Selectivity
The ability of a catalyst to direct a reaction towards one particular product among several possible ones.
Formula / equation
selectivity = desired product / total product
Unit
Dimensionless
In depth
Selectivity is often more valuable commercially than raw activity, because separating unwanted by-products is expensive and wasteful. Shape-selective catalysts such as zeolites achieve it by admitting only molecules that fit their pore geometry, while enzymes achieve it through precise binding.
Examples in the real world
ZSM-5 zeolite producing para-xylene almost exclusively; silver catalysing ethene to ethylene oxide rather than CO₂.