Page:The New International Encyclopædia 1st ed. v. 04.djvu/647

* CHEMISTRY. The symbols CH,, 20;. CO,, and 2HjO, represent, respectively, certain relative weights of marsh gas, oxygen, carbonic acid, and water: and the sign of equality denotes that the total mass of the carbonic acid and water yielded is i)recisely the same as the total mass of the marsh gas and oxygen that have disappeared as Such. Simi- larly, the fact that the total number of C's, IVa, and O's is respectively equal in the two mem- bers of the equation reminds us of the principle of conservation of the elements. Further, CH,, O;. CO.. and lUO represent equal volumes of the four substances, and hence the equation expresses that 2 volumes of oxygen are required to bum completely 1 volume of marsh gas, and that the products of the combustion are a volume of carbonic acid equal to that of the marsh gas burned and twice that volume of water vapor. Revek.siulk Reactions. Tn mathematics, the members of an equation can always be trans- posed at pleasure, and o = 6 may as well be written 6 = a. Not so in chemistry. A chem- ical equation represents not merely an equality of quantities, but an actual reaction of sub- stances. And if, for example, marsh gas and oxygen react to form carbonic acid and water, it does not by any means follow that carbonic acid and water will react to form marsh gas and oxygen, which would be expressed by the above equation if written in the form CO, + 2H,0 = CH, + 20, There are. however, reactions that can actually be reversed. For instance, ordinary ethyl alco- hol and acetic acid react to form ethyl-acetic ester and water. But ethyl-acetic ester and water also react to form ethyl alcohol and acetic acid. In this case we are, of course, justified in writing the equation in either of the following forms: C.H.OH + CH.COjH = CH,CO,C.Hs + H,0 Alcohol .cetic acid Ester Water CH3COAH5 + H,0 = CjH.OH + CHgCO,H Eeter Water .lcohol .cetic acid Reactions of this nature are termed reversible reactions, and are now denoted by expressions in which the sign of equality is replaced by two arrows pointing in opposite directions. Thus, the rea«tions just mentioned would be represent- ed as follows: C.H.OH + CHjCO.H ♦=> CH3COAHJ + H,0 Alcohol Acetic acid Ester Water The investigation of reversible reactions ha.s re- sulted in the establishment of the so-called law of mass action, which is at the basis of modem chemical statics and dynamics. This will be dis- cussed at some length in the article Reactiox. Ther.iochemic.l Equations. Every given quantity of matter carries with it a certain quan- tity of energy. Heat being a form of energj', it is clear that the hotter the body the greater its cnergj'. To cool it. we must abstract some of its heat by bringing it into contact with some cooler body: and then, by determining the rise of tem- perature in the latter, we 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 much energy it contains altogether, for we have no way of abstracting its energy entirely. Never- theless, we know that different substances gen- erally contain different amounts of energj-, even 56." CHEMISTRY. if their temperatures are precisely the same. This is plainly shown by the fact that different substances have different "specific heats' — that is to say, that different amounts of heat are re- quired to cause an e<jual rise of temperature in equal masses of them. Now, since during chemical reactions the given substances disappear as such and new ones arise in their place, it is evident that chemical reac- tions must be accomjianied by cither evolution or absorption of heat. For, liko the mass of matter, a (piantity of energy can be 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 some energy to be given oil'; thus, lij'drogen gas and oxygen gas contain much more energy than the water vapor that may bo formed from them, and hence their combination (the 'burning' 1 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 energj- is given off are called exothermal reactions ; those in which energy is taken up are called endother- mal rcnctions. It must not be thought, however, that transfers of energy are involved in chemical reactions alone. Thus, the evaporation of water is a purely phj-sical transformation, and j'et it involves the absor])tion 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, he must make a tliorough study of the physical changes accom- pany ing the reaction and of the transfers of enorg}' caused by those changes. In this respect the most important form of physical change 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 re])re- sent transformations of matter without stilting what changes of energj' accompany them. When- ever, therefore, questions of energv are of mo- ment, whether in theoretical discussions or in problems dealing with foods, fuels, etc., chem- ists use a more complete form of equations — viz. 'thermochemical equations.' In writing these, Ostwald has adopted the following notation: Ga,ses are denoted by their chemical formulas inclosed in parentheses ; .solids by their formu- las inclosed in brackets: liquids simplv by their formulas ; substances dissolved in a great deal of water by the svmbol Aq (i.e. aqua, water) affixed to their chemical formula's. Thus, (H,0) denotes water vapor; [H,0] denotes ice; H,0 denotes liquid water: KClAq denotes potassium chloride in very dilute aqueous solution. Ost- wald also proposes to denote the energy taken up or given off during reactions in terms of kilojoules. denoted by the sjnnbol J. One kilo- joule (= lOOOnonnnno ergs) "is the same as 230.1 calories, a calorie being here the amount of heat required to raise by 1° C. the' temiierature of one gram of water of 18° C. For example, the neutralization of hvdrochloric acid by potash in dilute solution, which is ordinarily represented bv the equation HCl + KOH = KCl -f H,0,