Conversion, yield and selectivity are three different numbers
These three terms get used interchangeably on the plant floor, and they should not be. Conversion is the share of the charged reactant that actually reacted. Yield is the share of the theoretical product you ended up holding. Selectivity is the share of the consumed reactant that went to the product you wanted. They are tied together by one relationship: yield equals conversion times selectivity.
| Metric | Formula | What it tells you |
|---|---|---|
| Conversion | (charged − unreacted) ÷ charged | How far the reaction went |
| Yield | product ÷ theoretical yield | How much you actually kept |
| Selectivity | yield ÷ conversion | How much leaked to by-products |
Charge 100 mol, find 20 mol unreacted and recover 70 mol of product, and you get 80 % conversion, 70 % yield and 87.5 % selectivity. The gap says 10 of the 80 mol consumed went somewhere other than your product. Chasing yield without splitting it into these two parts usually means fixing the wrong variable.
The stoichiometric factor is how many moles of product one mole of the limiting reactant can make. Use 1 for a simple A + B to C reaction, or 0.5 when two molecules combine into one. If you track batches by weight, divide by molecular weight first so every field is in moles.
Low selectivity usually points at catalyst, temperature and residence time, while low conversion points at reaction time and molar ratio. Scale-up decisions should still rest on analytical data reviewed with a process engineer rather than a single batch calculation.
Frequently Asked Questions
The inputs disagree with each other. Either the stoichiometric factor is wrong or the unreacted reactant was overstated by the analysis. Close the material balance and re-enter the numbers.
If selectivity is high and conversion is low, push the reaction harder. If conversion is high and selectivity is low, suppress the side reaction first, because unreacted feed can be recycled and by-products generally cannot.