Limiting Reagents

All of the calculations in Section 1.7.1 assumed the reactants were supplied in exactly the stoichiometric ratio, so that everything reacts completely. In practice this is rarely true -- one reactant usually runs out before the others. The reactant that is completely consumed first is called the limiting reagent: it limits how much product can form, because once it is gone the reaction stops, no matter how much of the other reactants remain. The reactants left over are called excess reagents.

The kesari analogy again: the recipe needs 2 cups of sugar per cup of rava. If you have 8 cups of sugar but only 3 cups of rava, the recipe only lets you use 3 cups of rava with 6 cups of sugar (making 18 cups of kesari) -- the leftover 2 cups of sugar cannot be used, because there is no more rava. Here rava is the limiting reagent and sugar is the excess reagent, even though sugar was the more plentiful ingredient in absolute terms.

Chemical example: for S + 3F₂ → SF₆, if 3 moles of sulphur are reacted with 12 moles of fluorine, the stoichiometric requirement is 3 × 3 = 9 moles of fluorine for all 3 moles of sulphur -- so all the sulphur is used up (limiting reagent) while 12 − 9 = 3 moles of fluorine remain unreacted (excess reagent). …