What does selectivity measure in reaction engineering?

• A. Selectivity equals conversion.
• B. Selectivity equals overall yield.
• C. Selectivity S is the moles of desired product formed per mole of undesired byproduct formed.
• D. Selectivity equals rate constant.

Answer: (C)

Selectivity measures how much the reaction favors forming the desired product over undesired byproducts, i.e., the distribution of products when the reaction occurs. It is defined as the ratio of moles of the desired product formed to the moles of the undesired byproduct formed. This focuses on which products you actually end up with, not on how much of the reactant reacts (conversion) or the total amount of desired product relative to feed (yield). It’s also not a kinetic parameter like a rate constant. For example, if you form 4 moles of the desired product and 1 mole of the byproduct, the selectivity is 4, showing a strong preference for the desired product.