Diathermic

Consider two distinct closed thermodynamic systems each consisting of n moles of a specific substance in a volnme Vand at a pressure p. These two distinct systems are separated by an idealized wall that may be either adiabatic (lieat-impemieable) or diathermic (lieat-condncting). Flowever, becanse the concept of heat has not yet been introdnced, the definitions of adiabatic and diathemiic need to be considered carefiilly. Both kinds of walls are impemieable to matter a permeable wall will be introdnced later. 

Warme-durchlassigkeit,/. diathermancy heat conductance, -dynamik, /. thermodynamics, -effckt, m. heat effect, -einfluss, m. inftuence of heat heat influx, -einheit, /. heat unit, thermal unit. 

Description of a thermodynamic system requires specification of the way in which it interacts with the environment. An ideal system that exchanges no heat with its environment is said to be protected by an adiabatic wall. To change the state of such a system an amount of work equivalent to the difference in internal energy of the two states has to be performed on that system. This requirement means that work done in taking an adiabatically enclosed system between two given states is determined entirely by the states, independent of all external conditions. A wall that allows heat flow is called diathermal. 

Consider a closed composite system consisting of two compartments separated by a rigid impermeable diathermal wall. The volumes and mole numbers of the two simple systems are fixed, but the energies fid1) and JV> may change, subject to the restriction f/0) + UV> = constant, imposed on the composite closed system. At equilibrium the values of f/0) and IfV) are such as to maximize the entropy. 

Consider the same composite system as before, but with the impermeable diathermal wall no longer fixed. Both internal energy as well as the volume and V may now change, subject to the extra closure condition, RP) + VP) = constant. 

Reconsider the equilibrium state of two systems separated by a rigid diathermal wall, but now permeable to one type of material (Ar1) and impermeable to all others. 

For two systems separated by a diathermal wall and with Xk = U, the affinity... 

It is most important to know in this connection the compressibility of the substances concerned, at various temperatures, and in both the liquid and the crystalline state, with its dependent constants such as change of. melting-point with pressure, and effect of pressure upon solubility. Other important data are the existence of new pol3miorphic forms of substances the effect of pressure upon rigidity and its related elastic moduli the effect of pressure upon diathermancy, thermal conductivity, specific heat capacity, and magnetic susceptibility and the effect of pressure in modif dng equilibrium in homogeneous as well as heterogeneous systems. 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS DIATHERMIC WALL ADIABATIC WALL... 

We shall later deal with adiabatic, diathermal, semipermeable, and other types of partitions. 

Figure 3.9 Schematic representation of (a) an adiabatic container, allowing PV work by movement of a piston, but unaffected by other changes in the surroundings (b) a diathermal (nonadiabatic) container, allowing thermal equilibration with the surroundings.

An enclosure, such that the equilibrium of a system contained within it can only be disturbed by mechanical means, is adiabatic, otherwise it is diathermic. For instance, stirring, or the passage of an electric current, constitute mechanical means." A system Kf] in an adiabatic enclosure is adiabatically isolated, but this does not preclude mechanical interactions with the surroundings. Its transitions are then called adiabatic. 

Such a system will be called a standard system (n — 1 enclosures in diathermic contact, each containing a simple fluid, may serve as example. x being any one of the pressures)... 

Suppose two systems K pi x) and KB(y) to be in mutual diathermic contact Experience shows that the states SJ and fp, cannot be assigned arbitrarily. 

When K is in equilibrium, 0 must be constant in time, and this will be die case if il is a function of llie (time-indepeiidem) Hamiltonian H of K. Ensemble averages are now assumed to coincide with temporal averages. When, in particular. K is in diathermic equilibrium with its surroundings one can show that

[Pg.1607]

G. D. Liveing and J. Dewar examined the absorption spectrum of liquid air. H. Buff reported that dry air absorbs 50 to 60 per cent, of the heat rays from a source at about 100°, and that the absorptive power of dry air exceeds that of moist air. J. Tyndall, on the contrary, found air to be quite diathermous. N. Egorofl examined the absorption of sunlight by the atm., and H. Wild showed that if dust be excluded, dry or moist air absorbs very little light. 

It is observed experimentally that, when two bodies having different temperatures are brought into contact with each other for a sufficient length of time, the temperatures of the two bodies approach each other. Moreover, when we form the contact between the two bodies by means of walls constructed of different materials and otherwise isolate the bodies from the surroundings, the rate at which the two temperatures approach each other depends upon the material used as the wall. Walls that permit a rather rapid rate of temperature change are called diathermic walls, and those that permit only a very slow rate are called adiabatic walls. The rate would be zero for an ideal adiabatic wall. In thermodynamics we make use of the concept of ideal adiabatic walls, although no such walls actually exist. 

We describe the phenomenon that the temperatures of the two bodies placed in diathermic contact with each other approach the same value by saying that heat has transferred from one body to another. This is the only concept of heat that is used in this book. It is based on the observation of a particular phenomenon, the behavior of two bodies having different temperatures when they are placed in thermal contact with each other. 

We define a heat reservoir as any body, used as part of the surroundings of a particular system, whose only interaction with the system is across a diathermic boundary. A heat reservoir is then used to transfer heat to or from a thermodynamic system and to measure these quantities of heat. It may consist of one or more substances in one or more states of aggregation. In most cases a heat reservoir must be of such a nature that the addition of any finite amount of heat to the system or the removal of any finite amount of heat from the system causes only an infinitesimal change in the temperature of the reservoir. 

We consider an isolated, homogenous system, and imagine that a part of the single phase is separated from the rest of the phase by a diathermal, nonrigid, permeable wall. By this device we can consider variations of the entropy, volume, and mole numbers of the two parts of the system subject to the conditions of constant entropy, volume, and mole numbers of the... 

We have already defined Dalton s pressures and the partial pressures. There is still one more pressure of a gas in a gas mixture that must be considered. This is the equilibrium pressure, and was first discussed by Gibbs [15]. The equilibrium pressure of a component is the pressure exerted by the pure component when it is in equilibrium with a gas mixture through a rigid, diathermic membrane that is permeable only to that component. The temperature of the pure phase and that of the gas mixture must be the same, but the pressure of the two phases will be different. We may conceive of two phases at the same temperature separated by such a membrane, one phase being the pure fcth component and the other the gas mixture. The condition of equilibrium is that the chemical potential of the fcth component must be identical in each phase. If P is the equilibrium pressure, then by Equation (7.69)... 

The measurement of osmotic pressure and the determination of the excess chemical potential of a component by means of such measurements is representative of a system in which certain restrictions are applied. In this case the system is separated into two parts by means of a diathermic, rigid membrane that is permeable to only one of the components. For the purpose of discussion we consider the case in which the pure solvent is one phase and a binary solution is the other phase. The membrane is permeable only to the solvent. When a solute is added to a solvent at constant temperature and pressure, the chemical potential of the solvent is decreased. The pure solvent would then diffuse into such a solution when the two phases are separated by the semipermeable membrane but are at the same temperature and pressure. The chemical potential of the solvent in the solution can be... 

A binary system consisting of two parts separated by a diathermic, rigid membrane has three degrees of freedom. The particular system under discussion can be made univariant by fixing the temperature and pressure of the pure solvent the equilibrium pressure on the solution is then a function of the composition of the solution. The condition of equilibrium is... 

Katchalsky and Curran [2], for example, consider a system separated from the environment by a rigid adiabatic wall. The system consists of two compartments 1 and 2, separated by a diathermal, elastic barrier that is permeable to one of the components in the system (Figure 4.1). It can be shown that the entropy generation rate is given by... 

The absence of heat flow may be a result of the walls not permitting the transfer of thermal energy. Boundaries of this kind are called adiabatic. (Adiabatic walls are infinitely good thermal insulators.) If the walls are non-adiabatic (sometimes called diabatic or diathermal) and do permit heat transfer, but it does not occur, we say that the system is at thermal equilibrium with its surroundings. 

---