It is often the case that additional (unwanted) reactions may occur in our reactor that either divert reactants to something else or use our product after it is made.
A -> B (desired product)
2B -> C
Because there is now multiple possible outcomes, we need some terminology to describe the products we get. In other words, if the world were perfect, we would always only have the reactions we want happening. But the world is not perfect (that's why we ChE's have to strive to get it closer!), so we must quantify how close to perfect we can get!
The yield of a reaction is the ratio of the desired product formed (in moles) to the total amount that could have been made (if conversion of limiting reactant was 100% and no side reactions occurred).
The selectivity of a reaction is the ratio of the desired product formed (in moles) to the undesired product formed (in moles).
Define and calculate the yield and selectivity for reaction networks