Stoichiometry of Limiting and Excess Quantities

What we have assumed so far In all our reactions, we assume all reactants get used up We assume all reactants provided are pure We rarely see these two and thus we have Limiting reagents Percent Purity Percent Yield So in a chemical reaction, we typically only use up one reactant and the others are usually left over (excess reagents)

Example – 1 You have one 1L bag of flour. You also have 1L bag of sugar. Which one will you use up first?

Simulation Sandwich making

Why do we want to have excess amounts? We have excess amounts so that the one that we force one of the limiting (less) amounts to be used up If it is too expensive, we want to use it up and not have any remaining to spoil If it is harmful to the environment, we want to react it so it won’t be harmful It can also be unavoidable as we usually do have chemical that is in high quantities Air is all around us and if the reaction involves air, the air will always be in excess

Some terms to know Excess Reagent = reagent that is in excess, will not get used up in the reaction Limiting Reagent = reagent that will get used up fully in the reaction Theoretical yield = the amount of product we expect to get from calculations Actual yield = The amount of product we actually get from experiments

Steps to finding Convert the mass/volume of both reactants to their respective moles Convert both their moles to the moles of ONE product Whichever one produces the smaller moles of product is the limiting reagent Whichever one is in excess will have some remaining

How to find excess remaining We can calculate what is leftover if We convert the limiting reagents moles to moles of excess that will be used up in reaction Subtract the moles of excess from what will be used up in reaction Convert moles remaining to mass to find how much mass will remain

Note! We refer to moles, not mass for limiting reagent. Just because something has a smaller mass doesn’t mean it is limiting! A limiting reagent question will always show enough information to get to moles for both reactants!

Example - 1 2Al + 3I2 → AlI3 Determine the limiting reagent and theoretical yield of the product if A) 1.20 mol Al and 2.40 mol iodine B) 1.20g of Al and 2.40g of iodine C) How many grams of Al are left over in part b?

Example - 2 15.00g of aluminum sulfide and 10.00g of water react until the limiting reagent is used up Al2S3 + 6H2O → 2Al(OH)3 + 3H2S Which is the limiting reagent? What is the mass of H2S which can be formed? How much excess reagent remains after the reaction is complete?

Example - 3 If 60.7g of FeCl2, 15.2g of KNO3 and 42.6g of HCl are mixed and allowed to react 3FeCl2 + KNO3 + 4HCl → 3FeCl3 + NO + 2H2O +KCl Which chemical is the limiting reagent? How many grams of each excess reactants will be present after the reaction?

Practice - 1 Page 133 - #26-32