« PreviousContinue »
after the formula of a compound or element it means that that body is in solution in a large quantity of water.
Chemical formulae express other facts regarding chemical 83 changes; these we shall learn as we advance. It is advisable to note here that these formulae and equations do not say anything regarding the conditions under which the chemical interactions occur. Thus Fe+S= FeS only tells us that a certain mass of iron combines with a certain mass of sulphur to produce the sum of these masses of iron sulphide. So Na + H2O + Aq = NaOHAq + H tells that certain masses of sodium and water interact to produce certain masses of sodium hydroxide (which is dissolved in the excess of water (s. ante par. 56)] and hydrogen, and that the sum of the masses of sodium and water is equal to the sum of the masses of sodium hydroxide and hydrogen. The equations in no way indicate the facts that iron and sulphur only combine when heated, but that sodium and water interact at ordinary temperatures. The equation Zn + H SO, Aq=ZnSO Aq + 2H expresses certain definite quantitative facts (s. ante); but it does not indicate or even suggest that the compositions of the products of the interaction of zinc and sulphuric acid vary with variations in the temperature at which the interaction occurs, and that the interaction proceeds according to the representation given by the equation only at the ordinary temperature.
Chemical equations evidently give very incomplete representations of chemical changes. But nevertheless chemical formulae are of the greatest value, inasmuch as they enable us to exhibit, in a simple and intelligible way, the composition of compounds, and those changes of composition, the study of which forms one part of chemical science.
We have learned that the symbol of an element represents 84 a definite mass, and also one combining weight, of that element.
The formula of a compound also represents a definite mass of the compound, and tells the composition of that definite mass, both in parts by weight, and also in combining weights, of each of the elements by the combination of which the compound has been formed. The following are the formulae of some well-known compounds ;Water
Formic acid H,CO, Hydrogen peroxide H, Hydrogen sulphide H, S. Oxalic acid H.C.O.
Sulphur chloride S,či,.
Hydrogen chloride HCl. Antimony chloride SbC1z.
Why should we choose to represent hydrogen peroxide as composed of 2 combining weights of hydrogen with 2 c. ws. of oxygen? Water is represented as produced by the union of 1 c. w. of oxygen with 2 c. ws. of hydrogen ; why should not the composition of peroxide of hydrogen be represented by the formula HO? The ratio H: 0 is the same ratio H,: , Again, why should the formula of ferric chloride be Feci, rather than FeCl, ? Chloride of antimony, SbCl,, is represented as formed by the union of 1 c. w. of antimony with 3 c. ws. of chlorine; why should we choose a formula for ferric chloride which represents the composition of that mass of this compound which is formed by the union of 2 c. ws. of iron with 6 c. ws. of chlorine ?
Similar questions are suggested by the other formulae. In some cases the formula appears to be the simplest that could be given to the compound, e.g. H,O, H,S, HCI, SbCl,; in other cases a needless and foolish complication seems to be introduced. Why not HCO, in place of H.CO.; HC in place of H.C.; SCl in place of S,Ci, ; HO in place of H,O,; FeCl, in place of Fe Clo? Or if the more complex formulae are. to be used, why should such formulae not be always used ? Why not H.O, or 1,0, in place of H,0; H S, or HS, or H, Se in place of HS; Šb ci, in place of SbCl, ; &c. ? There must
be reason for these apparent incon-sistencies. There are several reasons; but we are not yet in a position fully to understand and appreciate these reasons. We may however gain some notion of the kind of reasoning employed in determining which of several possible formulae best represents the composition and reactions of a compound. The gist of the matter, as we shall hereafter find, is in the conception expressed by the words composition and reactions. So long as we look only at the composition of compounds we cannot find answers to our questions. If we disregard the composition and look only at the reactions of compounds we cannot find answers to our questions.
The symbol of an element represents a certain mass of 86 that element usually called its combining weight. Elements combine in the ratios of their combining weights, or in ratios bearing a simple relation to these.
The formula of a compound represents the composition of a certain mass of that compound; this mass we propose to call the reacting weight of the compound. Compounds interact in the ratios of their reacting weights, or in ratios bearing a simple relation to these.
The reacting weight of water is 18 (H, = 2 + 0 = 16).
The combining weight of sodium is 23"(this, as a matter of fact, is the mass of sodium which combines with 8 parts by weight of oxygen). Let us examine the interaction of water and sodium.
When sodium is thrown into water a reaction immediately occurs; the sodium rapidly disappears and hydrogen gas is produced. When the reaction is finished, let the solution be evaporated; water passes away as steam, and a white solid (caustic soda) remains. The composition of this solid is represented by the formula NaOH (Na = 23,0 = 16), that is to say, this compound is produced by the combination of one combining weight of sodium, one c. w. of hydrogen, and one c. w. of oxygen. We know that water is a compound of hydrogen and oxygen, and that sodium is an element. Hence the oxygen and hydrogen which form part of the caustic soda must have come from the water. But besides caustic soda, hydrogen was produced; this must also have come from the water. Hence when sodium and water interact, a portion of the hydrogen which was combined with oxygen is evolved as hydrogen gas, and another portion enters into combination with the sodium and the oxygen to produce caustic soda. When this experiment is made quantitative, it is found that 23 parts by weight of sodium interact with 18 parts by weight of water, and there are produced 40 parts by weight of caustic soda and 1 part by weight of hydrogen. The 40 parts of caustic soda are composed of 23 parts of sodium, 16 parts of oxygen, and 1 part of hydrogen.
The conclusion from these experiments is, that, as regards the interaction of water with sodium, 18 is the reacting weight of water, and that the decomposition of one reacting weight of water results in the production of 2 combining weights of hydrogen and 1 c. w. of oxygen. But if H= 1 and 0 = 16, the formula H (18) summarises the results of this experiment. M. E. C.
A quantitative study of the reactions of water, carried out in the way thus briefly indicated, leads to the conclusion that the mass of water which interacts with other compounds and with elements can always be represented as 18, or as a whole multiple of 18.
The composition of the hydrocarbon benzene is most simply represented as one c. w. of carbon combined with one c. w. of hydrogen; therefore the smallest value that can be given to the reacting weight of benzene is 13 (CH; C= 12, H=1).
Is this the best value to adopt for the reacting weight of benzene?
Benzene and chlorine react to form a series of compounds, each composed of carbon, hydrogen, and chlorine; the formation of each of these is accompanied by the formation of hydrogen chloride (HCI). The first of these compounds is composed of 35.5 parts by weight of chlorine, 72 of carbon, and 5 of hydrogen; therefore (as C= 12, and Cl = 35.5) the simplest formula to be given to this compound is CH.C. The composition of the next compound cannot be represented by a simpler formula than Cuci, The other compounds have compositions which cannot be expressed by formulae simpler than CH CI, C.HCl, C.HCl,, and C Cin, respectively. Now, as C = 12, and H = 1, and as carbon and hydrogen combine to form benzene in the ratio 12:1, the simplest formula which we can use to express the composition of the reacting weight of benzene is C.H. = 78. When we extend our quantitative study of the reactions of benzene we find that the mass of this compound which interacts with other compounds and with elements is either 78 or a whole multiple of 78.
These examples give some notion of the methods used for determining the value to be given to the reacting weight of a compound. There is no generally applicable chemical method. Each compound must be considered apart from other compounds. The object of the inquiry is to find the relative weight of the smallest mass of the compound which interacts with other compounds, or with elements, in chemical changes. The composition of this mass is then expressed in the formula of the compound.
It will be noticed that in this inquiry the combining weights of the elements are assumed to be known.
But we know that great difficulties have to be overcome before the
combining weights of the elements can be determined ; indeed it was stated that the only satisfactory principle on which a method for finding these combining weights has been based is physical rather than chemical. We shall see later on that the same physical principle gives us
means for determining the reacting weights of compounds.
In addition to the three laws of chemical combination now 87 considered—the law of fixity of composition, the law of multiple proportions, and the law of reciprocal proportionsthere is another generalised statement regarding the volumes of gaseous elements or compounds which interact and the volumes of the gaseous products of these interactions.
The law of volumes, or the law of Gay Lussac, states that the volume of a gaseous compound produced by the interaction of gaseous elements or compounds bears a simple relation to the volumes of the gases from which it is produced, and the volumes of the interacting gaseous elements or compounds bear a simple relation to each other,
All volumes are measured under the same conditions of temperature and pressure.
Vols. of gaseous elements or compounds.
products. 1 vol. hydrogen and 1 vol.
2 vols, hydrogen chlorine
H + Cl = HCI. 2 vols. hydrogen and 1 vol.
2 vols. water-gas. oxygen
H, + () = H,O. 3 vols. hydrogen and 1 vol.
2 vols. ammonia. nitrogen
3H+ N = HN. 2 vols. carbon oxide and 2 vols.
2 vols. carbonyl chlorine
CO + Cl, = COCI, 2 vols. hydrogen iodide and
2 vols. hydrogen 1 vol. chlorine
chloride and 1
HI + Cl = HCl + I. vol. iodine-gas. 2 vols. ethane and 2 vols.
2 vols. chlorchlorine
ethane and 2 CH, + Cl, = CH,Cl + HCl. vols, hydrogen