Limiting Reagents. We have considered reactions with just the perfect amount of each reactant.
Lecture 12.3- Limiting Reagents and Percent Yield
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Bellwork- Make 1cup of water H 2 + O 2 H 2 O How many liters of H 2 gas and O 2 gas at STP are required to make a cup of water? One cup (240mL) has a mass of 240g for pure water. 240g H 2 O x 1mol = 13.3mol H 2 O 18g 13.3 mol H 2 O x 13.3 mol H 2 O x 2 2 2 moles H 2 2 moles H 2 O 1 mole O 2 . 2 moles H 2 O = 13.3mol H 2 = 6.67mol O 2 x 22.4L/mol = 299L H 2 x 22.4L/mol = 149L O 2
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Transcript of Lecture 12.3- Limiting Reagents and Percent Yield
Bellwork- Make 1cup of water
H2 + O2 H2OHow many liters of H2 gas and O2 gas at STP are required to make a cup of water? One cup (240mL) has a mass of 240g for pure water.
240g H2O x 1mol = 13.3mol H2O 18g
13.3 mol H2O x
13.3 mol H2O x
22
2 moles H 2
2 moles H2O
1 mole O 2 . 2 moles H2O
= 13.3mol H2
= 6.67mol O2
x 22.4L/mol = 299L H2
x 22.4L/mol = 149L O2
Limiting Reagent and Percent Yield
If a carpenter had two tabletops and seven table legs, he could only build one four-legged table.
The number of table legs is the limiting factor.
Similarly, in chemistry, the amount of product made in a chemical reaction may be limited by the amount of one or more of the reactants.
2 H2 + O2 → 2H2O
When reactants combine in their stoichiometric ratio (a.k.a. mole ratio), both reactants will be completely used.
In any other ratio, one of the reactants will be used up and the other will have some left over.
In a chemical reaction, an insufficient quantity of any of the reactants will limit the amount of product that forms.
The limiting reagent is the reagent that determines the amount of product that can be formed by a reaction.
N2 + 3H2 2NH3
You’ve got 2mole N2 3mol H2
How much NH3 can you make?
What is the limiting reactant?
N2 + 3H2 2NH3
You’ve got 2mole N2 3mol H2
How much NH3 can you make? Only 2 moles
What is the limiting reactant? Hydrogen, because there is not enough H2 to react with the 2nd mole of N2.
Nitrogen is the reagent that is not completely used up in the reaction.
The reagent that is not used up is called the excess reagent.
N2 + 3H2 2NH3
You’ve got 2mole N2 3mol H2
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for Sample Problem 12.7
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for Sample Problem 12.8
The percent yield is a measure of the efficiency of a reaction carried out in the laboratory.
A batting average is actually a percent yield.
The theoretical yield is the maximum amount of product that could be formed from given amounts of reactants.
In contrast, the amount of product that actually forms when the reaction is carried out in the laboratory is called the actual yield.
comes from stoichiometry
comes from the lab
The percent yield is the ratio of the actual yield to the theoretical yield expressed as a percent.
What you get in the lab
What you get mathematically
The actual yield is often lower than the theoretical yield because not all reactions go to completion.
The actual yield should never be higher than your theoretical yield.
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for Sample Problem 12.10
12.3 Section Quiz.
1. In the reaction 3NO2 + H2O → 2HNO3 + NO, how many grams of HNO3 can form when 1.00 g of NO2 and 2.25 g of H2O are allowed to react?
a. 0.913 g
b. 0.667 g
c. 15.7 g
d. 1.37 g
12.3 Section Quiz.
2. How many grams of H2O can be formed from 24.0 g O2 and 6.00 g H2?
a. 30.0 g
b. 27.0 g
c. 54.0 g
d. 13.5 g
3. Octane burns according to the following equation.
2C8H18 + 25O2 → 16CO2 + 18H2O
What is the percent yield if 14.6 g of CO2 are produced
when 5.00 g of C8H18 are burned?
a. 106%
b. 94.8%
c. 34.2%
d. 62.5%
12.3 Section Quiz.
