Title: fractional selectivity
Long Title: IUPAC Gold Book - fractional selectivity
DOI: 10.1351/goldbook.F02496
Status: current

Definition
The term selectivity (\(S\)) is used to describe the relative rates of two or more competing reactions on a catalyst. Such competition includes cases of different reactants undergoing simultaneous reactions 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 selectivity, \(S_{\rm{F}}\), for each product by the equation \[S_{\rm{F}}=\frac{\xi _{i}}{\sum \xi _{i}}\] The second defines relative selectivities, \(S_{\rm{R}}\), for each pair of products by \[S_{\rm{R}}=\frac{\xi _{i}}{\xi _{j}}\] In each case, \(\xi _{i}\) and \(\xi _{j}\) are the rates of increase of the extent of reactions 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.

Related Terms
- catalyst: https://goldbook.iupac.org//terms/view/C00876
- competition: https://goldbook.iupac.org//terms/view/C01198
- extent of reactions: https://goldbook.iupac.org//terms/view/E02283
- selectivity: https://goldbook.iupac.org//terms/view/S05563
- simultaneous reactions: https://goldbook.iupac.org//terms/view/S05680

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 (https://doi.org/10.1351/pac197646010071)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/F02496/html
- json: https://goldbook.iupac.org/terms/view/F02496/json
- xml: https://goldbook.iupac.org/terms/view/F02496/xml

Citation: Citation: 'fractional selectivity' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.F02496
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Accessed: 2026-05-09T09:00:02+00:00