A general expression is derived for the apparent ``thermal conductivity'' of reacting gas mixtures in which a single chemical reaction proceeds at a finite rate. In contrast to cases where reaction rates are either very high or very low, it is found that the ``thermal conductivity'' depends on the geometry and scale of the systemmdash;hence the quotes. This expression applies to the three geometries that can be described onehyphen;dimensionally: parallel plates, concentric cylinders, and concentric spheres. The results are limited to systems with small temperature differences, such as thermal conductivity cells, wherein reaction rate can be assumed constant. In addition to the scale and geometry, the ``thermal conductivity'' is determined by three types of constants having the dimensions of reciprocal length. Gas phase reaction is characterized by phgr;, which is proportional to the square root of the chemical reaction rate at equilibrium. (This is the total rate in either direction, not the net rate, which is, of course, zero.) Heterogeneous chemical reaction at the two bounding surfaces is characterized by psgr;1and psgr;2, while khgr;1and khgr;2account for the temperature accommodation. The nature of the ``thermal conductivity'' is discussed for several specific systems. The expression is shown to approximate a numerical calculation of the effect of chemical reaction rate on stagnation point heat transfer. Experimental thermal conductivity data for the N2O4rlarr2;2NO2system are adequately described in terms of this theory. An upper limit for the rate constant for the bimolecular dissociation of N2O4is estimated to be 5.3times;109cm3molemdash;1secmdash;1at 296deg;K.
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