How To Find Limiting Reagent

Overview

When we mix reactants in mole ratios according to the coefficients in a balanced equation, we say that reactants are in stoichiometric proportions. All initial reactants are totally consumed when the reaction goes to the completion. However, we carry out most reactions with a limited amount of one reactant and excess quantities of others. We do this to minimize the extent to which reverse reactions can occur and thus maximize the yield of products.

Introduction

The following scenario illustrates the significance of limiting reagent. The situation is much like the task, that packaging a snack meal for airline passengers. Each package consists of a sandwich, two cookies, and an orange; that is, the "stoichiometric" proportions are 1:2:1. If we have 100 sandwiches, 200 cookies, and 100 oranges, we can prepare 100 snack meals and have nothing left over. But from 98 sandwiches, 202 cookies, and 102 oranges we can assemble only 98 packages. The sandwiches are the "limiting reagents", and the cookies and the oranges are reactants in excess. From 105 sandwiches, 202 cookies, and 107 oranges we can assemble 101 packaged meals. Here the cookies are the "limiting reagent". In effect, what we do is determine how much of the product can be made from each "reactant" assuming we have enough of the others. We then pick the smallest answer.

Definition

The limiting reagent is the reactant that is completely used up in a reaction, and thus determines when the reaction stops. From the reaction stoichiometry, we can calculate the exact amount of reactant needed to react with another element, If the reactants are not mixed in the correct stoichiometric proportions. then one the reactant will be entirely consumed while other will be left over. So the limiting reagent is the one which that totally consumed, and limits the reaction from continuing because there is none left to react with the in-excess reactant.

Rules to finding limiting reagent.

There are two ways to determine limiting reagent, which is listed below.

Method1: Find the limiting reagent by looking at the number of moles of each element.

1. Determine the balanced chemical equation for the chemical reaction.
2. Convert all given information into moles ( through the use of molar mass as a conversion factor).
3. Calculate the mole ratio from the given information. Compare the calculated ratio to the actual ratio.
4. Use the amount of limiting reactant to calculate to calculate the amount of product produced.
5. If necessary, calculate how much is left in excess of the non-limiting reagent.

Method2: Find the limiting reagent by calculating and comparing the amount of product each reactant will be produced.

1. Balanced the chemical equation for the chemical reaction.
2. Convert the given information into moles.
3. Use stoichiometry for each individual reactant to find the mass of product produced.
4. The reactant that produces lesser amount of product is the limiting reagent.
5. The reactant that produces a large amount of product is the excess reagent.
6. To find the amount of remaining excess reactant, subtract the mass of reagent consumed from the total mass of excess reagent, given.