What is the limiting reagent in a reaction? Why is it necessary to determine the limiting reagent?

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bellw's profile pic

bellw | (Level 1) Adjunct Educator

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The limiting reagent of a chemical reaction is always the Reactant that has the lowest molar concentration as determined through the balanced chemical reaction and quantities given:

Mass (grams) Given / Coefficient x GFW of Reactant = LR value

Volume (liters) Given / Coefficient x 22.4L of Reactant = LR value

Moles Given (mol) / Coefficient of Reactant = LR value

The reactant with the lowest LR value is always the limiting reactant

As the limiting reactant is always completely consumed in the reaction; the reaction will come to a stop at that point. Therefore the amounts of products produced must be based solely on  the quantity of the limiting reagent origionally present.

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ncchemist | eNotes Employee

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The limiting reagent is the reagent in a multicomponent reaction that will be exhausted as the reaction proceeds to completion.  In other words, if a chemical reaction has multiple reagents, the limiting reagent is the reagent that will become depleted first, leaving some amount of the other reactants remaining.  This is important to determine because the amount of the limiting reagent that is present in the reaction will determine how much product is produced.  Specifically, the number of moles of the limiting reagent will determine the number of moles of product. 

Determining which reagent is the limiting reagent is easy.  Take the amounts of each reagent and convert them from grams to moles.  Then divide the number of moles of each reagent by the corresponding coefficient for each reagent as found in the balanced chemical equation.  This puts each reagent on an equimolar basis.  The smallest number will show which reagent is the limiting reagent.