Electric Enthalpy

To illustrate the notation and meaning of the third-order coefficients let us discuss the electric Enthalpy H2 (6.16) and the electric Gibbs function G2 (6.17). With Ek and Vab as independent variables they are important for experiments involving elastic waves arrd vibrations. 

A general prerequisite for the existence of a stable interface between two phases is that the free energy of formation of the interface be positive were it negative or zero, fluctuations would lead to complete dispersion of one phase in another. As implied, thermodynamics constitutes an important discipline within the general subject. It is one in which surface area joins the usual extensive quantities of mass and volume and in which surface tension and surface composition join the usual intensive quantities of pressure, temperature, and bulk composition. The thermodynamic functions of free energy, enthalpy and entropy can be defined for an interface as well as for a bulk portion of matter. Chapters II and ni are based on a rich history of thermodynamic studies of the liquid interface. The phase behavior of liquid films enters in Chapter IV, and the electrical potential and charge are added as thermodynamic variables in Chapter V. 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

This reaction has been carefully studied with the aim of obtaining the enthalpy of combustion as electrical energy, and successful hydrazine-air fuel cells have been developed using potassium hydroxide as the electrolyte. The hydrazine fuel, however, has the disadvantage that it is expensive and poisonous. 

As expected from the enthalpy of formation, water is thermally very stable but when steam is heated to above 1300 K slight dissociation to the elements does occur. Pure water is almost a nonconductor of electricity but slight ionic dissociation occurs ... 

In some instances, however, pai t of the chemical energy bound in relatively high-enthalpy compounds can be converted directly to electricity as these reactants are converted to produc ts of lower enthalpy (galvanic action). A process in the opposite direc tion also is possible for some systems an elec tric current can be absorbed as the increased chemical energy of the higher-enthalpy compounds (electrolytic action). The devices in which electrochemical energy conversion processes occur are called cells. 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

The driving force for the transport of all particles is a change in the electrochemical potential /i, which is related to the partial molar free enthalpy /i, and the electric potential 0 as follows ... 

Sensible heating only Cionstant moisture content increase in specific volume, dry- and wet-bulb temperature, and specific enthalpy decreased % saturation Steam, hoc-water coils, or electric heating... 

Some physical properties of the elements are compared in Table 10,2. Germanium forms brittle, grey-white lustrous crystals with the diamond structure it is a metalloid with a similar electrical resistivity to Si at room temperature but with a substantially smaller band gap. Its mp, bp and associated enthalpy changes are also lower than for Si and this trend continues for Sn and Pb which are both very soft, low-melting metals. 

Theoreticians did little to improve their case by proposing yet more complicated and obviously unreUable parameter schemes. For example, it is usual to call the C2 axis of the water molecule the z-axis. The molecule doesn t care, it must have the same energy, electric dipole moment and enthalpy of formation no matter how we label the axes. I have to tell you that some of the more esoteric versions of extended Hiickel theory did not satisfy this simple criterion. It proved possible to calculate different physical properties depending on the arbitrary choice of coordinate system. 

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

As noted earlier, for a reaction at constant pressure, such as that taking place in an open coffee-cup calorimeter, the heat flow is equal to the change in enthalpy. If a reaction is carried out at constant volume (as is the case in a sealed bomb calorimeter) and there is no mechanical or electrical work involved, no work is done. Under these conditions, with w = 0, the heat flow is equal to the change in energy, AE. Hence we have... 

For the electrochemical cell reaction, the reaction free energy AG is the utilizable electrical energy. The reaction enthalpy AH is the theoretical available energy, which is increased or reduced by an amount TAS. The product of the temperature and the entropy describes the amount of heat consumed or released reversibly during the reaction. With tabulated values for the enthalpy and the entropy it is possible to obtain AG. ... 

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