3/18/2014

Today:

◦ Stoichiometric Analysis:
Next Meeting
◦ Reading for Monday:
 Gram to Gram Conversions:
 Use MOLAR MASS to get to moles
 Start Chapter 7, Sections 7.1-7.5, pp.
238-248
 Limiting Reagents:
 Method 1
 Method 2
 Actual Yield & Percent Yield
◦ Combustion Analysis
◦ Titrations
iClicker Participation Question:
Predicting the Volume of Gases Produced
Which mixture will yield the MOST CO2 GAS?
CH3CO2H(aq) + NaHCO3(s)
A
3.0 g (0.05 mol)
4.2 g (0.05 mol)
B
6.0 g (0.10 mol)
4.2 g (0.05 mol)
C
3.0 g (0.05 mol)
8.4 g (0.10 mol)
NaCH3CO2(aq) + H2O(l) + CO2(g)
D. Each mixture will yield the same volume of gases
E. Both B & C will yield the same amount of gas,
which will be more than A
When the ions switch places, be sure
write the formulas of the products so
that NEUTRAL compounds form (with the
smallest whole number ratio of atoms).
Nitrogen Dioxide: A major component of smog
•
Nitrogen monoxide (aka nitric oxide, NO) is produced in internal
combustion engines.
•
Nitric oxide released into the atmosphere can react with O2 to
form nitrogen dioxide (aka nitrous oxide, NO2).
2 NO + O2
2 NO2
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3/18/2014
Determining the Limiting Reagent: METHOD 1
While calculating the THEORETICAL YIELD
1.
2.
Convert the starting quantity of each reactant to the quantity of product that
could potentially form.
The limiting reagent is the reactant that would produce LESS product and
determines the THEORETICAL YIELD.
4 mol O2
4 mol NO
O2 is in EXCESS
Nitric oxide
is LIMITING
(some remains
unreacted in the end)
(it is consumed &
limits the formation
of product)
Animations adapted from McGraw Hill Publishers
Determining the Limiting Reagent: METHOD 2
Comparing one reactant to another
1. Start with Reactant A. Given the starting quantity of A, calculate the quantity
of the other reactant (B) needed to FULLY react with A.
2. Compare the available quantity of B to the amount needed to react with A. If
more is available than is needed, B is in EXCESS & A is the LIMITING.
2 mol O2
6 mol NO
Nitric oxide
is in EXCESS
Oxygen is
LIMITING (it is
(some remains
unreacted in the end)
consumed & limits the
formation of product)
Animations adapted from McGraw Hill Publishers
A Stoichiometric Mixture:
Neither reactant is limiting or in excess
•
When the reactants are in the proper stoichiometric ratio, each can FULLY
react with the other. Neither starting material remains in the end of the
reaction. Neither is considered a limiting reagent.
3 mol O2
6 mol NO
Animations adapted from McGraw Hill Publishers
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3/18/2014
iClicker Participation Question:
Stoichiometric Analysis
If 10 moles of iron (III) oxide are mixed with 10 moles of zinc
metal and react according to the equation below, which
component will be consumed first?
A. Fe2O3
In other words, which is the limiting reagent?
B. Zn
Fe2O3 + 3 Zn
3 ZnO + 2 Fe
C. ZnO
D. Fe
E. There are no limiting reagents here. Everything reacts fully
Reactions are NOT 100% efficient
• If only a portion of the limiting reagent reacts, only a fraction of
the theoretical yield will be obtained. The quantity of the
product that is produced in reality is called the ACTUAL YIELD.
• The PERCENT YIELD measures how much of the limiting reagent
actually reacted.
• PERCENT YIELD =
Actual Yield x 100
Theoretical Yield
(This number should always be LESS than or equal to 100 %)
Given the Actual Yield & the Theoretical Yield, it is possible to
calculate the PERCENT YIELD.
OR:
Given the PERCENT YIELD & the Theoretical Yield, it is possible
to calculate the Actual Yield.
iClicker Participation Question:
Mole-to-Mole Conversions—From Reactants to Products
30 moles of hydrogen gas reacted with sufficient nitrogen gas
to produce 5 moles of NH3. What is the percent yield?
N2(g) + 3 H2(g)
A. 100 %
B. 75 %
C. 50 %
D. 25 %
2 NH3(g)
“The expansion of the world's population
from 1.6 billion people in 1900 to today's
seven billion [in 2012] would not have been
possible without the [industrial] synthesis
of ammonia.“ –MIT Press
E. None of the above
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3/18/2014
Percent Yield Practice Problems:
Hydrazine, once used as a rocket propellant, reacts with oxygen:
N2H4 + O2 → N2 + 2 H2O
A. How many grams of water could be obtained from 50.0 g N2H4
reacting with excess oxygen?
B. How many grams of water would be obtained from part A
above if the percent yield of the reaction was only 85.5 %?
Combustion Analysis using Stoichiometry:
Determining Empirical Formulas of Organic Compounds
•
The empirical formula for an organic compound can be
determined by burning it and analyzing the products (CO2 &
H2O)
CXHYOZ + O2(g) → X CO2 + Y/2 H2O
Combustion Analysis using Stoichiometry:
Determining Empirical Formulas
Caproic acid, the compound responsible for the unpleasant aroma
of dirty socks, contains only C, H, and O.
When a 0.450-g sample of the compound is
burned in oxygen, 1.023 g of CO2 and 0.418 g
of H2O are collected.
What is the empirical formula of the compound?
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3/18/2014
Molarity: Linking Volume & Moles
 Concentration is the measure of the amount of solute
in a solution (part / whole).
 Molar concentration, or molarity is a measure of the
moles of a solute in one liter of solution.
Molarity 
n
moles of solute
mol solute


M
V volume of solution L solution
 Brackets around a formula indicate the concentration
of the substance is being discussed:
 [NaCl] means “the molarity of NaCl”
Solution Stoichiometry
Instead of using grams to find the moles of a component:
Volume and Molarity can be used to determine MOLES
& this information can be worked into stoichiometry
problems.
Solution Stoichiometry
Example Problem 1
Consider the reaction of HCl
with KOH.
If a 0.298 M solution of HCl(aq)
reacts with 0.0592 moles of
KOH, what volume of HCl
solution would be need to
complete react with the
KOH?
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3/18/2014
Solution Stoichiometry
Example Problem 2
Consider the reaction of 20.0
mL of a sulfuric acid (H2SO4)
solution with a 0.127 M
solution of sodium hydroxide
(NaOH). If 32.66 mL of NaOH
is required to fully react with
the H2SO4 solution, what is
the concentration of the
original H2SO4 solution?
Practice Problems:
Urea (CH4N2O), a common fertilizer, can be synthesized by the reaction of
ammonia (NH3) with carbon dioxide.
2 NH3(aq) + CO2(aq)
CH4N2O(aq) + H2O(l)
An industrial synthesis of urea begins with 35.8 kg of aammonia and 89 kg of
carbon dioxide. What mass of urea could theoretically be produced from this
starting mixture?
An emergency breathing apparatus placed in mines or caves works via the
chemical reaction below:
4 KO2(s) + 2 CO2(g)
2 K2CO3 + 3 O2(g)
If the oxygen supply becomes limited, a worker can use the apparatus to
breathe while exiting the mine. Notice that the reaction produces O2, which can
be breathed, and absorbs CO2, a product of respiration. What minimum amount
of KO2 is required for the apparatus to provide enough oxygen to allow the user
15 minutes to exit the mine? Assume 4.4 g of O2 are needed for 15 minutes of
normal breathing.
Solution Stoichiometry
Example Problem 3
Consider the reaction of NaOH with
phosphoric acid (H3PO4).
If 50.5 mL of 0.54 M H3PO4
solution is mixed with 15.9 mL of
0.61 M NaOH solution, which
would be considered the limiting
reagent? In other words, which
What would be the
reactant would be consumed
final concentration of
first?
the reagent in
excess?
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