when we do chemical reactions it is very unlikely that two substances will be present in precisely the right amounts in order to both react completely inevitably one substance will run out first leaving some of the other unable to react the reactant that runs out first is called the limiting reactant or limiting reagent and the one that has some left over will be called the reactant in excess let's look at a simple analogy to see how this works if we are making cheese sandwiches for a family lunch and we make them according to the following reaction one slice of cheese plus two slices of bread will make one cheese sandwich so the stoichiometric coefficients of this reaction will be one to two to one let's say we look in the fridge and we see that we have 28 slices of bread and 11 slices of cheese how many sandwiches can we make 28 slices of bread could potentially make 14 sandwiches but 11 slices of cheese can only make 11 sandwiches so in this case cheese will be the limiting reagent it will get used up completely in order to make the eleven sandwiches and bread will be our reagent in excess there will be six slices of bread left over make sure to realize that the limiting reagent will not always be the one present in the smaller amount if we had 15 slices of cheese along with the 28 slices of bread then all of a sudden bread would become the limiting reactant since that could only produce 14 sandwiches while 15 slices of cheese can produce 15 sandwiches we must consult the stoichiometric ratio of a reaction in order to figure out which substance is the limiting reactant let's look at this concept applied to some real chemistry silicon nitride is produced according to this equation if we have 2 grams of silicon and 1.5 grams of nitrogen which will be the limiting reagent well the grams tell us nothing we have to convert to moles let's use the molar masses of silicon and nitrogen to convert these into their respective molar quantities now that we have these one strategy we could use would be to see how many moles of product each of these could potentially generate by using the stoichiometric factors for each substance this many moles of silicon could potentially generate 0.0237 moles of product this many moles of nitrogen could potentially generate 0.0268 moles of product since the silicon cannot produce as much silicon must be the limiting reactant and that is the quantity we will use to do stoichiometric calculations for the product