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element such as chlorine is attended with an increase in the affinity-constants; thus the relative affinities of the acids. H,C,0,, H CIC,0,, HCl,C,0,, HCl,C,, (acetic, monochloracetic, dichloracetic, trichloracetic, acid) are in the ratio 51 : 38 : 76 : 79.

Increase of the number of oxygen atoms relatively to that of the other atoms forming the molecule of an acid is then frequently accompanied by an increase in the affinity of the acids. And similarly, an increase in the affinity accompanies the replacement of atoms of hydrogen by atoms of more negative elements, such as chlorine or bromine.

These facts concerning the connexion between the compositions and the properties of acids establish the existence of definite relations between the acidic or non-acidic character of compounds of hydrogen with oxygen and a third element, the basicities of acids, and the values of the affinity-constants of acids, on the one hand, and the nature, number, and probably the relative arrangement, of the atoms in the molecules of these compounds, on the other hand.

If we suppose that every atom in a molecule, or reacting atomic aggregate, is directly related in some way to a limited number of other atoms, then it appears that an atom of hydrogen which is directly related to strongly negative atoms is usually easily replaceable by atoms of positive elements; or, to use a shorter form of words, we may say that this atom of hydrogen performs an acidic function in the molecule, or is acidic. It also appears that an atom of hydrogen which is directly related to strongly positive atoms, e.g. atoms of potassium or sodium, is not acidic, that is cannot be replaced by the atoms of positive elements. But we are not yet in a position to discuss this subject of the connexion of properties with molecular structure otherwise than in the most general way (s. Chap. XVII.).

In Chap. XII. we made a slight examination of some of the circumstances which condition the course and final results of a chemical change. We then arrived at the conception of chemical equilibrium as the result of direct and reverse processes of change occurring simultaneously in the changing system (comp. pars. 233 to 236).

The general representation which the molecular and atomic theory puts before us of a system of substances free to act and react chemically-for instance of a mixture in dilute aqueous solution of equivalent quantities of potash, soda, and nitric acid

332

-is that of a great many small particles moving about freely, colliding and exchanging parts so as to produce new particles, which also collide with each other and with those of the original particles which remain unchanged, and some of which in so doing are decomposed with the re-formation of the original particles. This redistribution of atoms is accompanied by a redistribution of energy; energy is degraded in some of the molecular changes, and raised to a higher form in other parts of these changes, but the net result is a degradation of a portion of the energy of the whole system. The strife of molecules proceeds until equilibrium results; this equilibrium may be overthrown by introducing a fresh number of molecules of one of the original constituents of the system, or by altering the physical conditions under which the equilibrium was attained.

In Chap. XII. we briefly examined some cases of chemical 333 change wherein less complex substances, one at least of which was a gas, were produced from a more complex substance, by the action of heat alone. These changes, which were classed together under the name dissociation, were found to be reversible ; that is to say, the original more complex body was re-formed when the products of the change were allowed to cool in contact with each other. We learned that there was a certain distribution of the changing substances at any specified temperature and pressure, but that change of temperature or pressure was attended with chemical change either in the direct or reverse direction.

The explanation which the molecular and atomic theory gives of dissociation can only be indicated here. Dissociation is regarded by this theory as essentially a change of one kind of molecules into two or more kinds of simpler molecules, brought about by adding heat-energy to the system. The explanation is based on a deduction from the fundamental assumptions of the theory, to the effect that there must be differences in the states of motion of individual molecules in a mass of gaseous molecules of one kind. The kinetic energy of the molecules is made up of two parts; the energy of the motion of the molecules as wholes, and the energy of the rotation of the parts of the molecules. Although the sum of these must be constant as long as temperature is unchanged, yet the distribution of the two motions, and hence of the two energies, may differ much, as regards the individual molecules. The energy due to the rotation of the parts of

some of the molecules may be so great, that collision between
these molecules may cause them to separate into parts; the
energy due to the motion of other molecules as wholes may be
so much greater than the energy due to the rotation of their
parts that a considerable quantity of energy must be added to
these molecules from a source external to the system before
they separate into parts. When the system is heated, those
molecules whose kinetic energy is chiefly due to the motion of
rotation of their parts will be at once separated into parts;
they will be dissociated. Some of the heat-energy added to
the system will be used in increasing the motion of rotation of
parts of other molecules, until these molecules also are dis-
sociated. The process of dissociation will proceed rapidly for
a time, but as the number of molecules which are not separated
into parts becomes fewer so will the rate of dissociation be-
come less, as temperature rises. But it will be possible for
some parts of molecules to reunite and reproduce some of the
original kinds of molecules, the rotational energy of which
will not be greater than that which brings about the separa-
tion of molecules into parts. Reunion of parts of molecules
will therefore occur to some extent. If temperature is kept
constant, the processes of separation of molecules into parts,
and of recombination of parts of molecules, will proceed until
both the kinetic energy of the system, and the atoms which
form the molecules of the system, are so distributed that
equilibrium results.
In
par.

318 we learned that the specific gravity of gaseous nitrogen tetroxide at low temperatures is about half what it is at high temperatures. The most probable explanation of this fact, in terms of the molecular theory, is that gaseous nitrogen tetroxide has two molecular weights, one half as great as the other. The formulae N, O, and No, express the atomic compositions of the two molecules. The action of heat on the molecules N, O, is to convert them into the mole cules NO,; the change is represented in an equation thus N, 0,= 2NO, But at any temperature between that at which only N, O, molecules exist and that at which only NO, molecules exist, the gas must be composed of a mixture of both NO, and NO, molecules. If we adopt this explanation of the action of heat on nitrogen tetroxide, then it is evident that determinations of the spec. gravity of the gas at varying temperatures give data from which the relative number of molecules of each kind (N., and NO,) at that temperature

334

can be calculated. The change from N, O, to NO, is a process of dissociation (s. pars. 233 to 236).

The only other explanation of the change of spec. gravity 335 of nitrogen tetroxide is that which asserts that this gas

does not even approximately obey the ordinary law of the expansion of gases by heat. This explanation obliges us to assume that the rate of expansion of gaseous nitrogen tetroxide as temperature rises differs widely from the normal rate of expansion of gases; and also that the rate of expansion of nitrogen tetroxide itself is very different at different temperatures. To make this assumption is to go against the mass of evidence concerning the relations of the volumes of gases to temperature; whereas the assumption which is made in the statement of the molecular explanation of the facts is wholly in keeping with a large mass of evidence, and at the same time brings the apparently abnormal behaviour of nitrogen tetroxide within the number of those occurrences which find a simple explanation in terms of the molecular and atomic theory.

Many other instances of so-called abnormal vapour-densities 336 have been observed. For instance, the gas obtained by heating sulphuric acid, H,SO,, is about 24.5 times heavier than hydrogen. Now the molecular weight of a gaseous compound is twice the spec. gravity of that compound referred to hydrogen as unity; therefore, reasoning only from determinations of the spec. gravity of the gas obtained by heating sulphuric acid, we should conclude that the molecular weight of gaseous sulphuric acid is approximately 49. But the simplest formula which can express the composition of a molecule of sulphuric acid is H 80,= 98, if the atomic weights of sulphur and oxygen are 32 and 16 respectively. The density of the vapour of sulphuric acid seems then to be abnormal. As the definition of the molecular weight of a gas is deduced from the law of Avogadro, we begin to doubt this so-called law. But our doubts are laid when experiment proves that the gas obtained by heating sulphuric acid is a mixture of equal volumes of water-gas and sulphur trioxide. The specific gravity of such a mixture is calculated thus: the formulae H.O and So, represent the compositions of molecules of gaseous water and gaseous sulphur trioxide, respectively ; but H,0=18, and So, = 80; therefore the molecular weights of the two gases are 18 and 80, respectively; but the molecular weight of a gas is the weight of 2 volumes of that gas (comp.

this gas.

337

Chap. xv. par. 294); therefore a mixture of equal volumes of

18 + 80 H,O and SO, is = 24.5 times heavier than hydrogen.

4 Therefore the observed specific gravity of the gas obtained by heating sulphuric acid is identical with the specific gravity calculated from the experimentally determined composition of

Much discussion at one time took place as to the specific gravity of phosphorus pentachloride in the gaseous state. Analyses of this compound, and determinations of the atomic weights of phosphorus (31) and chlorine (35.5), shewed clearly that the simplest formula which could be given to the solid compound was PCI. The specific gravity of this compound in the state of gas, referred to hydrogen as unity, must be 31 + (35.5 x 5)

104:25:

; or, referred to air as unity, the spec. 2 gravity must be 7.2.

The following table gives the observed spec. gravities (air = 1) at different temperatures of the gas obtained by heating phosphorus pentachloride, PCI,. In each case the pressure was 760 mm.

Ratio of

PC1z to
Spec.
Temp.

total
gravity

Temp. possible

PC12 1820 5.08

.42

2300 190 4.99 .45 250 200 4.85 •49 274

288

Ratio of
Spec.

PC1, to

total gravity

Temp.
possible

PC12
4:30 .68 2890
3.99 .80 300
3.84 .98 327
3.67 .97 336

Ratio of Spec.

PClz to

total gravity

possible

PCI, 3.69 3.65 3.65

.98 3.65 .98

•96) .98

The specific gravity, and therefore probably the molecular weight, of the gaseous compound varies; but at no temperature does the specific gravity approximate to that required by the formula PCI. Here again we have an instance of so-called abnormal vapour-density. Further investigation however shewed that the numbers are not really abnormal, but that they find a ready and simple explanation in terms of the molecular and atomic theory. Experiments proved that phosphorus pentachloride is gradually dissociated by heat into phosphorus trichloride and chlorine; PCI =PC1, +Či,. The gas obtained by heating PCl, is therefore a mixture of more than one substance. The specific gravity of a mixture of phosphorus

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