Limiting Reagents & Percent Yield Chapter 9 Notes Part III.

Limiting ReagentsLimiting Reagents& Percent Yield& Percent Yield

Chapter 9 Notes Part IIIChapter 9 Notes Part III

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.

For Example:

How many sandwiches can you make from a full jar of peanut butter, a full jar of jelly and 4 slices of bread?

For Example:


+

For Example:


+ +

For Example:


+ + =

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 is the one not

completely used up.completely used up.

How do you find what the limiting reagent is?

To find the limiting reagent, use dimensional analysis to determine how much of the product will be formed by both reactants.

The smaller of the two results will indicate which reactant is the limiting reagent.

Here’s an example:Here’s an example:

In the reaction: In the reaction:

Mg + HCl Mg + HCl MgCl MgCl22 + H + H22

you combine 35.5 grams of you combine 35.5 grams of magnesium with 28.1 grams of magnesium with 28.1 grams of

hydrochloric acid. Which one is hydrochloric acid. Which one is the limiting reagent, and how the limiting reagent, and how much magnesium chloride is much magnesium chloride is

produced?produced?

Mg + Mg + 22 HCl HCl MgCl MgCl22 + H + H22


35.5 g Mg x 35.5 g Mg x 1 mole Mg1 mole Mg x x 1 mole MgCl1 mole MgCl22 x x 95.3 g MgCl95.3 g MgCl22 = 139 g MgCl = 139 g MgCl22

24.3 g Mg 1 mole Mg 1 mole MgCl24.3 g Mg 1 mole Mg 1 mole MgCl22




28.1 g HCl x 28.1 g HCl x 1 mole HCl1 mole HCl x x 1 mole MgCl1 mole MgCl22 x x 95.3 g MgCl95.3 g MgCl22 = 36.6 g MgCl = 36.6 g MgCl22

36.5 g HCl 2 moles HCl 1 mole MgCl36.5 g HCl 2 moles HCl 1 mole MgCl22




28.1 g HCl x 28.1 g HCl x 1 mole HCl1 mole HCl x x 1 mole MgCl1 mole MgCl22 x x 95.3 g MgCl95.3 g MgCl22 = 36.6 g MgCl = 36.6 g MgCl22

36.5 g HCl 2 moles HCl 1 mole MgCl36.5 g HCl 2 moles HCl 1 mole MgCl22

HCl is the limiting reagent and HCl is the limiting reagent and

36.6 g of MgCl36.6 g of MgCl2 2 are produced.are produced.

Percent YieldIn the real world, you never form In the real world, you never form

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

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

Percent Yield %Yield= actual yield x 100

theoretical yield

Practice ProblemIn the reaction of nitrogen gas In the reaction of nitrogen gas

and hydrogen gas producing and hydrogen gas producing ammonia (NHammonia (NH33), 250.2g H), 250.2g H22

produces 1095.1g NHproduces 1095.1g NH33. What . What

is the % yield of this reaction?is the % yield of this reaction?

N2 + 3 H2 → 2 NH3

First find out how much NH3 should be produced:

N2 + 3 H2 → 2 NH3


250.2 g H2 x 1 mole H2 x 2 moles NH3 x 17 g NH3 = 2836 g NH3

1 g H2 3 moles H2 1 mole NH3

N2 + 3 H2 → 2 NH3




Then calculate the percent yield:

N2 + 3 H2 → 2 NH3




Then calculate the percent yield:% Yield = Actual x 100 % Yield = 1095.1 g x 100

Theor. 2836 g

% Yield = 38.6 %

Reaction #1

KK22CrOCrO44 + Pb(NO + Pb(NO33))22 KNO KNO33 + PbCrO + PbCrO44

If 0.947g KIf 0.947g K22CrOCrO44 are combined are combined

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

will be the limiting reagent, will be the limiting reagent, and how much lead and how much lead chromate is produced?chromate is produced?

KK22CrOCrO44 + Pb(NO + Pb(NO33))22 2 KNO 2 KNO33 + PbCrO + PbCrO44

0.947g K2CrO4 x 1 mole K2CrO4 x 1 mole PbCrO4 x 323.2 g PbCrO4 = 1.97 g PbCrO4

155.1 g K2CrO4 1 mole K2CrO4 1 mole PbCrO4

0.331g Pb(NO0.331g Pb(NO33))22 x x 1 mole Pb(NO1 mole Pb(NO33))22 x x 1 mole PbCrO4 x 323.2 g PbCrO4 = 0.323 g PbCrO4

331.2 g Pb(NOPb(NO33))22 1 mole Pb(NO 1 mole Pb(NO33))2 2 1 mole PbCrO4

Pb(NOPb(NO33))22 is the limiting reagent and is the limiting reagent and

0.323 g PbCrO0.323 g PbCrO44 is produced. is produced.

Reaction #2

CC33HH88O + OO + O22 CO CO22 + H + H22OO

If 3.78 L of OIf 3.78 L of O22 are combined with are combined with

7.9 g of C7.9 g of C33HH88O, what is the O, what is the

limiting reagent, and how much limiting reagent, and how much water is produced?water is produced?

2 C2 C33HH88O + 9 OO + 9 O22 6 CO 6 CO22 + 8 H + 8 H22OO

3.78 L of O3.78 L of O2 2 x x 1 mole O1 mole O22 x x 8 moles H8 moles H22OO x x 18 g H18 g H22OO = 2.7 g H = 2.7 g H22OO

22.4 L O22.4 L O22 9 moles O 9 moles O22 1 mole H 1 mole H22OO

7.9 g of C7.9 g of C33HH88O x O x 1 mole C1 mole C33HH88OO x x 8 moles H8 moles H22OO x x 18 g H18 g H22OO = 9.48 g H = 9.48 g H22OO

60 g C60 g C33HH88O 2 moles CO 2 moles C33HH88O 1 mole HO 1 mole H22OO

OO22 is the limiting reagent and 2.7 g of H is the limiting reagent and 2.7 g of H22O are produced.O are produced.

Reaction #3

KK22CrOCrO44 + AgNO + AgNO33 KNO KNO33 + Ag + Ag22CrOCrO44

If 1.17g KIf 1.17g K22CrOCrO44 are combined with are combined with

1.23g AgNO1.23g AgNO33, what is the limiting , what is the limiting

reagent and how much Agreagent and how much Ag22CrOCrO44 is is

produced?produced?

KK22CrOCrO44 + 2 AgNO + 2 AgNO33 2 KNO 2 KNO33 + Ag + Ag22CrOCrO44

1.17g K1.17g K22CrOCrO44 x x 1 mole K1 mole K22CrOCrO44 x x 1 mole Ag1 mole Ag22CrOCrO44 x x 331.8 g Ag331.8 g Ag22CrOCrO44 = 1.99 g Ag = 1.99 g Ag22CrOCrO44

194.2 g K194.2 g K22CrOCrO44 1 mole K 1 mole K22CrOCrO44 1 mole Ag 1 mole Ag22CrOCrO44

1.23g AgNO1.23g AgNO3 3 x x 1 mole AgNO1 mole AgNO33 x x 1 mole Ag1 mole Ag22CrOCrO44 x x 331.8 g Ag331.8 g Ag22CrOCrO44 = 1.20 g Ag = 1.20 g Ag22CrOCrO44

169.9 g AgNO169.9 g AgNO33 2 moles AgNO 2 moles AgNO33 1 mole Ag 1 mole Ag22CrOCrO44

AgNOAgNO33 is the limiting reagent and is the limiting reagent and

1.20 g Ag1.20 g Ag22CrOCrO44 are produced. are produced.

Reaction #4

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

What is the limiting reagent and What is the limiting reagent and how many liters of hydrogen how many liters 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?

Zn + 2 HCl Zn + 2 HCl ZnCl ZnCl22 + H + H22

15.5 g Zn x 1 mole Zn x 1 mole H2 x 22.4 L H2 = 5.31 L H2

65.4 g Zn 1 mole Zn 1 mole H2

26.3 g HCl x 1 mole HCl x 1 mole H2 x 22.4 L H2 = 8.07 L H2

36.5 g HCl 2 moles HCl 1 mole H2

Zn is the limiting reagent and 5.31 Liters of H2 are produced.

Reaction #5

Na + HNa + H22O O NaOH + H NaOH + H22

If 2.1g of sodium is added to 3.8g If 2.1g of sodium is added to 3.8g of water, what is the limiting of water, what is the limiting reagent, and how many grams of reagent, and how many grams of sodium hydroxide are produced?sodium hydroxide are produced?

2 Na + 2 H2 Na + 2 H22O O 2 NaOH + H 2 NaOH + H

22

2.1 g Na x 1 mole Na x 2 moles NaOH x 40 g NaOH = 3.7 g NaOH

23 g Na 2 moles Na 1 mole NaOH

3.8 g H2O x 1 mole H2O x 2 moles NaOH x 40 g NaOH = 8.4 g NaOH

18 g H2O 2 moles H2O 1 mole NaOH

Na is the limiting reagent and 3.7 g NaOH are produced.

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

Limiting Reagents & Percent Yield Chapter 9 Notes Part III.

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