Chemistry is Imperfect
Problem #1. Many possible reaction paths.
We wrote: CH4 + 2O2  CO2 + 2H2O
But methane can also do this:
CH4 + O2  CO + 2H2O
The second reaction makes deadly carbon
monoxide; the first produces only benign CO2
and H2O.
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Imperfection happens.
Problem #2. A second problem is that
we may inefficiently isolate a product.
•product can stick to glassware.
•it can vaporize.
•it can get dropped on floor.
•it can stick to filter paper, etc.
•it can re-react (e.g., isolating K compared to Fe)
•Like gambling, there are many ways to lose!
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Theory vs. Reality: We actually quantify the amount of
imperfection.
Theoretical Yield: what God would get.
Percent Yield: what you would get compared
to what God would get as a chemist,
expressed as a percentage.
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Example 1 involves production of iron
from iron ore.
If you had 2 tons of rust (Fe2O3) how
many tons of iron could you get from it?
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Fe2O3  2Fe +
O
2
2
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Let’s do it by percent!
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Or you can do it the hard way. (Having
learned conversions, might as well use them.)
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Suppose your metal processing plant loses some
iron and you only get 1.35 tons.
% Yield = 100 x 1.35/1.40 = 96 %
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Example 2
Soda lime glass is made from this reaction*:
Na2CO3 + SiO2  Na2(SiO3) + CO2
If we collect 200 g of CO2 from 1000 g of
sodium carbonate, what percent yield is that?
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Reactions do not go all the way.
It’s not really A + B  C + D
It’s more like: A + B  C + D
At any one time:
zillions of A’s
zillions of B’s
zillions of C’s
zillions of D’s
Reactions that “go”
More zillions of C’s and D’s
Reactions that don’t “go”
Less zillions of C’s and D’s
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Energy helps to determine whether
reaction goes or not
(it is not the ultimate determinant, though)
Energy
A+B
C+D
Time (“reaction progress”)
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2 questions about reactions:
how fast?
how far?
Energy
How fast
A+B
C+D
How far
Time (“reaction progress”)
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Things that affect how fast include:
Temperature: rate  as T 
Pressure: rate  as P 
Concentration: rate  as c 
Catalyst: rate 
Energy
A+B
C+D
With catalyst: lowers energy of activation.
http://en.wikipedia.org/wiki/Enzyme
Time (“reaction progress”)
Biocatalyst = Enzyme. Enzymes are important!
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Things that affect how far
Chemists and, especially, chemical engineers
who try to manipulate equilibrium.
Let me try to explain…..
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Equilibrium
It’s more like: A + B  C + D
zillions of A’s
zillions of B’s
zillions of C’s
zillions of D’s
The molecules know what
balance they want between A, B,
C and D.
Problem is: we may not
necessarily agree!
Western cultures (and chemists of
all cultures) try to manipulate
equilibrium, as if it is our
manifest destiny to do so!
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One of your first encounters with equilibrium was physical
equilibrium between gases & liquids & solids.
The Hot Soup Problem (It’s alphabet soup—hence the letters).
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Why is equilibrium such a foreign concept?
Because we are used to fairly small numbers in
our daily lives. There are things that go like
equilibrium, though.
Example: attentiveness of spectators at a
football game.
"Concession customers"  "Active spectators"
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Can we manipulate this? Yes, by realizing that the
equation is not complete.
Concession customers + Scoring Drive

Active spectators
+ Hunger
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We can characterize this equilibrium by
a number, the
“equilibrium” constant, that shows the
ratio of “product” to “reactant”.
concession customers 72303
K

 18
watching game
4118
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Limits to Equilibrium
What happens if we run out of Tiger Dogs?
What happens if the food lines are too long?
What happens if the oceans run out of capacity
to buffer all the CO2 we are producing?
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Returning to the soup problem…
Hot soup  Cold soup + Heat released
If we remove heat from the soup in the form of hot
vapor, the system will try to make more heat in the
space above the soup.
When it does, we get more cold soup.
Blowing on soup is manipulating equilibrium!
Heat Energy + H2O(l)  H2O(g)
Equilibrium between water
liquid and water vapor.
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Le Chatelier’s principle
Shift To Right
Add reactants
Remove
Products
Shift To Left
Remove
reactants
Add Products
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Spontaneity!
Who decides what's equilibrium?
Which way to equilibrium?
Observation: often, the reactions that occur
spontaneously release heat energy
(exothermic).
But not always! Some endothermic reactions
also occur spontaneously.
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Entropy
Reactions (changes) occur if they
increase the disorderliness of the
universe.
“Disorderliness" is called "entropy"
No one knows why this law holds true.
Like any law, it's the sum total of our
EXPERIMENTAL observations.
You actually take much of this for
granted, whether you realize it or not,
when you use time: Entropy is time's
arrow.
"Forward" in times means more
disordered!
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