12.3 limiting reagent and percent yield > · 12.3 limiting reagent and percent yield >...

12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield >

Chapter 12Stoichiometry

12.1 The Arithmetic of Equations

12 2 Chemical Calculations12.2 Chemical Calculations

12.3 Limiting Reagent and Percent Yield

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12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > CHEMISTRY & YOU

What determines how much product you can make?you can make?

If a carpenter had two tabletops and seven t bl l h ldtable legs, he would have difficulty building more than one functionalmore than one functional four-legged table.


12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > Limiting and Limiting and Excess ReagentsExcess Reagents

Limiting and Excess ReagentsHow is the amount of product in a reaction affected by an yinsufficient quantity of any of the reactants?



To make tacos, you need enough meat, cheese lettuce tomatoes sour creamcheese, lettuce, tomatoes, sour cream, salsa, and seasonings.



To make tacos, you need enough meat, cheese lettuce tomatoes sour creamcheese, lettuce, tomatoes, sour cream, salsa, and seasonings.• If you have only 2 taco shells the quantity of• If you have only 2 taco shells, the quantity of

taco shells will limit the number of tacos you can make.



To make tacos, you need enough meat, cheese lettuce tomatoes sour creamcheese, lettuce, tomatoes, sour cream, salsa, and seasonings.• If you have only 2 taco shells the quantity of• If you have only 2 taco shells, the quantity of

taco shells will limit the number of tacos you can make.

• Thus, the taco shells are the limiting ingredient.



In a chemical reaction, an insufficient quantity of any of the reactants will limit the amount of product that forms.



A balanced chemical equation is a chemist’s recipe.Ch i l E tiChemical Equations

N2(g) + 3H2(g) 2NH3(g)

“Microscopic recipe” 1 molecule N2 + 3 molecules H2 2 molecules NH3

“Macroscopic recipe” 1 mol N + 3 mol H 2 mol NH

• What would happen if two molecules (moles) of N2( ) f ?

Macroscopic recipe 1 mol N2 + 3 mol H2 2 mol NH3

reacted with three molecules (moles) of H2?

Experimental ConditionsReactants Products

Before reaction2 molecules N2 3 molecules H2 0 molecules NH3


2 molecules N2 3 molecules H2 0 molecules NH3




After reaction1 molecule N2 0 molecules H2 2 molecules NH3

• Before the reaction takes place, N2 and H2 are present in a 2:3 molecule (mole) ratio.

• As the reaction takes place, one molecule (mole) of N2reacts with 3 molecules (moles) of H2 to produce two molecules (moles) of NH


molecules (moles) of NH3.





• All the H2 has now been used up, and the reaction stops.

• One molecule (mole) of unreacted N2 is left in addition toOne molecule (mole) of unreacted N2 is left in addition to the two molecules (moles) of NH3 that have been produced by the reaction.






• In this reaction, only the hydrogen is completely used up.

• H2 is the limiting reagent, or the reactant that 2 g g ,determines the amount of product that can be formed by a reaction.






• The reactant that is not completely used up in a reaction is called the excess reagent.

• In this example, nitrogen is the excess reagent because some nitrogen remains unreacted.



What determines how much product you can make in a chemical reaction?you can make in a chemical reaction?



What determines how much product you can make in a chemical reaction?you can make in a chemical reaction?

A limited quantity of any ofA limited quantity of any of the reactants that are needed to make a product will limit the amount of product that forms.


12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > Percent YieldPercent Yield

Percent YieldWhat does the percent yield of a reaction measure?

• A batting iaverage is

actually a percent yieldpercent yield.



When a balanced chemical equation is used to calculate the amount of product that will form during a reaction, the calculated value represents the theoretical yield.



When a balanced chemical equation is used to calculate the amount of product that will form during a reaction, the calculated value represents the theoretical yield.• The theoretical yield is the maximum amount y

of product that could be formed from given amounts of reactants.

• The amount of product that actually forms when the reaction is carried out in the


laboratory is called the actual yield.


The percent yield is the ratio of the actual yield to the theoretical yield expressed as a percent.

percent yield =actual yield

100%percent yield = theoretical yield 100%

Because the actual yield of a chemical reaction is often less than the theoretical yield, the

t i ld i ft l th 100%


percent yield is often less than 100%.


The percent yield is a measure of the efficiency of a reaction carried

t i th l b tout in the laboratory.



The percent yield is a measure of the efficiency of a reaction carried

t i th l b tout in the laboratory.

The mass of

The mass of the

one of the products, the actual yieldof the

reactant is measured.

actual yield, is measured. The percent yield is calculated.

Th t t


The reactant is heated.


Many factors cause percent yields to be less than 100%.• Reactions do not always go to completion; when a

reaction is incomplete, less than the calculated amount f d t i f dof product is formed.

• Impure reactants and competing side reactions may cause unwanted products to formcause unwanted products to form.

• Actual yield can be lower than the theoretical yield due to a loss of product during filtration or in transferringto a loss of product during filtration or in transferring between containers.

• If reactants or products have not been carefully


eacta ts o p oducts a e ot bee ca e u ymeasured, a percent yield of 100% is unlikely.

12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > Sample Problem 12.10Sample Problem 12.10

Calculating the Theoretical Yield of a Reaction

Calcium carbonate, which is found in seashells, is decomposed byseashells, is decomposed by heating. The balanced equation for this reaction is

CaCO3(s) CaO(s) + CO2(g)

What is the theoretical yield of CaO if 24.8 g CaCO3 is heated?



Analyze List the knowns and the unknown.1C l l t th th ti l i ld i th f thCalculate the theoretical yield using the mass of the reactant.

KNOWNSmass of CaCO3 = 24.8 g CaCO3

1 l C CO 100 1 C CO ( l )1 mol CaCO3 = 100.1 g CaCO3 (molar mass)

1 mol CaO = 56.1 g CaO (molar mass)

1 mol CaO/1 mol CaCO3 (mole ratio from balanced equation)

UNKNOWN


theoretical yield = ? g CaO


Calculate Solve for the unknown.2

Start with the mass of the reactant and convert to moles of the reactant.

1 mol CaCO324.8 g CaCO3 100.1 g CaCO3

1 mol CaCO3




Next, convert to moles of the product using the mole ratio.

1 mol CaCO3 1 mol CaO24.8 g CaCO3 100.1 g CaCO3

1 mol CaCO31 mol CaCO3

1 mol CaO




Finish by converting from moles to mass of the productof the product.

1 mol CaCO3 1 mol CaO 56.1 g CaO24.8 g CaCO3 100.1 g CaCO3

1 mol CaCO31 mol CaCO3

1 mol CaO1 mol CaO56.1 g CaO

= 13.9 g CaOIf there is an excess of a reactant, then there is more than enough of that reactant and it will not limit the


t at eacta t a d t ot t t eyield of the reaction.


Evaluate Does the result make sense?3

• The mole ratio of CaO to CaCO3 is 1:1. The ratio of their masses in the reaction should be the same as the ratio of their molar masses, which is slightly greater th 1 2than 1:2.

• The result of the calculations shows that the mass of CaO is slightly greater than half the mass of CaCO3.



Wh t i th t i ld if 13 1 C O iCalculating the Percent Yield of a ReactionWhat is the percent yield if 13.1 g CaO is actually produced when 24.8 g CaCO3 is h t d?heated?

Calculate the theoretical yield first. Then you can

calculate the percent yield.

CaCO3(s) CaO(s) + CO2(g)



Analyze List the knowns and the unknown.1U th ti f t i ld Th th ti l i ldUse the equation for percent yield. The theoretical yield for this problem was calculated in Sample Problem 12.10.

KNOWNSactual yield = 13.1 g CaO

theoretical yield = 13.9 g CaO (from sample problem 12.10)

actual yield100%

UNKNOWNt i ld ? %

percent yield = y

theoretical yield 100%


percent yield = ? %



Substitute the values for actual yield and theoretical yield into the equation for percent yield.

percent yield = 100% = 94.2%13.1 g CaO13 9 g CaOp y 13.9 g CaO



Evaluate Does the result make sense?3

• In this example, the actual yield is slightly less than the theoretical yield.

• Therefore, the percent yield should be slightly less than 100%.slightly less than 100%.



Carbon tetrachloride, CCl4, is a solvent that was once used in large amounts in dry cleaning. One reaction that produces carbon tetrachloride is

CS2 + 3Cl2 CCl4 + S2Cl2What is the percent yield of CCl4 if 617 kg p y 4 gis produced from the reaction of 312 kg of CS2?



What is the percent yield of CCl4 if 617 kg is produced from the reaction

CS + 3Cl CCl + S Cl

g pof 312 kg of CS2?

CS2 + 3Cl2 CCl4 + S2Cl2

3 12 105 CS 1 mol CS2 1 mol CCl4 153 81 g CCl43.12 105 g CS2 76.142 g CS2

1 mol CS2 1 mol CCl41 mol CS2

153.81 g CCl41 mol CCl4

6 30 105 CCl 630 k CCl= 6.30 105 g CCl4 = 630 kg CCl4

617 kg CCl4


Percent yield = 100% = 97.9%617 kg CCl4630 kg CCl4

12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > Key Concepts and Key Concepts and Key EquationKey Equation

In a chemical reaction, an insufficient quantity of any of the reactants will limit the amount of yproduct that forms.

The percent yield is a measure of theThe percent yield is a measure of the efficiency of a reaction performed in the laboratory.y

percent yield =actual yield

theoretical yield 100%p y theoretical yield


12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > Glossary TermsGlossary Terms

• limiting reagent: any reactant that is used fi t i h i l ti it d t iup first in a chemical reaction; it determines

the amount of product that can be formed in the reactionin the reaction

• excess reagent: a reagent present in a tit th t i th ffi i t t tquantity that is more than sufficient to react

with a limiting reagent; any reactant that remains after the limiting reagent is usedremains after the limiting reagent is used up in a chemical reaction


12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > Glossary TermsGlossary Terms

• theoretical yield: the amount of product that could form during a reaction calculated from a b l d h i l ti it t thbalanced chemical equation; it represents the maximum amount of product that could be formed from a given amount of reactantformed from a given amount of reactant

• actual yield: the amount of product that forms when a reaction is carried out in the laboratorywhen a reaction is carried out in the laboratory

• percent yield: the ratio of the actual yield to the theoretical yield for a chemical reactionthe theoretical yield for a chemical reaction expressed as a percentage; a measure of the efficiency of a reaction


12.3 Limiting Reagent and Percent Yield >12.3 Limiting Reagent and Percent Yield > BIG IDEA

The Mole and Quantifying MatterThe percent yield of a reaction can be calculated from the actual yield and theoretical yield of the reaction.


12.3 limiting reagent and percent yield > · 12.3 limiting reagent and percent yield >...

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