorough study of the physical changes ac•om pant ing, the reaction and of the transfers of energy caused by those changes. In this respect the most important form of physical (-flange is the dissolution of solids in liquids, especially in water, because many important reactions take place in aqueous solutions.
The above considerations make it evident that the chemical equations discussed in preceding paragraphs are really incomplete: for they repre sent transformations of matter stating what changes of energy accompany them. When ever, therefore. questions of energy are of mo ment, whether in theoretical discussions or in problems dealing with foods, fuels. etc., (lpin ists use a more complete form of equations—viz. 'thernmeheinical equations.' In writing these, Ostwald has adopted the following notation: Gases are denoted by their chemical formulas inclosed in parentheses; solids by their formu las inclosed in brackets; liquids simply by their formulas: substances dissolved in a great deal of water by the symbol Aq (i.e. aqua, water) affixed to their chemical formulas. Thus, (IT.,0) denotes water vapor; denotes ice; denotes liquid water: KCIAq denotes potassium chloride in very dilute aqueous solution. 1 kt• wald also proposes to denote the energy taken up or given off during reactions in terms of ki/ojou/cs. denoted by the symbol .7. One kilo joule (= 10000000000 ergs) is the same as •39.1 calories, a calorie being here the amount of heat required to raise by 1° C. the temperature of one gram of water of 18° C. For example, the neutralization of h•droehlorie acid by potash in dilute solution, which is ordinarily repress oled by the equation