Reaction in a Gas Mixture

If the pressure p of the equilibrium mixture is low enough, then the factor exp. .. in equation (102) may be omitted. A value of can then be calculated from measured values of the equilibrium mole fractions xg and of the equilibrium pressure p . If the initial amoimts n] are known, then it is sufficient to determine p plus any one of the x s, chosen for the availability of a convenient method of chemical or physical (e.. spectro-photometric) analysis. Provided that 27 56 0, the x s can be determined (but still only if the pressure is small enough) if the volume F is measured, in addition to the pressure, from the relation  

Instead of the static methods implied above for determining the equilibrium mole fractions and pressure, flow methods can be used in which the reactant gases are made to flow at measured rates through a reaction chamber (for example, a long tube) in which they are allowed a sufficient residence time to reach equilibrium. The effluent gas is then analysed. Flow methods are especially useful for reactions which are slow in the absence of a solid catalyst the reaction chamber is then loosely packed with solid catalyst. 

Experimental methods for the study of equilibria in gas reactions will be reviewed in a subsequent volume. 

If the pressure cannot be made low enough to justify the omission of the factor exp. ., from equation (102), then in addition to an analysis of the equilibrium mixture, measurements are needed of the molar volume of the gas mixture over a range of mole fractions of each of the substances round the equilibrium mole fractions, and all this over a range of pressures between 0 and the equilibrium pressure p . Such a formidable programme has not to the author s knowledge been carried right through for any mixture relevant to a chemical reaction. 

If the pressure is low, but not quite low enough, then the molar volume Vra may be written in the form  

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