Stoichiometry in Chemical Reactions
Stoichiometry
(Chapter 12)
Dr. Walker
Stoichiometry
(stoy-KEY-ah-muh-tree)
Definition: predicting the amounts of reactants
and/or products that will be involved in a
reaction
Chocolate Chip Cookie Recipe
•
Similar to using recipes to cook
•
When cooking, recipes require specific amounts of each
ingredient to make a set amount of food.
•
Example:
–
Chocolate Chip Cookies
–
2 ½ cups flour
½ cup sugar
–
¾ teaspoon salt
2 eggs
–
½ cup butter
1 teaspoon vanilla extract
–
1 cup brown sugar
1 pkg chocolate chips
–
1 cup shortening
1 cup chopped nuts
•
Makes 6 dozen cookies
Recipes = Chemical Formulas
•
When cooking, recipes require specific amounts of
each ingredient to make a set amount of food.
•
Chemical reactions are the exact same thing!!!
•
We need to know how much material can be made
from existing amounts (i.e., how much food is in the
pantry) OR how much we need to make a specific
amount (how much of everything do I need to make
10 dozen cookies)
Chemical Reactions
•
Chemical change involves a reorganization of the
atoms in one or more substances.
C
C
2
2
H
H
5
5
OH +
OH +
3
3
O
O
2
2
2
2
CO
CO
2
2
+
+
3
3
H
H
2
2
O
O
reactants
reactants
products
products
1
1
mole of ethanol
mole of ethanol
reacts with
reacts with
3
3
moles of
moles of
oxygen
oxygen
to produce
to produce
2
2
moles of carbon
moles of carbon
dioxide
dioxide
and
and
3
3
moles of water – Pay
moles of water – Pay
attention to the coefficients!!!!!
attention to the coefficients!!!!!
When the equation is balanced it has
When the equation is balanced it has
quantitative significance:
quantitative significance:
Chemical Reactions
•
Chemical change involves a reorganization of the
atoms in one or more substances.
C
C
2
2
H
H
5
5
OH + 3O
OH + 3O
2
2
2CO
2CO
2
2
+ 3H
+ 3H
2
2
O
O
reactants
reactants
products
products
How many moles of ethanol could react
How many moles of ethanol could react
with 9
with 9
moles of oxygen?
moles of oxygen?
If this happened,
If this happened,
what products would we get?
what products would we get?
When the equation is balanced it has
When the equation is balanced it has
quantitative significance:
quantitative significance:
Chemical Reactions
C
C
2
2
H
H
5
5
OH + 3O
OH + 3O
2
2
2CO
2CO
2
2
+ 3H
+ 3H
2
2
O
O
reactants
reactants
products
products
How many moles of oxygen could react with 9
How many moles of oxygen could react with 9
moles of oxygen? If this happened, what products
moles of oxygen? If this happened, what products
would we get?
would we get?
If 1 mole of ethanol reacts with 3 moles of
If 1 mole of ethanol reacts with 3 moles of
oxygen, then it would require 3 moles of ethanol to
oxygen, then it would require 3 moles of ethanol to
react with 9 moles of oxygen.
react with 9 moles of oxygen.
If 1 mole of ethanol gives 2 moles CO
If 1 mole of ethanol gives 2 moles CO
2
2
and 3 moles
and 3 moles
H
H
2
2
O, 3 moles of ethanol would yield 3 x 2 = 6
O, 3 moles of ethanol would yield 3 x 2 = 6
moles CO
moles CO
2
2
and 3 x 3 = 9 moles H
and 3 x 3 = 9 moles H
2
2
O
O
Mole Ratios
•
Using the coefficients we can write
mole
ratios.
–
Definition: A mole ratio gives the relative amounts
of reactants and products using a fraction
•
The coefficients you used in the recipes are
basically mole ratios. You’re doing the same
thing.
Using Mole Ratios
•
Example: N
2
+ 3 H
2
2 NH
3
–
Give the mole ratio of N
2
to NH
3
Using Mole Ratios
•
Example:
1
N
2
+ 3 H
2
2
NH
3
–
Give the mole ratio of N
2
to NH
3
–
Use the coefficients
•
1 N
2
to 2 NH
3
Using Mole Ratios
•
Example: N
2
+ 3 H
2
2 NH
3
–
Give the mole ratio of H
2
to NH
3
Using Mole Ratios
•
Example: N
2
+
3
H
2
2
NH
3
–
Give the mole ratio of H
2
to NH
3
–
Use the coefficients
•
3 H
2
to 2 NH
3
Mole-Mole Conversions
•
Example: N
2
+ 3 H
2
2 NH
3
•
How many moles of ammonia could be
produced from 6 moles of N
2
and excess H
2
?
Write all
Mole-Mole Conversions
•
Example: N
2
+ 3 H
2
2 NH
3
•
How many moles of ammonia could be
produced from 6 moles of N
2
and excess H
2
?
•
What are we starting with?
•
6 moles of N
2
•
Whenever you have excess, you have more
than enough – you won’t use it in the
problem!
6
m
o
l
e
s
?
?
?
?
Mole-Mole Conversions
•
Example: N
2
+ 3 H
2
2 NH
3
•
How many moles of ammonia could be
produced from 6 moles of N
2
and excess H
2
?
•
Use the coefficients in a proportion
6
m
o
l
e
s
?
?
?
?
-----------
1
m
o
l
e
N
2
-----------
2
m
o
l
e
s
N
H
3
Mole-Mole Conversions
•
Example: N
2
+ 3 H
2
2 NH
3
•
How many moles of ammonia could be
produced from 6 moles of N
2
and excess H
2
?
6
m
o
l
e
s
?
?
?
?
-----------
1
m
o
l
e
N
2
-----------
2
m
o
l
e
s
N
H
3
?
?
?
=
1
2
m
o
l
e
s
N
H
3
Another Mole-Mole Example
•
2 C
8
H
18
+ 25 O
2
16 CO
2
+ 18 H
2
O
•
How many moles of water can be produced
from 1.2 moles of octane and excess O
2
?
Another Mole-Mole Example
•
2 C
8
H
18
+ 25 O
2
16 CO
2
+ 18 H
2
O
•
How many moles of water can be produced
from 1.2 moles of octane and excess O
2
?
1
.
2
m
o
l
e
s
C
8
H
1
8
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1
8
m
o
l
e
s
H
2
O
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
2
m
o
l
e
s
C
8
H
1
8
?
?
?
?
?
Another Mole-Mole Example
•
2 C
8
H
18
+ 25 O
2
16 CO
2
+ 18 H
2
O
•
How many moles of water can be produced
from 1.2 moles of octane and excess O
2
?
1
.
2
m
o
l
e
s
C
8
H
1
8
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1
8
m
o
l
e
s
H
2
O
?
?
?
?
=
1
0
.
8
m
o
l
e
s
H
2
O
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
2
m
o
l
e
s
C
8
H
1
8
?
?
?
?
?
You try…
•
2 Al + 3 H
2
SO
4
Al
2
(SO
4
)
3
+ 3 H
2
–
How many moles of hydrogen can be produced
from 0.24 moles of aluminum?
•
2 Al
2
O
3
4 Al + 3 O
2
–
How many moles of aluminum can be made from
0.35 moles of Al
2
O
3
You try…
•
2 Al + 3 H
2
SO
4
Al
2
(SO
4
)
3
+ 3 H
2
–
How many moles of hydrogen can be produced
from 0.24 moles of aluminum?
•
2 Al
2
O
3
4 Al + 3 O
2
–
How many moles of aluminum can be made from
0.35 moles of Al
2
O
3
0.24 moles Al
----------------
3 moles H
2
-----------------
2 moles Al
?????
?
?
?
?
=
0
.
3
6
m
o
l
e
s
H
2
0.35 moles Al
2
O
3
----------------
4 moles Al
-----------------
2 moles Al
2
O
3
?????
?
?
?
?
=
0
.
7
0
m
o
l
e
s
A
l
The Next Step…
•
Mass-Mass Stoichiometry
•
When we measure materials for chemical
reactions, the scales measure in grams, not
moles
•
Why can’t we just multiply the original mass
times the mole ratio?
The Next Step…
•
Mass-Mass Stoichiometry
•
When we measure materials for chemical
reactions, the scales measure in grams, not
moles
•
Why can’t we just multiply the original mass
times the mole ratio?
–
We must account for the difference in molar
masses between the two materials we are dealing
with
Proportion Method
•
“Plug and chug”
•
Plug in the known numbers
•
Solve for the unknown
Mass-Mass Example
•
C
3
H
8
+ 5 O
2
3 CO
2
+ 4 H
2
O
•
How many grams of H
2
O could be produced from the
combustion of 100 g of propane?
Mass-Mass Example
•
C
3
H
8
+ 5 O
2
3 CO
2
+ 4 H
2
O
•
How many grams of H
2
O could be produced from the
combustion of 100 g of propane?
Mass (start) = 100 g
Coefficient (start) = 1
Molar Mass (start) = 44.11 g/mole
Mass (finish) = unknown
Coefficient (finish) = 4
Molar Mass (start) = 18.02 g/mole
Remember – cross multiply, then divide for solving proportions
(100 g x 4 moles x 18.02 g/mole)
/
(1 mole x 44.11 g/mole) = ???
1
0
0
g
?
?
?
?
Mass-Mass Example
•
C
3
H
8
+ 5 O
2
3 CO
2
+ 4 H
2
O
•
How many grams of H
2
O could be produced from the
combustion of 100 g of propane?
Mass (start) = 100 g
Coefficient (start) = 1
Molar Mass (start) = 44.11 g/mole
M
a
s
s
(
f
i
n
i
s
h
)
=
1
6
3
.
4
1
g
(
a
n
s
w
e
r
!
!
)
Coefficient (finish) = 4
Molar Mass (start) = 18.02 g/mole
100 g C
3
H
8
1 mole C
3
H
8
x 44.11 g/mole C
3
H
8
Grams H
2
O
4 mole H
2
O x 18.02 g/mole H
2
O
1
0
0
g
?
?
?
?
Some helpful hints…
•
Only use the materials provided in the problem
–
The compounds not listed are irrelevant to the problem,
even if they’re in the reaction
•
The coefficients and the molar masses are separate
–
Do not count the coefficient in your molar mass!!!
•
If you see the word “excess”, cross out that chemical – you will
not use it!!!
–
The word excess just means you’ve got plenty of material
to react with the amounts that you have.
Another Example
•
6 Na + Fe
2
O
3
3 Na
2
O + 2 Fe
•
How many grams of Fe can be produced by
the reaction of 10 g Na and excess iron oxide?
Another Example
•
6 Na + Fe
2
O
3
3 Na
2
O + 2 Fe
•
How many grams of Fe can be produced by
the reaction of 10 g Na and excess iron oxide?
1
0
g
?
?
?
?
Another Example
•
6 Na + Fe
2
O
3
3 Na
2
O + 2 Fe
•
How many grams of Fe can be produced by
the reaction of 10 g Na and excess iron oxide?
1
0
g
N
a
6
m
o
l
e
N
a
x
2
2
.
9
9
g
/
m
o
l
e
N
a
G
r
a
m
s
F
e
2
m
o
l
e
F
e
x
5
5
.
8
5
g
/
m
o
l
e
F
e
Another Example
•
6 Na + Fe
2
O
3
3 Na
2
O + 2 Fe
•
How many grams of Fe can be produced by
the reaction of 10 g Na and excess iron oxide?
10 g Na
6 mole Na x 22.99 g/mole Na
Grams Fe
2 mole Fe x 55.85 g/mole Fe
G
r
a
m
s
F
e
=
8
.
1
0
g
(
a
n
s
w
e
r
!
!
)
Volume-Volume Stoichiometry
•
It is difficult to measure gases with a mass, so
they are usually measured with a volume
•
Instead of using the molar mass, use 22.4 L
–
1 mole of any gas = 22.4 L volume (at 0
o
C and 1
atm pressure, known as STP)
Example
•
N
2
+ 3 H
2
2 NH
3
•
How many liters of ammonia can be made
from 15 liters of hydrogen and excess
nitrogen?
Example
•
N
2
+ 3 H
2
2 NH
3
•
How many liters of ammonia can be made
from 15 liters of hydrogen and excess
nitrogen?
A
n
s
w
e
r
=
1
0
l
i
t
e
r
s
N
H
3
1
5
L
3
m
o
l
e
H
2
x
2
2
.
4
L
/
m
o
l
e
H
2
L
i
t
e
r
s
N
H
3
2
m
o
l
e
N
H
3
x
2
2
.
4
L
/
m
o
l
e
N
H
3
Mixed Stoichiometry
•
Sometimes, we need to start with one unit and finish with
another.
•
If you’re converting between mass and moles, use the molar
mass
•
If you’re converting between volume and moles, use 1 mole =
22.4 L
•
If the conversion involves molecules, use Avogadro’s number!
Mixed Example
•
2 KClO
3
(s) 2 KCl (s) + 3 O
2
(g)
•
How many liters of oxygen are produced from the
decomposition of 244 g KClO
3
?
Mixed Example
•
2 KClO
3
(s) 2 KCl (s) + 3 O
2
(g)
•
How many liters of oxygen are produced from
the decomposition of 244 g KClO
3
?
•
Remember, since we’re dealing with
liters
of
oxygen, we’ll use 22.4 L instead of the molar
mass.
Mixed Example
•
2 KClO
3
(s) 2 KCl (s) + 3 O
2
(g)
•
How many liters of oxygen at STP are produced from the
decomposition of 244 g KClO
3
?
2
4
4
g
K
C
l
O
3
2
m
o
l
e
K
C
l
O
3
x
1
2
2
.
5
5
g
/
m
o
l
e
K
C
l
O
3
L
i
t
e
r
s
O
2
3
m
o
l
e
O
2
x
2
2
.
4
L
O
2
Mixed Example
•
2 KClO
3
(s) 2 KCl (s) + 3 O
2
(g)
•
How many liters of oxygen at STP are produced from the
decomposition of 244 g KClO
3
?
2
4
4
g
K
C
l
O
3
2
m
o
l
e
K
C
l
O
3
x
1
2
2
.
5
5
g
/
m
o
l
e
K
C
l
O
3
L
i
t
e
r
s
O
2
3
m
o
l
e
O
2
x
2
2
.
4
L
O
2
L
i
t
e
r
s
O
2
=
6
6
.
9
0
L
(
a
n
s
w
e
r
!
!
)
Another Mixed Example
•
2 Ca (s) + O
2
(g) 2 CaO
•
How many grams of calcium are required to
completely react with 4.48 liters of O
2
at STP?
Another Mixed Example
•
2 Ca (s) + O
2
(g) 2 CaO
•
How many grams of calcium are required to
completely react with 4.48 liters of O
2
at STP?
4
.
4
8
L
O
2
1
m
o
l
e
O
2
x
2
2
.
4
L
/
m
o
l
e
O
2
X
G
r
a
m
s
C
a
2
m
o
l
e
C
a
x
4
0
.
0
8
g
/
m
o
l
e
C
a
X
=
1
6
.
0
3
g
C
a
Other Possible Mixing
•
Sometimes you may start with moles and go
to mass or liters
–
How will THAT work in the proportion?
–
If you are dealing with MOLES for a particular
compound, simply leave out the second number
on the bottom.
Mole-Mass Example
•
How many grams of iron could be made from
6 moles of carbon?
Mole-Mass Example
•
How many grams of iron could be made from
6 moles of carbon?
6
m
o
l
e
s
C
3
m
o
l
e
C
Grams Fe
2 mole Fe x 55.85 g Fe
Notice that since we’re only
dealing with moles, you don’t
need a second number on the bottom!
G
r
a
m
s
F
e
=
2
2
3
.
4
g
F
e
Volume – Mole Example
•
How many moles of B
5
H
9
are required to react
with 100 L of O
2
?
Volume – Mole Example
•
How many moles of B
5
H
9
are required to react
with 100 L of O
2
?
1
0
0
L
O
2
1
2
x
2
2
.
4
L
M
o
l
e
s
B
5
H
9
2
m
o
l
e
s
B
5
H
9
M
o
l
e
s
B
5
H
9
=
0
.
7
4
m
o
l
e
s
Notice that since we’re only
dealing with moles, you don’t
need a second number on the bottom!
Percent Yield
•
Theoretical Yield
–
The amount of material you can make from a
reaction
if it works perfectly….in theory
–
Everything you have calculated so far has been a
theoretical yield
•
Yes, you’ve been doing THIS already
•
Actual Yield
–
The amount of stuff you
actually
make in a
reaction
Percent Yield
•
Percent Yield
–
Notice (again) that we multiply by 100 to turn a
decimal to a percentage
–
The percent yield should NEVER be > 100%
•
If this occurs, you either made a math error or your
sample is impure (wet).
Percent Yield Example (Easy)
•
Some problems will give you a theoretical and
percent yield
–
A student produces 50 g of NaCl in a reaction
yield where the theoretical yield is 60 g
Percent Yield Example (Easy)
•
Some problems will give you a theoretical and
percent yield
–
A student produces 50 g of NaCl in a reaction
yield where the theoretical yield is 60 g
5
0
g
-----------
6
0
g
x
1
0
0
=
8
3
.
3
%
Percent Yield Example (Not As
Easy…)
Calculate theoretical yield first
Then, plug into actual/theoretical x 100
Percent Yield Example (Not As
Easy…)
4
5
.
8
g
K
2
C
O
3
1
m
o
l
e
K
2
C
O
3
x
1
3
8
.
0
3
g
/
m
o
l
e
K
2
C
O
3
X
G
r
a
m
s
K
C
l
2
m
o
l
e
K
C
l
x
7
4
.
5
5
g
/
m
o
l
e
K
C
l
T
h
e
o
r
e
t
i
c
a
l
Y
i
e
l
d
=
4
9
.
5
g
K
C
l
4
6
.
3
g
-----------
4
9
.
5
g
x
1
0
0
=
9
3
.
5
%
3
rd
Percent Yield Example
2 Al(s) + 3 Cl
2
(g) 2 AlCl
3
(s)
40.5 g of aluminum is reacted with excess chlorine gas to
produce 180.0 g of aluminum chloride. Give the percent yield of
this process.
3
rd
Percent Yield Example
2 Al(s) + 3 Cl
2
(g) 2 AlCl
3
(s)
40.5 g
?????
actual yield = 180.0 g
40.5 g of aluminum is reacted with excess chlorine gas to
produce 180.0 g of aluminum chloride. Give the percent yield of
this process.
3
rd
Percent Yield Example
2 Al(s) + 3 Cl
2
(g) 2 AlCl
3
(s)
40.5 g
?????
actual yield = 180.0 g
40.5 g of aluminum is reacted with excess chlorine gas to
produce 180.0 g of aluminum chloride. Give the percent yield of
this process.
4
0
.
5
g
A
l
2
m
o
l
e
A
l
x
2
6
.
9
8
g
/
m
o
l
e
A
l
X
G
r
a
m
s
A
l
C
l
3
2
m
o
l
e
s
A
l
C
l
3
x
1
3
3
.
3
3
g
/
m
o
l
e
A
l
C
l
3
T
h
e
o
r
e
t
i
c
a
l
Y
i
e
l
d
=
2
0
0
.
1
g
A
l
C
l
3
1
8
0
.
0
g
-----------
2
0
0
.
1
g
x
1
0
0
=
9
0
.
0
%
Limiting Reagents
•
We previously compared chemical reactions to
recipes
•
If we’re lacking one ingredient, we can’t make the
entire recipe.
–
This is called the
limiting reagent
(material that
controls how much material we can make)
–
Example: we have 4 hamburgers, 4 slices of
cheese, and 3 buns. We’re limited by the number
of buns – this is our
limiting reagent
–
We have an excess number of burgers and cheese
slices
Limiting Reagents
Non-chemical examples
https://www.chem.tamu.edu/class/majors/tutorialnotefiles/limiting.htm
https://ka-perseus-images.s3.amazonaws.com/f574961ecc30ebb45ee67a64aeb3ddacdb06d1fd.svg
Limiting Reagents
•
https://phet.colorado.edu/sims/html/reactant
s-products-and-leftovers/latest/reactants-
products-and-leftovers_en.html
http://chemistrygroupvivzara.weebly.com/uploads/1/2/7/0/12703729/927436.png?551
Limiting Reagents
C
3
H
8
+ 5 O
2
3 CO
2
+ 4 H
2
O
How many moles of carbon dioxide can be
made from 2.5 moles of propane and 10 moles
of oxygen?
Notice that you are given amounts for BOTH
reactants!
Limiting Reagents
C
3
H
8
+ 5 O
2
3 CO
2
+ 4 H
2
O
2.5 moles
10 moles
??????
How many moles of carbon dioxide can be made
from 2.5 moles of propane and 10 moles of
oxygen?
Find the moles of
CO
2
from both
reactants!
Limiting Reagents
C
3
H
8
+ 5 O
2
3 CO
2
+ 4 H
2
O
2.5 moles
10 moles
??????
How many moles of carbon dioxide can be
made from 2.5 moles of propane and 10 moles
of oxygen?
2
.
5
m
o
l
e
s
C
3
H
8
?
?
?
?
----------------
1
m
o
l
e
C
3
H
8
-----------
3
m
o
l
e
s
C
O
2
=
?
?
?
=
7
.
5
m
o
l
e
s
C
O
2
F
r
o
m
C
3
H
8
1
0
m
o
l
e
s
O
2
?
?
?
?
----------------
5
m
o
l
e
s
O
2
-----------
3
m
o
l
e
s
C
O
2
=
?
?
?
=
6
m
o
l
e
s
C
O
2
F
r
o
m
O
2
Limiting Reagents
C
3
H
8
+ 5 O
2
3 CO
2
+ 4 H
2
O
2.5 moles
10 moles
??????
How many moles of carbon dioxide can be
made from 2.5 moles of propane and 10 moles
of oxygen?
2
.
5
m
o
l
e
s
C
3
H
8
?
?
?
?
----------------
1
m
o
l
e
C
3
H
8
-----------
3
m
o
l
e
s
C
O
2
=
?
?
?
=
7
.
5
m
o
l
e
s
C
O
2
F
r
o
m
C
3
H
8
1
0
m
o
l
e
s
O
2
?
?
?
?
----------------
5
m
o
l
e
s
O
2
-----------
3
m
o
l
e
s
C
O
2
=
?
?
?
=
6
m
o
l
e
s
C
O
2
F
r
o
m
O
2
L
i
m
i
t
i
n
g
r
e
a
g
e
n
t
G
a
v
e
l
e
s
s
p
r
o
d
u
c
t
!
Limiting Reagents
Mass-Mass Problem
CaO (s) + CO
2
(g) CaCO
3
(s)
How many grams of calcium carbonate are
produced from 2.8 g calcium oxide and 4.48 L
carbon dioxide at STP?
2
.
8
g
C
a
O
1
m
o
l
e
C
a
O
x
5
6
.
0
8
g
/
m
o
l
e
C
a
O
X
G
r
a
m
s
C
a
C
O
3
1
m
o
l
e
s
C
a
C
O
3
x
1
0
0
.
0
9
g
/
m
o
l
e
C
a
C
O
3
2
.
8
g
C
a
O
4
.
4
8
L
C
O
2
?
?
?
?
?
?
4
.
4
8
L
C
O
2
1
m
o
l
e
C
O
2
x
2
2
.
4
L
/
m
o
l
e
C
O
2
X
G
r
a
m
s
C
a
C
O
3
1
m
o
l
e
s
C
a
C
O
3
x
1
0
0
.
0
9
g
/
m
o
l
e
C
a
C
O
3
X
=
5
.
0
0
g
C
a
C
O
3
X
=
2
0
.
0
2
g
C
a
C
O
3
Terms To Know, Skills To Master
•
Terms to know
–
Mole Ratio
–
Percent Yield
–
Limiting Reagent
•
Skills to master
–
Calculate mole ratios
–
Perform mole-mole, mass-mass, mixed
stoichiometry calculations
–
Determine percent yield from a reaction
–
Determine the limiting reagent of a reaction
Stoichiometry is the concept of predicting the amounts of reactants and products in a chemical reaction, similar to following a recipe in cooking. It involves balancing chemical equations and determining the quantities of substances involved. By paying attention to coefficients, one can calculate how many moles of each substance are required and produced. Through examples like the reaction of ethanol with oxygen to form carbon dioxide and water, stoichiometry helps in understanding the quantitative aspects of chemical changes.
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Presentation Transcript
Stoichiometry (Chapter 12) Dr. Walker
Stoichiometry (stoy-KEY-ah-muh-tree) Definition: predicting the amounts of reactants and/or products that will be involved in a reaction
Chocolate Chip Cookie Recipe Similar to using recipes to cook When cooking, recipes require specific amounts of each ingredient to make a set amount of food. Example: Chocolate Chip Cookies 2 cups flour teaspoon salt cup butter 1 cup brown sugar 1 cup shortening Makes 6 dozen cookies cup sugar 2 eggs 1 teaspoon vanilla extract 1 pkg chocolate chips 1 cup chopped nuts
Recipes = Chemical Formulas When cooking, recipes require specific amounts of each ingredient to make a set amount of food. Chemical reactions are the exact same thing!!! We need to know how much material can be made from existing amounts (i.e., how much food is in the pantry) OR how much we need to make a specific amount (how much of everything do I need to make 10 dozen cookies)
Chemical Reactions Chemical change involves a reorganization of the atoms in one or more substances. C2H5OH + 3O2 reactants When the equation is balanced it has quantitative significance: 2CO2 + 3H2O products 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water Pay attention to the coefficients!!!!!
Chemical Reactions Chemical change involves a reorganization of the atoms in one or more substances. C2H5OH + 3O2 reactants When the equation is balanced it has quantitative significance: 2CO2 + 3H2O products How many moles of ethanol could react with 9 moles of oxygen? If this happened, what products would we get?
Chemical Reactions C2H5OH + 3O2 reactants How many moles of oxygen could react with 9 moles of oxygen? If this happened, what products would we get? 2CO2 + 3H2O products If 1 mole of ethanol reacts with 3 moles of oxygen, then it would require 3 moles of ethanol to react with 9 moles of oxygen. If 1 mole of ethanol gives 2 moles CO2 and 3 moles H2O, 3 moles of ethanol would yield 3 x 2 = 6 moles CO2 and 3 x 3 = 9 moles H2O
Mole Ratios Using the coefficients we can write mole ratios. Definition: A mole ratio gives the relative amounts of reactants and products using a fraction The coefficients you used in the recipes are basically mole ratios. You re doing the same thing.
Using Mole Ratios Example: N2 + 3 H2 2 NH3 Give the mole ratio of N2 to NH3
Using Mole Ratios Example: 1 N2 + 3 H2 2 NH3 Give the mole ratio of N2 to NH3 Use the coefficients 1 N2 to 2 NH3
Using Mole Ratios Example: N2 + 3 H2 2 NH3 Give the mole ratio of H 2 to NH3
Using Mole Ratios Example: N2 + 3 H2 2 NH3 Give the mole ratio of H 2 to NH3 Use the coefficients 3 H2 to 2 NH3
Write all Mole-Mole Conversions Example: N2 + 3 H2 2 NH3 How many moles of ammonia could be produced from 6 moles of N2 and excess H2?
Mole-Mole Conversions Example: N2 + 3 H2 2 NH3 6 moles ???? How many moles of ammonia could be produced from 6 moles of N2 and excess H2? What are we starting with? 6 moles of N2 Whenever you have excess, you have more than enough you won t use it in the problem!
Mole-Mole Conversions Example: N2 + 3 H2 6 moles ----------- 1 mole N2 2 NH3 ???? ----------- 2 moles NH3 How many moles of ammonia could be produced from 6 moles of N2 and excess H2? Use the coefficients in a proportion
Mole-Mole Conversions Example: N2 + 3 H2 6 moles ----------- 1 mole N2 2 NH3 ???? ----------- 2 moles NH3 ??? = 12 moles NH3 How many moles of ammonia could be produced from 6 moles of N2 and excess H2?
Another Mole-Mole Example 2 C8H18 + 25 O2 16 CO2 + 18 H2O How many moles of water can be produced from 1.2 moles of octane and excess O2?
Another Mole-Mole Example 2 C8H18 + 25 O2 16 CO2 + 18 H2O ????? 1.2 moles C8H18 ----------------- 2 moles C8H18 ---------------- 18 moles H2O How many moles of water can be produced from 1.2 moles of octane and excess O2?
Another Mole-Mole Example 2 C8H18 + 25 O2 16 CO2 + 18 H2O ????? 1.2 moles C8H18 ----------------- 2 moles C8H18 ---------------- 18 moles H2O ???? = 10.8 moles H2O How many moles of water can be produced from 1.2 moles of octane and excess O2?
You try 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2 How many moles of hydrogen can be produced from 0.24 moles of aluminum? 2 Al2O3 4 Al + 3 O2 How many moles of aluminum can be made from 0.35 moles of Al2O3
You try 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2 0.24 moles Al ----------------- 2 moles Al ????? ???? = 0.36 moles H2 ---------------- 3 moles H2 How many moles of hydrogen can be produced from 0.24 moles of aluminum? 2 Al2O3 4 Al + 3 O2 0.35 moles Al2O3 ----------------- 2 moles Al2O3 ????? ???? = 0.70 moles Al ---------------- 4 moles Al How many moles of aluminum can be made from 0.35 moles of Al2O3
The Next Step Mass-Mass Stoichiometry When we measure materials for chemical reactions, the scales measure in grams, not moles Why can t we just multiply the original mass times the mole ratio?
The Next Step Mass-Mass Stoichiometry When we measure materials for chemical reactions, the scales measure in grams, not moles Why can t we just multiply the original mass times the mole ratio? We must account for the difference in molar masses between the two materials we are dealing with
Proportion Method Plug and chug Plug in the known numbers Solve for the unknown
Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O How many grams of H2O could be produced from the combustion of 100 g of propane?
Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O 100 g ???? How many grams of H2O could be produced from the combustion of 100 g of propane? Mass (start) = 100 g Coefficient (start) = 1 Molar Mass (start) = 44.11 g/mole Remember cross multiply, then divide for solving proportions (100 g x 4 moles x 18.02 g/mole) / (1 mole x 44.11 g/mole) = ??? Mass (finish) = unknown Coefficient (finish) = 4 Molar Mass (start) = 18.02 g/mole
Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O 100 g ???? How many grams of H2O could be produced from the combustion of 100 g of propane? Grams H2O 100 g C3H8 1 mole C3H8 x 44.11 g/mole C3H8 4 mole H2O x 18.02 g/mole H2O Mass (start) = 100 g Coefficient (start) = 1 Molar Mass (start) = 44.11 g/mole Mass (finish) = 163.41 g (answer!!) Coefficient (finish) = 4 Molar Mass (start) = 18.02 g/mole
Some helpful hints Only use the materials provided in the problem The compounds not listed are irrelevant to the problem, even if they re in the reaction The coefficients and the molar masses are separate Do not count the coefficient in your molar mass!!! If you see the word excess , cross out that chemical you will not use it!!! The word excess just means you ve got plenty of material to react with the amounts that you have.
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe 10 g ???? How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe Grams Fe 10 g Na 2 mole Fe x 55.85 g/mole Fe 6 mole Na x 22.99 g/mole Na How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe 10 g Na Grams Fe 2 mole Fe x 55.85 g/mole Fe 6 mole Na x 22.99 g/mole Na Grams Fe = 8.10 g (answer!!) How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Volume-Volume Stoichiometry It is difficult to measure gases with a mass, so they are usually measured with a volume Instead of using the molar mass, use 22.4 L 1 mole of any gas = 22.4 L volume (at 0oC and 1 atm pressure, known as STP)
Example N2 + 3 H2 2 NH3 How many liters of ammonia can be made from 15 liters of hydrogen and excess nitrogen?
Example N2 + 3 H2 2 NH3 Liters NH3 15 L 2 mole NH3 x 22.4 L/mole NH3 3 mole H2 x 22.4 L/mole H2 Answer = 10 liters NH3 How many liters of ammonia can be made from 15 liters of hydrogen and excess nitrogen?
Mixed Stoichiometry Sometimes, we need to start with one unit and finish with another. If you re converting between mass and moles, use the molar mass If you re converting between volume and moles, use 1 mole = 22.4 L If the conversion involves molecules, use Avogadro s number!
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) How many liters of oxygen are produced from the decomposition of 244 g KClO3?
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) How many liters of oxygen are produced from the decomposition of 244 g KClO3? Remember, since we re dealing with liters of oxygen, we ll use 22.4 L instead of the molar mass.
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) Liters O2 244 g KClO3 3 mole O2 x 22.4 L O2 2 mole KClO3 x 122.55 g/mole KClO3 How many liters of oxygen at STP are produced from the decomposition of 244 g KClO3?
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) Liters O2 244 g KClO3 3 mole O2 x 22.4 L O2 2 mole KClO3 x 122.55 g/mole KClO3 Liters O2 = 66.90 L (answer!!) How many liters of oxygen at STP are produced from the decomposition of 244 g KClO3?
Another Mixed Example 2 Ca (s) + O2 (g) 2 CaO How many grams of calcium are required to completely react with 4.48 liters of O2 at STP?
Another Mixed Example 2 Ca (s) + O2 (g) 2 CaO X Grams Ca 4.48 L O2 2 mole Ca x 40.08 g/mole Ca 1 mole O2 x 22.4 L/mole O2 X = 16.03 g Ca How many grams of calcium are required to completely react with 4.48 liters of O2 at STP?
Other Possible Mixing Sometimes you may start with moles and go to mass or liters How will THAT work in the proportion? If you are dealing with MOLES for a particular compound, simply leave out the second number on the bottom.
Mole-Mass Example How many grams of iron could be made from 6 moles of carbon?
Mole-Mass Example Grams Fe 6 moles C 2 mole Fe x 55.85 g Fe 3 mole C Grams Fe = 223.4 g Fe Notice that since we re only dealing with moles, you don t need a second number on the bottom! How many grams of iron could be made from 6 moles of carbon?
Volume Mole Example How many moles of B5H9 are required to react with 100 L of O2?
Volume Mole Example Moles B5H9 100 L O2 2 moles B5H9 12 x 22.4 L Notice that since we re only dealing with moles, you don t need a second number on the bottom! Moles B5H9 = 0.74 moles How many moles of B5H9 are required to react with 100 L of O2?
Percent Yield Theoretical Yield The amount of material you can make from a reaction if it works perfectly .in theory Everything you have calculated so far has been a theoretical yield Yes, you ve been doing THIS already Actual Yield The amount of stuff you actually make in a reaction
Percent Yield Percent Yield Notice (again) that we multiply by 100 to turn a decimal to a percentage The percent yield should NEVER be > 100% If this occurs, you either made a math error or your sample is impure (wet).
Percent Yield Example (Easy) Some problems will give you a theoretical and percent yield A student produces 50 g of NaCl in a reaction yield where the theoretical yield is 60 g
