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, 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)

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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!13

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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