Thermochemical and Kinetic Theory Calculations

Thermochemical and Kinetic Theory Calculations 1. Type Reactions 

For our purposes the chemical reactions which occur between gases and metals, gases and solids, and solids and solids can be classified (1) in four groups (a) reactions in which a gas is formed as one of the products, (b) reactions in which one of the reactants is a gas and is therefore removed from the atmosphere or from the vacuum, (c) reactions in which one gas is exchanged for another gas as a result of reaction, and (d) reactions between solids in which a new phase or phases are formed but no gas is involved. We are not classifying reactions in which one of the phases involved is a liquid. This classification may be rather arbitrary, but we have found it very useful. 

For purposes of simplicity, in the case to be discussed, reactions involving sulfur, phosphorus, and other volatile components are assumed to be absent. Materials containing these elements must be avoided in vacuum systems which are to be used at high temperatures. The following reactions will be considered. 

It is important to calculate from thermochemical data the direction and equilibrium concentrations of the components involved in the reactions previously outlined. The recent work of Kubaschewski and Evans (2) is of special interest in this regard. It should be recognized that these calculations are limited in many cases by kinetic and rate considerations and the presence of reaction inhibitors such as protective films. 

The free energy of formation of many of the oxide systems and the several reacting gases is well known. However, the data for the nitrides, hydrides, and carbides of the metals have only in a few cases been determined. The vapor pressure data for the metals with which we are concerned are available with varying degrees of accuracy. However, the data on the vapor pressure of the oxides of the metals are incomplete. 

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