announcements…monday april 15

Announcements…Monday April 15

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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!

Here’s what people said they learned.

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Here’s what people said they wanted to know better.

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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.

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Maybe this never happened; click the story for a link.

More AnnouncementsWednesday, March 27

Ch. 8,9There 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.

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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.

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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

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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

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The mole is the chemist’s dozen.

Avogadro's number

= 602202035873921029561369 

= 6.02202035873921029561369 x 1023 This is a lie!!!!

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•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 = 1.660 x 10-24 g.

A single atom of weighs 12 x 1.660 x 10-24 g.

Atomic Mass Unit. Pay attention—this is where Avogadro's

number comes from.

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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 )

= 6.022 x 1023 atoms

1 Atom 12 x 1.660 x 10-24

grams ( )

It’s the same as the number of atoms of hydrogen in one gramof hydrogen (assuming only the “normal” hydrogen isotope).

It’s the same as the number of atoms of iodine in 126.90 gram ofiodine (assuming the natural abundance of iodine isotopes)

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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

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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.

Properly speaking, Avogadro’s number is a conversion factor, not

just a number.

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Number: 12 Conversion factor: 12 as in 12 doughnuts/dozenRewrite this: 12/dozenRewrite it again: 12 dozen-1

Number: 6.022 x 1023

Conversion factor: 6.022 x 1023 as in 6.022 x 1023 per molRewrite this: 6.022 x 1023 /dozenRewrite it again: 12 dozen-1

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.

Your share: $32,654.52

$10,205,557,724,099.29

National debt as of October 31, 2011

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Taxes per person for balanced budget: $14,000/year

http://www.odec.ca/projects/2008/stan8e2/hatom.jpg

A tirade on the irrelevance of detail…

Wouldn’t $ x 1012 be close enough?

Your share: $3.3 x 104 = $33,000

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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.

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Get Molecular weight:

Example #1 for Louisiana: CH4

Example #2 for Louisiana: benzene, C6H6

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Get Formula weight for :

Ba(HCO3)2

Ba: 137.327

H: 1.008

C: 12.011

O: 15.999

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What % of Ba(HCO3)2

is made of carbon?

Ba: 137.327

H: 1.008

C: 12.011

O: 15.999

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How many carbon atoms in 7.8 grams of benzene, C6H6

How many hydrogen atoms?

How many MOL of H atoms?

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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 = 1622

Let's do another, perhaps a bit harder.   1. What is molecular mass of penicillin, C16H17N2O5SK 2. What is mass of 0.45 mol of penicillin?3. How many C atoms in 19.5 g of penicillin?4. What percentage of penicillin, by weight, is oxygen?

Actually, I don’t think thisis pencillin—maybe a derivative.

Dorothy Crowfoot Hodgkin, Nobel Prize 1964

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Reactions are like recipes.CH4 + O2 CO2 + H2OUnbalanced: a list of ingredients & results

CH4 + 2O2 CO2 + 2H2OBalanced: a correct recipe

Wheels + Pedals + Handlebar BicycleUnbalanced: a list of ingredients & results

2 Wheels + 2 Pedals + 1 Handlebar BicycleBalanced: a correct recipe

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Chemical Equivalence: 

The 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  2 KCl which is the same as: 1 KClO3   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: 

2 KClO3 2 KCl + 3 O2

74.54 g of KCl 122.45 g of KClO3

We can also write:  2 KClO3    3 O2 

Or: 244.9 g KClO3 96 g O2

This means we get 96 g of oxygen for every 244.9 

g of KClO3.

KClO3: 122.45 g/molKCl: 74.45 g/molO2: 32 g/mol

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What do chemists really use?

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Another question on the same theme:

How much KClO3 will I need to produce 1 ton of O2?

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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.

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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.

Chemistry is Imperfect

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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?

Fe2O3 2Fe

+

O2

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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 and unlimited amounts of SiO2, what percent yield is that?

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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”.

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You actually ARE used to problems just like this. Consider a car.

1965 Plymouth Barracuda’s weigh 3,000 pounds.

How many tires per 9,000 pounds of Fishcar?

Answer: 12 (not counting the spare)

How many cubic inches of engine displacement per 15,000 pounds of Fishcar?5 x 273 = 1365 cubic inches

How many horsepower per 18,000 pounds of Fishcar?About 6 cars X 270 hp = 1620 hp

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Supply Limits. Suppose you had 17 tires; what is the most number of cars you can make?

Supply Limits.

If you have 17 tires and 3 motors, how many cars can you make?

Supply Limits.

It’s a V8 engine (8 cylinders). If you have 51 pistons, 12 engine blocks and 29 tires, how many cars can you make?

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). If 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 0coefficient: 51/8 12/1 29/4 =6.375 =12 =7.25

*Limit = lowest

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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?

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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)

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A+B

C+D

Energy

Time (“reaction progress”)

How far

How fast

2 questions about reactions: how fast? how far?

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A+B

C+D

Energy

Time (“reaction progress”)

Things that affect how fast include:

Temperature: rate as T Pressure: rate as P Concentration: rate as c Catalyst: rate

Biocatalyst = Enzyme. Enzymes are important!47

A+B

C+D

Energy

Time (“reaction progress”)

With catalyst: lowers energy of activation.http://en.wikipedia.org/wiki/Enzyme

Things that affect how far

Chemists and, especially, chemical engineers who try to manipulate equilibrium.

Let me try to explain…..

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EquilibriumIt’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”.

184118

72303

game watching

customers concessionK

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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

Add reactantsRemove Products

Remove reactantsAdd Products

Shift To Right Shift To Left

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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.

Spontaneity!

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EntropyReactions (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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