Limiting Reactants Limiting Reactant Analogy

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Limiting Reactants
• Many reactions are carried out with a limited
amount of one reactant and a plentiful amount of
the other(s).
• The reactant that is completely consumed in the
reaction limits the amounts of products and is
called the limiting reactant, or limiting reagent.
• The limiting reactant is not necessarily the one
present in smallest amount.
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Limiting Reactant Analogy
If we have 11
sandwiches, 16
cookies, and 10
oranges …
… how many
packaged meals
can we make?
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Limiting Reactant Analogy
…totally
consumed!
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Molecular View of the Limiting Reactant Concept
1. Why is ethylene left
over, when we started
with more bromine
than ethylene? (Hint:
count the molecules.)
2. What mass of ethylene
is left over after
reaction is complete?
(Hint: it’s an easy
calculation; why?)
When 28 g (1.0
mol) ethylene
reacts with …
… 128 g (0.80 mol)
bromine, we get …
… 150 g of 1,2dibromoethane, and
leftover ethylene!
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Recognizing and Solving Limiting
Reactant Problems
•
We recognize limiting reactant problems by the
fact that amounts of two (or more) reactants are
given.
•
One way to solve them is to perform a normal
stoichiometric calculation of the amount of
product obtained, starting with each reactant.
•
The reactant that produces the smallest amount
of product is the limiting reactant.
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Magnesium nitride can be formed by the reaction of
magnesium metal with nitrogen gas.
a) How many grams of magnesium nitride can be made
in the reaction of 35.00 g of magnesium and 15.00 g of
nitrogen?
b) How many grams of the excess reactant remain after
the reaction?
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Yields of Chemical Reactions ‫ניצולת‬
• The theoretical yield of a chemical reaction is the
calculated quantity of product in the reaction.
• The actual yield is the amount you actually get
when you carry out the reaction.
• Actual yield will be less than the theoretical yield,
for many reasons … can you name some?
actual yield
Percent yield = ––––––––––––– × 100
theoretical yield
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Actual yield of ZnS is less than the
theoretical yield…
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Solutions and
Solution Stoichiometry
• Solute: the substance being dissolved.
‫מומס‬
• Solvent: the substance doing the dissolving. ‫ממס‬
• Concentration of a solution: the quantity of a
solute in a given quantity of solution (or solvent).
– A concentrated solution contains a relatively large
amount of solute vs. the solvent (or solution).
– A dilute solution contains a relatively small
concentration of solute vs. the solvent (or solution).
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Molar Concentration
Molarity (M), or molar concentration, is the amount
of solute, in moles, per liter of solution:
moles of solute
Molarity = ––––––––––––––
liters of solution
• A solution that is 0.35 M sucrose contains 0.35
moles of sucrose in each liter of solution.
• Keep in mind: molarity signifies moles of solute
per liter of solution, not liters of solvent.
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Preparing 0.01000 M KMnO4
Weigh 0.01000
mol (1.580 g)
KMnO4.
Dissolve in water. How much
water? Doesn’t matter, as long
as we don’t go over a liter.
Add more water
to reach the
1.000 liter mark.
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You want to prepare a solution of 6.68 M NaOH.
(a) How many moles of NaOH are required to prepare 0.500
L of 6.68 M NaOH?
(b) How many liters of 6.68 M NaOH can we prepare with
2.35 kg NaOH?
The label of a stock bottle of aqueous ammonia indicates
that the solution is 28.0% NH3 by mass and has a density of
0.898 g/mL. Calculate the molarity of the solution.
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Dilution of Solutions ‫מיהול‬
• Dilution is the process of preparing a more dilute
solution by adding solvent to a more concentrated
one.
• Addition of solvent does not change the amount of
solute in a solution but does change the solution
concentration.
• It is very common to prepare a concentrated stock
solution of a solute, then dilute it to other
concentrations as needed.
Visualizing the Dilution of a Solution
We start and
end with the
same amount of
solute.
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Addition of
solvent has
decreased the
concentration.
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Dilution Calculations
• Moles of solute does not change on dilution.
• Moles of solute = C × V
– remains unchaged!
⇒ Cconc × Vconc = Cdil × Vdil
OR…
Cdil = Cconc ×
Vconc
Dilution
factor
Vdil
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How many milliliters of a 2.00 M CuSO4 stock
solution are needed to prepare 0.250 L of 0.400 M
CuSO4?
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Solutions in Chemical Reactions
• Molarity provides an additional tool in
stoichiometric calculations based on
chemical equations.
• Molarity provides factors for converting
between moles of solute (either reactant or
product) and liters of solution.
Adding to the previous stoichiometry scheme …
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If substance A is a
solution of known
concentration …
… we can start with
molarity of A times
volume (liters) of
the solution of A to
get here.
If substance B is in
solution, then …
… we can go from
moles of substance B
to either volume of B
or molarity of B.
How?
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‫אבן גיר תוססת כאשר מטפטפים עליה חומצה כלורית )כך מזהים‬
‫ התסיסה היא למעשה שחרור גז פחמן דו‬.(‫אותה בטיולים שנתיים‬
.‫חמצני‬
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
? 3.25M HCl ‫ תמיסת‬225 mL ‫ יגיב עם‬CaCO3 ‫כמה גרם‬
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Summary of Concepts
• The limiting reactant determines the amounts of products
in a reaction.
• The actual yield of the reaction is lower than the
theoretical yield because of various factors.
• The molarity of a solution is the number of moles of a
solute per liter of solution.
⇒ Can be used as a conversion factor from moles to
volume (or vice versa).
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