Limiting ReagentsLimiting Reagents

Chemistry NotesChemistry Notes

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:

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:

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

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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:

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:

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:

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:

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

+ + =

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: Mg + HCl In the reaction: Mg + HCl MgCl MgCl22 + H + H22

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

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.

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

If 0.947g KIf 0.947g K22CrOCrO44 are combined with 0.331g are combined with 0.331g

Pb(NOPb(NO33))22 what will be the limiting reagent, and what will be the limiting reagent, and

how much lead chromate is produced?how much lead 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.

CC33HH88O + OO + O22 CO CO22 + H + H22OO

If 3.78 L of OIf 3.78 L of O22 are combined with 7.9 g of are combined with 7.9 g of

CC33HH88O, what is the limiting reagent, and howO, what is the limiting reagent, and how

much water is produced?much 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.

KK22CrOCrO4 4 + AgNO + AgNO33 KNO KNO33 + Ag + Ag22CrOCrO44

If 1.17g KIf 1.17g K22CrOCrO44 are combined with 1.23g AgNO are combined with 1.23g AgNO33,,

what is the limiting reagent and how much Agwhat is the limiting reagent and how much Ag22CrOCrO44

is produced?is 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.

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

What is the limiting reagent and how manyWhat is the limiting reagent and how many

liters of hydrogen are produced if 15.5 grams\liters of hydrogen are produced if 15.5 grams\

of zinc are combined with 26.3 grams ofof zinc are combined with 26.3 grams of

hydrochloric acid?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.

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

If 2.1g of sodium is added to 3.8g of water, what isIf 2.1g of sodium is added to 3.8g of water, what is

the limiting reagent, and how many grams ofthe limiting 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.