Limiting Reagents/% Limiting Reagents/% YieldYieldChapter 9 Notes, part IIChapter 9 Notes, part II

What are limiting reagents?• Up until now, we have

assumed that all reactants are used up in a reaction.

• In actuality, usually one chemical runs out first, stopping the reaction—that chemical is called the limiting reagent.

The The limiting reagentlimiting reagent limits or limits or determines the amount of product determines the amount of product that can be formed in a reaction.that can be formed in a reaction.

On the flip side, the On the flip side, the excess excess reagentreagent is the one not completely is the one not completely

used up.used up.

How do you find what the limiting reagent is?

• Set up as many sets of dimensional analysis as pieces of data you are given.

• Solve each, and the one with the lowest amount of product formed is the correct answer.

• The chemical you started with is the limiting reagent.

Here’s an example:Here’s an example:In the reaction Mg + HCl In the reaction Mg + HCl MgCl MgCl22 + H + H22 you you

combine 35.5 grams of magnesium with 28.1 combine 35.5 grams of magnesium with 28.1 grams of hydrochloric acid. Which one is the grams of hydrochloric acid. Which one is the limiting reagent, and how much magnesium limiting reagent, and how much magnesium

chloride is produced?chloride is produced?

Percent Yield• In a real world setting, you never In a real world setting, you never

form 100% of the products that form 100% of the products that you think you should based on you think you should based on theoretical calculations.theoretical calculations.

• By comparing how much of a By comparing how much of a product forms and how much product forms and how much should formed based on should formed based on stoichiometry, you can find the stoichiometry, you can find the percent yield.percent yield.

Percent Yield• %Yield= actual yield x100

theoretical yield

Practice Problem

• In the reaction of nitrogen In the reaction of nitrogen gas and hydrogen gas gas and hydrogen gas producing ammonia (NHproducing ammonia (NH33), ),

250.2g H250.2g H22 produces 1095.1g produces 1095.1g

NHNH33. What is the % yield of . What is the % yield of

this reaction?this reaction?

Practice Problem

• KK22CrOCrO44 + Pb(NO + Pb(NO33))22

KNOKNO33 + PbCrO + PbCrO44

• If 0.947g KIf 0.947g K22CrOCrO44 are combined are combined

with 0.331g Pb(NOwith 0.331g Pb(NO33))22 what will what will

be the limiting reagent, and how be the limiting reagent, and how many grams of lead chromate is many grams of lead chromate is produced?produced?

Practice Problem

• Zn + HCl Zn + HCl ZnCl ZnCl22 + H + H22

• What is the limiting reagent and What is the limiting reagent and how many how many litersliters of hydrogen of hydrogen are produced if 15.5 grams of are produced if 15.5 grams of zinc are combined with 26.3 zinc are combined with 26.3 grams of hydrochloric acid?grams of hydrochloric acid?

Tougher Practice Problem

• If 12.2mL of ethanol (C2H5OH) are added to 3.78L of O2 and the ethanol undergoes combustion, how many milliliters of water are formed? Density of water is 1.00g/mL and density of ethanol is 0.779g/mL.

Practice Problem

• In the reaction:

Mg3N2 + 6H2O 3Mg(OH)2 + 2NH3

• If the initial amount of Mg3N2 is 58.1g and the initial amount of H2O is 20.4g, and 26.2g of Mg(OH)2 are produced, what is the percent yield of the reaction?

In Review:In Review:What is a limiting reagent?What is a limiting reagent?

What is the excess reagent?What is the excess reagent?

How does a limiting reagent control the How does a limiting reagent control the amount of product?amount of product?

What is percent yield?What is percent yield?