Reaction Yield

Yield is the third leg of the stool, after conversion and selectivity, and the three are routinely confused because all of them are fractions that are better when they are bigger. Conversion counts the reactant that disappeared. Selectivity compares two rates. Yield counts product actually made against reactant actually consumed — $Y = n_D/(n_{A0} - n_A)$ — and it is the number a plant's economics ultimately rest on.

Keeping conversion and yield apart is not pedantry. A reactor pushed to 99% conversion at a yield of 0.4 has turned most of its expensive feed into something that now has to be separated, treated and disposed of, and it will usually lose to a reactor running 70% conversion at a yield of 0.9 with the unreacted feed recycled. High conversion is easy to buy with volume or temperature; yield generally is not, and the two frequently move in opposite directions because the conditions that push a reaction to completion also give the by-product routes more time and more heat.

Two conventions travel under the same word, and quoting the wrong one is the commonest reporting error in the area. RELATIVE yield, which this page computes, is moles of product per mole of reactant CONSUMED. OVERALL yield is moles of product per mole of reactant FED, and it equals the relative yield multiplied by the conversion. The two agree only at complete conversion. Say which one you mean, especially in a report where somebody downstream will multiply it by something.

Yields above 1 are legitimate and should not be treated as an error. If one molecule of the reactant produces two molecules of the product, the stoichiometric ceiling is 2, and a well-behaved reactor will approach it. What matters is the ratio of the measured yield to the STOICHIOMETRIC yield the balanced equation permits — that ratio, expressed as a percentage, is what a laboratory calls percent yield, and it is the number that cannot honestly exceed 100 without something being wrong with the analysis, the drying, or the mass balance.