## fractional selectivity

in catalysis
https://doi.org/10.1351/goldbook.F02496
The term @S05563@ ($$S$$) is used to describe the relative rates of two or more competing reactions on a @C00876@. Such @C01198@ includes cases of different reactants undergoing @S05680@ or of a single reactant taking part in two or more reactions. For the latter case, $$S$$ may be defined in two ways. The first of these defines a fractional @S05563@, $$S_{\text{F}}$$, for each product by the equation $S_{\text{F}}=\frac{\xi _{i}}{\sum \xi _{i}}$ The second defines relative selectivities, $$S_{\text{R}}$$, for each pair of products by $S_{\text{R}}=\frac{\xi _{i}}{\xi _{j}}$ In each case, $$\xi _{i}$$ and $$\xi _{j}$$ are the rates of increase of the @E02283@i and j respectively, i.e. $\xi _{i}=\frac{\mathrm{d}\xi _{i}}{\mathrm{d}t}$ and $\xi _{j}=\frac{\mathrm{d}\xi _{j}}{\mathrm{d}t}$ where $$\xi _{i}$$ and $$\xi _{j}$$ are the extents of reactions $$i$$ and $$j$$ respectively.
Source:
PAC, 1976, 46, 71. (Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II: Heterogeneous Catalysis) on page 81 [Terms] [Paper]