Transcript SCH3U Limiting Reagent and Percent Yield ppt

SCH 3U1
Review
 In a chemical reaction, how do we relate moles of one
compound to moles of another?
2A + B
A2B
MOLE RATIO!
 What are the four steps to go from mass of one reactant or
product to the mass of another?
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1. Write the balanced equation
2. Convert mass of given to moles
3. Use the mole ratio from the balanced equation to find moles
of the unknown
4. Convert moles of the unknown to mass of
the unknown
Chemical Construction
 In your group, you have a beaker of hydrogen (H2) and a
beaker of carbon
 You need to make as many methane molecules (CH4) as
you can!
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Activity Analysis
 Which element limited the number of methane
molecules you could make?
 Which element was present in excess amounts?
 Did the element present in excess affect the number of
methane molecules you could make?
 Why doesn’t the carbon limit the number of methane
molecules you could make, even though they are present
in the smallest quantity?
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Limiting Reactant
 Reactants are rarely present in amounts that correspond
exactly to the mole ratios in the balanced equation
 Limiting reactant: a reactant that is completely
consumed during a chemical reaction and limits the
amount of product that can be formed
 Excess reactant: a reactant that remains after a
reaction is over
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Limiting Reactant
2 mol
1 mol
L.R. 2 mol
2 mol
4 mol
2 mol
1 mol L.R.
 The limiting reactant forms the smallest amount of
product
 Use stoichiometry to determine which reactant produces
the smallest amount of product
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Practice Problem
 Lithium nitride reacts with water to form ammonia and
lithium hydroxide, according to the following balanced
chemical equation:
If 4.87 g of lithium nitride reacts with 5.80 g of water, find the
limiting reactant.
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Yield
 Theoretical yield is the maximum amount of
product that can form in a chemical reaction
 Calculated by assuming that all of the limiting
reagent has reacted to form the product
 In reality:
 Many reactions do not go to completion
What are some things that might prevent us
from reaching the theoretical yield?
 Other reactions, called side reactions, may occur
 Some product may be lost during purification
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Yield
 The actual yield is the amount of a product that is
actually obtained from a chemical reaction
 The actual yield is almost always less than the
theoretical yield
 Actual Yield is an experimentally determined
quantity
 Percent yield:
% Yield =
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actual yield
x 100
theoretical yield
Crucial Information for Chemists
 Consider the synthesis of Diazonamide A
 Chemical
synthesis95%
in pharmaceutical industry or
82%
86%
industrial chemistry involves many steps
88%
 Low percent yield in each step results in a tiny over
all yield
88%
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50%
65%
67%
Wasted reactants85%
translates into loss 80%
of money and
resources
60%
R. R. Knowles, J. Carpenter, S. B. Blakey, A. Kayano, I. K. Mangion, C. J. Sinz, D. W. C. MacMillan, Chem. Sci., 2011, 2, 308-311
Practice Problem
 Ammonia can be prepared by reacting nitrogen gas with
hydrogen gas:
When 7.5 x 101 g of nitrogen reacts with sufficient hydrogen,
the theoretical yield of ammonia is 9.10g. If 1.72 g of
ammonia is obtained by experiment, what is the percentage
yield of this reaction?
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Homework
 Practice Problems
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