Stoichiometry – a SIMPLE idea.
Stoichiometry – the
process of relating one quantity of a
chemical to another within a chemical reaction
using molar ratios derived from the coefficients
of a balanced reaction!
HOW TO DO STOICHIOMETRY:
1.
START with a complete, balanced equation.
2.
Take whatever
you know in whatever units you have
and convert to moles.
3.
(is one reagent limiting? then choose
that one to move forward.)
4.
DO THE STOICHIOMETRY (IN UNITS OF MOLES)
5.
Convert back from moles to whatever units you need.
6.
(determine the theoretical yield, percent
yield etc…)
Why
does stoichiometry seem difficult?
Because of the
conversions, not the chemistry!
What units could you be given/what units might you need?
1.
moles – EASY; you’re already there!
2.
mass – need molar mass.
3.
volume (pure liquid) – you need the DENSITY of the liquid to
find the mass, then see #2.
4.
volume (solution) – need molar concentration.
5.
pH (can find concentration of H+ or OH–) –
but still need volume!
6.
mixture (percent composition) – still need mass, molar mass,
volume, pressure, temperature, etc.
7.
volume (gas) – need pressure AND temperature!
NOW ADD THE FOLLOWING IDEAS:
Limiting reactant (LR) – NOT the
reactant with the fewer number of moles; rather the one with fewer
“equivalents”
in
more concrete terms, the one that generates
less product!
Excess reactant (XS) – ALL the other
reactants…
Stoichiometric excess – the
other (NON-LIMITING) reactant(s) left over at the end of the reaction
Theoretical yield – the amount
of product calculated using the LIMITING reactant
Actual yield – the amount of
product actually measured at the end of the reaction (imperfect world!)
Percent yield = (Actual yield) /
(Theoretical yield)
Analogy: movies in a new
IKEA® bookshelf
180
DVDs total
7 shelves (18 DVDs per shelf)
34 pegs (4 pegs per shelf)
NOW,
IN CHEMICAL TERMS…
1 shelf + 4
pegs +
18 DVDs à 1 full shelf
we
have… 180 DVDs
18 DVDs
7
shelves * ---------------------- = 126 DVDs
1 shelf
18 DVDs
34
pegs *
------------------------- = 153 DVDs
4 pegs
HOW MANY DVDs can we put away???
Which
of the parts is limiting?
Which
of the parts is in excess?
What
is the theoretical yield (filled shelves of DVDs)?
What
is the stoichiometric excess of DVDs?
(number
started with – number put away)
180 – 126 = 54 DVDs (excess)
What
is the stoichiometric excess of pegs?
(number
of pegs – number actually used)
4 pegs
USED: 7
shelves * ---------------------- = 28 pegs (used)
1 shelf
34
– 28 = 6 pegs (excess)
WHEN YOU ACTUALLY put the DVDs on the shelf, you had some in
odd-shaped boxes, and only 118 fit on the shelf.
What
is the actual yield?
What
is the percent yield?
AN
EXAMPLE OF LIMITING AND EXCESS REACTANTS (THAT YOU’VE ALREADY DONE IN LAB):
Pb2+ + 2 NO3– + 2 K+
+ SO42– à PbSO4 (s) + 2 NO3– + 2 K+ (total ionic equation)
1.
Start
by adding 4X formula units of Pb(NO3)2 to a flask
2.
Add
K2SO4 gradually, 2X formula units at a time
3.
Observe
the progress of the reaction by conductivity
Pb2+: K+:
NO3–: SO42–:
A B
C D
|
|
A |
B |
C |
D |
|
|
Before
reaction |
Addition
of 2 K2SO4 |
Addition
of 4 K2SO4 |
Addition
of 6 K2SO4 |
|
K+ |
0 |
4 |
8 |
12 |
|
SO42– |
0 |
0 |
0 |
2 |
|
Pb2+ |
4 |
2 |
0 |
0 |
|
NO3– |
8 |
8 |
8 |
8 |
|
#
of PbSO4 precipitates |
0 |
2 |
4 |
4 |
|
#
of ions still in solution |
12 |
14 |
16 |
22 |
Questions:
1.
For
which column is the reaction at stoichiometric equality?
(the “correct number” of reactants
to products – equivalence point)
2.
In
column B, which reactant is limiting; which is in excess?
3.
In
column D, which reactant is limiting; which is in excess?
Manufacturing
of Freon:
2
HF (g) + CCl4 (l) à CCl2F2 (g) + 2 HCl (g)
1.0
kg HF +
1.0 kg CCl4 à 0.44 kg CCl2F2
Is one limiting – if so,
which?
What is the theoretical yield?
What was the percent yield?
How much HCl should have been
produced?
How much total product mass
should have been produced?
Why is this different than the
total mass of reactants (1 kg + 1 kg) and by how much?
WHAT makes up this difference?
Rocket
Science:
___N2O4 (l)
+ ___N2H4 (l)
à ___N2 (g) + ___H2O
(g)
(NOTE: These reactants are both
liquids at normal temperatures and reasonable pressures.
However,
the products are at high temperature and can be assumed to be gases.)
If
you start with: 5.0 x 104 g hydrazine “in the tank”:
NOTE: How large would the tank
carrying N2H4 have to be? (for 5.0 x 104
g) --
density = 1.01 g/cm3
How many moles of nitrogen
created?
How many moles of water are
produced?
How much mass of dinitrogen
tetroxide is needed? How much volume of
N2O4 is needed?
-- density = 1.44 g/cm3
20.5 mL of 0.234 M FeCl3 is added to 42.5 mL of
0.453 M NaOH
Which is limiting?
Which is in excess?
What mass (in g) of Fe(OH)3
will precipitate?
How many moles of the excess
reactant remain after the reaction is complete?
What is the molar concentration
of the excess reactant at the end of the reaction?
(remember M = moles solute / liters of
solution)
Ideal Gas Law
(in stoichiometry):
Revisit
rocket science example…
Start
with a 68 L tank of liquid hydrazine and another 68 L tank of dinitrogen
tetroxide
What
is the volume of gas produced in the reaction?
(assume sea level or
1 atm pressure, 3000 C temperature in exhaust plume)
Percent Composition
(in stoichiometry):
MOST OF SOCIETY IS
UNFAMILLIAR WITH MOLES AND MOLARITY! So
they use percent composition instead –
you see this all over the
place, especially in consumer goods, like toothpaste, bleach, etc.
THIS IS ALWAYS: the
part divided by whole (MUST be in same units) * 100%
This is often used for volumes (must have the same units!)
OR masses (yes – still the same units!)
…but it is ALWAYS specified (whether it is mass or
volume).
examples:
In your car’s radiator, you will find a solution that
contains approximately 600 mL of water and 800 mL of ethylene glycol, along
with some other additives. What volume
percent of the composition of the antifreeze is water?
A very explosive gas mixture contains 4.0 moles of propane
and 20.0 moles of oxygen. What weight
percent is the propane?
(C3H8 + 5 O2 à 4 H2O + 3 CO2)
The mineral chalcopyrite (CuFeS2) is the primary
ore of copper, and contains 34.63% Cu by mass.
How many grams of Cu can be refined from 5.11 x 103 kg of
this mineral? What percent (by mass) Fe
is in the ore? (NOTE: you can use the
formula to find the MOLAR ratios…)
Conversions between mass &
moles
conversion
of atoms to moles; moles to mass; mass to atoms
Conversions using solution
concentrations
conversions
between volume, mass, molarity and moles
Dilution equation: Mi Vi = Mf Vf
pH and the
concentration of H+ ion:
Note: pH = -log [H+]
AND 10-pH = [H+]
So what is the concentration of [H+] in a solution with a pH
of 5.5?
What is the pH of a solution containing 2.34 x 10-9
M H+?
What is the pH of the following solution: 0.0789 moles of HCl
dissolved in a volume of 7.58 x 104 L?
NOW USING pH with
Stoichiometry Calculations:
Going back to the combustion
of sulfur to create acid rain…
Burning 1.00 kg of FeS2 generated a volume of SO2.
FeS2 (s) + 2 O2
(g) à Fe (s)
+ 2 SO2 (g)
If
this SO2 was all oxidized to H2SO4 in the
atmosphere,
2 SO2 + O2 à 2 SO3
+ 2 SO3 + 2 H2O à 2 H2SO4
___________________________
NET:
2 SO2 + O2 + 2 H2O à 2 H2SO4
what
volume of water would it dissolve in to form acid rain with a pH of 4.8?