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Transcript
Announcements…Monday April 15
We are now in Chapters 8-9
all sort of jumbled together
Judge what matters by ABNG problems!
Chapter 8: Pretty much self study, not many ABNGs, I
don’t
care about solubility (yawn)
Chapter 9: MUCH harder; includes limiting Reagent…
that’s the hardest it gets! We saw in the banana
problem already, but now we have to figure it out!
1
Here’s what people said they learned.
2
Here’s what people said they wanted to know better.
3
Maybe this never happened; click the story for a link.
Moral of this story
•Do not smoke
•Insecticides can ignite
•Requires spark
•Gives off energy
•You have to be careful with
chemistry
Time to do REAL
chemistry: Starting
Chapter 8,9,10 now.
4
More Announcements
Wednesday, March 27
Ch. 8,9
There will be a LOT of assigned problems here
….because it really matters.
I don’t fuss much with “rules” of solubility—so many exceptions;
however, it’s good to know all nitrates are soluble.
I don’t care if a reaction is a decomposition or
a displacement or a metathesis. No wonder people
“despise” chemistry.
5
You already know some important
things.
Conservation of Numbers: in a chemical rxn, atoms aren't
changed, lost or made.
Same is true of electrons, protons & neutrons.
Conservation of Mass: mass isn't gained or lost either
(neglecting those tiny losses in nuclear reactions).
Chemistry goes by integer numbers – very large integers.
7
Zillions of molecules or atoms
are usually involved.
C + O  CO
We think: one atom at a time.
Reality: zillions of C’s, zillions of O’s,
zillion’s of CO’s
8
Practice with sub-Zillion Numbers makes it
easier for us….I hope.
One dozen cookies = 12 cookies
Two dozen eyes = 24 eyes
Four dozen yellow frosting zones =
= 48 squeezes of yellow frosting tube.
Other convenient numbers: Baker’s dozen = 13
Gross = dozen dozen = 144
9
The mole is the chemist’s dozen.
Avogadro's number
= 602202035873921029561369
= 6.02202035873921029561369 x 1023
This is a lie!!!!
10
Atomic Mass Unit. Pay attention—this is
where Avogadro's number comes from.
•Earlier we said "Let one atom of H have 1 atomic mass unit"
•Now, we have a problem, because H has 3 isotopes:
•So.....we cannot use "hydrogen" as it usually exists (mixed
isotopes) for our mass standard.
•We must purify it.
•Easier to purify carbon, so carbon became the mass standard:
Atomic mass unit:
1 a.m.u. = 1/12th the mass of isotope
A single atom of
= 1.660 x 10-24 g.
weighs 12 x 1.660 x 10-24 g.
11
Avogadro’s Number is the inverse of a.m.u.
How many atoms of are in 12.000 grams
of pure isotope
Answer: (12.000 grams of
1 Atom
)
12 x 1.660 x 10-24 grams
(
)
= 6.022 x 1023 atoms
It’s the same as the number of atoms of hydrogen in one gram
of hydrogen (assuming only the “normal” hydrogen isotope).
It’s the same as the number of atoms of iodine in 126.90 gram of
iodine (assuming the natural abundance of iodine isotopes)
12
Avogadro’s Number: Fixing that Lie.
We said: 602202035873921029561369
6.02202035873921029561369 x 1023
We really only know the first few digits, so:
6.02214129 x 1023
It does not look like an integer, but it has the
meaning of an integer, just as one dozen is
an integer. Because matter is discrete, it
comes in integer amounts. LARGE integers.
13
Properly speaking, Avogadro’s number is a
conversion factor, not just a number.
Number: 12
Conversion factor: 12 as in 12 doughnuts/dozen
Rewrite this: 12/dozen
Rewrite it again: 12 dozen-1
Number: 6.022 x 1023
Conversion factor: 6.022 x 1023 as in 6.022 x 1023 per mol
Rewrite this: 6.022 x 1023 /dozen
Rewrite it again: 12 dozen-1
14
Why did I lie about Avogadro Number?
To emphasize that Avogadro’s number is an integer CONCEPT,
just as a dozen or a gross are integer numbers. All the things we
do with ordinary integer numbers we also do with Avogadro’s
number. Compare the national debt, another “big” number
known to its smallest “quantum” of matter, which is the penny.
National debt as of October 31, 2011
$10,205,557,724,099.29
Your share: $32,654.52
Taxes per person for balanced budget: $14,000/year
http://www.odec.ca/projects/2008/stan8e2/hatom.jpg
15
A tirade on the irrelevance of detail…
Wouldn’t $ x 1012 be close enough?
Your share: $3.3 x 104 = $33,000
16
Compound Formula and Molecular Weights
follow the same basic prescription as atoms.
Molecular weight: how many a.m.u. per molecule
(same as how many grams per 6.022 x 1023 molecules)
Use for molecules!
Formula weight: how many a.m.u. per formula
(same as how many grams per 6.022 x 1023 repeats of the formula)
Use for compounds or molecules.
17
Get Molecular weight:
Example #1 for Louisiana: CH4
Example #2 for Louisiana: benzene, C6H6
18
Get Formula weight for :
Ba(HCO3)2
Ba: 137.327
H: 1.008
C: 12.011
O: 15.999
19
What % of Ba(HCO3)2
is made of carbon?
Ba: 137.327
H: 1.008
C: 12.011
O: 15.999
20
How many carbon atoms in 7.8 grams
of benzene, C6H6
How many hydrogen atoms?
How many MOL of H atoms?
21
Question:
Which is potentially worth more, in
terms of silver it can produce?
1000 g of AgCl
or..... 1000 g of AgI
or..... 1000 g of Ag NO3 ?
or..... 500 g of Ag2CO3
Atomic wts: Ag =108, I = 127, C = 12, O = 16
22
Let's do another, perhaps a bit harder.
1.
2.
3.
4.
What is molecular mass of penicillin, C16H17N2O5SK
What is mass of 0.45 mol of penicillin?
How many C atoms in 19.5 g of penicillin?
What percentage of penicillin, by weight, is oxygen?
Actually, I don’t think this
is pencillin—maybe a derivative.
Dorothy Crowfoot Hodgkin, Nobel Prize 1964
23
Reactions are like recipes.
CH4 + O2  CO2 + H2O
Unbalanced: a list of ingredients & results
CH4 + 2O2  CO2 + 2H2O
Balanced: a correct recipe
Wheels + Pedals + Handlebar  Bicycle
Unbalanced: a list of ingredients & results
2 Wheels + 2 Pedals + 1 Handlebar  Bicycle
Balanced: a correct recipe
24
Chemical Equivalence: d
The d symbol defines the relation between two compounds
according to the particular balanced chemical reaction being
considered.
It acts like a conversion factor!
Consider: 2 KClO3  2 KCl + 3 O2
2 KClO3 d 2 KCl which is the same as: 1 KClO3 d 1 KCl
This simply means that one KCl will be produced for every KClO3
present in this reaction (other reactions that produce KCl will be
different).
25
More about Chemical Equivalence: d
2 KClO3  2 KCl + 3 O2
KClO3: 122.45 g/mol
KCl: 74.45 g/mol
O2: 32 g/mol
74.54 g of KCl d 122.45 g of KClO3
We can also write: 2 KClO3 d 3 O2
Or: 244.9 g KClO3 d 96 g O2
This means we get 96 g of oxygen for every 244.9 g of KClO3.
26
What do chemists really use?
27
Another question on the same
theme:
How much KClO3 will I need to
produce 1 ton of O2?
29
Another Example (more complex, but
also more fun....and a little depressing!)
How many tons of CO2 are produced by
burning 1000 gallons of gasoline? Assume
density of gasoline is 0.692 g/ml and
formula is C8H18.
30
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.
31
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!
32
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.
33
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?
3
Fe2O3  2Fe +
O
2
2
34
Let’s do it by percent!
35
Or you can do it the hard way. (Having
learned conversions, might as well use them.)
36
Suppose your metal processing plant loses some
iron and you only get 1.35 tons.
% Yield = 100 x 1.35/1.40 = 96 %
37
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 and unlimited amounts of
SiO2, what percent yield is that?
38
Now it is time to talk about limits!
Limiting reagent problems are the
hardest of the whole course for many
students, but you can do them!
We saw one in the banana quiz, but it
was “easy”.
39
You actually ARE used to problems
just like this. Consider a car.
1965 Plymouth Barracuda’s weigh 3,000 pounds.
Supply
Limits.
Suppose
Supply
Limits.
How many tires per
9,000 pounds of Fishcar?
Supply
you
had 17Limits.
tires; what is
the
most
number
of and
cars 3
If
you
have
17
tires
Answer: 12 (not counting
the
spare)(8 cylinders).
It’scan
a V8
engine
you
make?
motors,
how many
cars 12
If you have
51 pistons,
youofblocks
make?
How many cubic can
inches
engineand
displacement
engine
29 tires,
per 15,000 poundshow
of Fishcar?
many cars can you
5 x 273 = 1365 cubicmake?
inches
How many horsepower per 18,000 pounds of Fishcar?
About 6 cars X 270 hp = 1620 hp
40
Sometimes people can do these in their heads—we are very
used to limits!—but let’s dissect it and see how we do it.
Here’s the question again: It’s a V8 engine (8 cylinders).
you have 51 pistons, 12 engine blocks and 29 tires, how many
cars can you make?
Write balanced equation:
8 pistons + 1 block + 4 tires = 1 car
Start:
51
12
29
0
 coefficient: 51/8
12/1
29/4
=6.375
=12
=7.25
*Limit = lowest
41
If
9.85 (ABNG)
Na reacts with H2 to make NaH.
A reaction mixture contains 10.0 g Na
and 0.0235 g H2.
When the reaction is performed, the
chemist gets 0.428 g of NaH.
What is her percent yield?
42
43
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
44
Energy helps to determine whether
reaction goes or not
(it is not the ultimate determinant, though)
Energy
A+B
C+D
Time (“reaction progress”)
45
2 questions about reactions:
how fast?
how far?
Energy
How fast
A+B
C+D
How far
Time (“reaction progress”)
46
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!
47
Things that affect how far
Chemists and, especially, chemical engineers
who try to manipulate equilibrium.
Let me try to explain…..
48
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!
49
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).
50
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"
51
Can we manipulate this? Yes, by realizing that the
equation is not complete.
Concession customers + Scoring Drive

Active spectators
+ Hunger
52
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
53
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?
54
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.
55
Le Chatelier’s principle
Shift To Right
Add reactants
Remove
Products
Shift To Left
Remove
reactants
Add Products
56
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.
57
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!
58