Limiting and Excess Reactants

Perhaps you have had an occasion where you went to the pantry to see if you had enough ingredients to make a certain recipe only to determine that you did not have enough of one thing or another. The ingredient that you do not have enough of is “limiting” how much of the recipe you can make. Perhaps you have even gone so far as to calculate just how much of the recipe you can make with the ingredients on hand. That is real life stoichiometry!

In chemistry (or in the kitchen), reactions will continue until one of the reactants or ingredients is used up. It can be very frustrating to be in the middle of cooking a great recipe, only to discover that you are “limited” by not having enough of an ingredient. Some simple calculations ahead of time can help avoid a social disaster when cooking.

When determining which substance in a given reaction will be used up first, calculate the amount of product for each substance. The substance which produces the least amount of product will be the limiting reactant and will be used up first.