stoichiometry and limiting reagent

[yui_96]

Advertisement - Members don't see this ad

Why is it not the absolute mole quantities of the reactants alone that determine which reactant is the limiting reagent, but rather the rate at which the reactants are consumed combined with the absolute mole quantities determines which reactant is the limiting reagent?

 

[BerkReviewTeach]

The limiting reagent is the one you run out of first. So consider a system where you have 3.0 moles of A and 2.0 moles of B, but they react in a ratio of 2 A to 1 B. You use up A twice as fast, so you need twice as much of A as you need of B. Because you have less than twice as much of A, you are going to run out of A first. So it is A that limits how much of the reaction can occur. I hope this helps.

 

[NextStepTutor_1]

Hi @yui_96 -

Just as a quick additional note, some students find cooking to be a helpful analogy. Imagine you're making sandwiches, and each sandwich needs two slices of bread, a slice of meat, three slices of cheese (let's imagine we really like cheese, I dunno), and a piece of lettuce. Following that recipe, if you only have two slices of bread but huge amounts of meat, cheese, and lettuce, you still only have the supplies for one sandwich (in this case, bread is your limiting reagent). On the other hand, if you only have three slices of cheese, you will similarly only have materials for one sandwich no matter how much bread, meat, and lettuce you have -- cheese would be the limiting reagent there.

Hope this helps & best of luck!!