A system of physical chemistry - Index of

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A system of physical chemistry - Index of

2 A SYSTEM OF PHYSICAL CHEMISTRY

dissolving some substance in the liquid ; the variation of the pressure

of saturated vapour with temperature in terms of the latent heat of

constant of a chemical

vaporisation ; the variation of the equilibrium

reaction with temperature and pressure ; the electromotive force of

reversible cells; the relation between the heat of reaction and the

chemical affinity of the process.

The laws of thermodynamics are not, however, in the first place

mechanical laws—

though we shall see later that the second law possesses

a statistical mechanical basis— but are founded essentially upon

experience ; they are taken to be true, because no phenomenon or

process in nature contradicts them. The characteristic feature of

thermodynamical treatment is, as we have already seen, that the results

obtained involve no assumption regarding the molecular structure

of the system under consideration. Thermodynamical deductions are

therefore perfectly general, in the sense that they hold good quite

apart from specific molecular theories. It is important to point out

that mechanics likewise furnishes a general mode of treatment, known

as generalised dynamics, by which certain physico-chemical results may

be obtained, the line of reasoning followed being to a certain extent

analogous to thermodynamic reasoning in that no particular assumption

is made regarding the molecular structure and molecular properties of

the system investigated. We treat the system as a whole and do not

attempt to deal with each molecule individually.

The best account of the applications of generalised dynamics to

physico-chemical problems is that of Sir J. J. Thomson, in his book

Applications of Dynamics to Physics and Chemistry. Among the problems

solved by this method are : the process of evaporation (including

the effect of electric charge upon the vapour pressure), the effect of

an inert gas upon the value of the saturated vapour pressure certain

;

properties of dilute solutions, such as the lowering of the vapour pressure

of the solvent by addition of solute, and the lowering of the

freezing point by the solute ; the deduction of the law of mass action,

as expressed

being shown

in the

to be

equilibrium

an explicit

constant, the equilibrium constant

function of the temperature the

;

principle of mobile equilibrium ;

the effect of pressure upon the freezing

point of a pure liquid, and the effect of pressure upon the solubility of

one substance in another ; the phenomenon of adsorption ; the relation

between the electromotive force and "the chemical change producing

it " and ; other problems.

It is evident from this enumeration that the methods of generalised

dynamics are of wide applicability. No is attempt made, however, to

pursue this method in the present volume. Suffice it to say that

generalised dynamics is based upon a general principle embodied in

Hamilton's and Lagrange's functions. These " hardly require a more

detailed knowledge of the structure of the system to which they are

applied than the conservation of energy—the first law of thermodynamics—

itself, and yet are capable of completely determining the

motion of the system". Thomson compares the thermodynamical

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