= Catalytic selectivity
<Catalytic selectivity> concerns which <reaction products> form when competing <reaction mechanisms> are available. Faster conversion is valuable when a process would otherwise be impractically slow; a suitable <catalyst> can also favour a useful product within the available time. In an illustrative pair of irreversible <first-order reactions> A to B and A to D, the initial fraction of converted A going to B is $k_B/(k_B+k_D)$. Multiplying both <rate constants> by the same factor changes speed but leaves this fraction unchanged; changing their relative values changes the kinetic product split. This is a finite-time kinetic illustration, not a new <equilibrium constant>.
A <catalyst> need not accelerate every competing <chemical reaction> equally. <Enzymes>, for example, can distinguish <substrates> through their <active sites>.<Chemistry 2e by Paul Flowers and others>[§12.7, Enzyme Structure and Function, Figure 12.25: substrate binding and specificity]{ref} If every relevant reversible channel eventually equilibrates in a <closed system>, its final <chemical equilibrium> still obeys the endpoint <thermodynamics>. Selective acceleration, product removal, or external driving can make an observed product distribution depend on time and operating conditions. A claim of improved yield must specify those conditions.
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