Transcript Kinds of Chemistry - Louisiana State University

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Today’s show is brought to you by
Chapters 13 & 14
Acids, bases and solutions
Part qualitative, part quantitative chapter
Very important chemistry,
your body uses it EVERY day!
Yes, it will be on the final!
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You are familiar with some acid
properties.
*Sour taste (be careful tasting strong acids!)
*React with otherwise unreactive
metals to produce hydrogen.
*Changes litmus to red.
*Produce CO2 when
added to limestone.
*Neutralize bases.
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Common
Acids
Vinegar
Tomatoes
Citrus fruits
Coke, Sprite, Pepsi, etc.
Black coffee
Gastric fluid
Vitamin C
Aspirin
http://www.tomato.com/home_files/444879_95747059.png
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http://www.phytomedical.com/Plant/Images/Aspirin_03.jpg
Base: opposite of Acid
http://www.geocities.com/napavalley/3227/images/pr_lutefisk.jpg
Properties:
Bitter, Slippery, Changes litmus to blue
Common examples: Lye, lutefisk,
Ammonia, baking soda, soap, detergents,
milk of magnesia, TUMS, oven cleaners,
drain cleaners.
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http://www.stlcommercemagazine.com/archives/february2001/images/tums.jpg
Acid-Base Factoids
Sulfuric acid is the #1 chemical (H2SO4)
Main use is fertilizer: the Earth has to be coaxed if we want
it to feed steak to 6 billion people.
Fertilizer is produced where energy costs are low.
http://www.energybulletin.net/1177.html
When you take a TUMS, it is an attempt to gently
neutralize a little too much acid by adding a weak base.
Other antiacids try to control how much acid your stomach
produces in the first place.
It’s pretty amazing that your stomach does not digest itself!
http://www.gi.alaska.edu/ScienceForum/ASF5/599.html
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The Arrhenius definition of acids & bases
is based on H+ and OH-.
Acid produces H+ in water
Base produces OH- in water
This view makes H+ and OH- the two opposites
H+ = proton
OH- = hydroxide
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Protons don't actually travel unescorted in water;
they go with a water escort and together this is
called hydronium ion, H3O+.
H+(aq) means H3O+
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The Bronsted-Lowry definition is based
totally on the proton.
Acid: produces protons
+ + ClHCl

H
hydrochloric acid
Base: accepts protons
CH3COO- + H+  CH3COOH
acetate
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Strong acids are compounds that fully
dissociate to spit out a hydrogen ion (proton).
e.g.
HCl
HNO3
H2SO4
These give protons almost completely
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Write these dissociations, including
production of hydronium
Identify conjugate pairs
Can you see WHY protons should fall off HCl, HNO3,
H2SO4?
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Weak acids often involve wimpy covalent bonds.
This COOH group is often responsible
for weak organic acids, including those
found in proteins, orange juice, etc.
Acetic acid give a less than 1:1 proton:acid ratio
The acetate anion acts as a base (conjugate base)
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Strong on left…..weak on right.
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Strong Bases
K OH  K+ + OH–
Ca (OH)2  Ca+2 + 2OH–
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Weak Bases
Example 1.
Potassium cyanide, a two-step
decomposition to produce some OH- ions
KCN  K+(aq) + CN -(aq)
Complete
dissociation
CN - + H2O  HCN (aq) + OH- (aq)
Partial
dissociation
This is the poison; how would
You make lots of it?
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Water is a special
acid....er....base....umm....whatever.
H2O + H2O  OH– + H3O+
acid + base
 base + acid
One water molecule supplies a
proton, the other accepts.
Water falls apart – about one in every 10 million does.
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Don’t worry--water falling apart is rare!
About 1 in ten million fall apart.
H3O+ and OH- really want to
form water!
This is extremely important!
To make that HCN reaction from two
slides back go, add acid to convert any
OH- ions produced into water. The
system responds by producing more
HCN! Le Chatelier’s principle in action.
So that’s how gas chambers work.
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Chemists often deliver chemicals
using solutions.
Now we must deal with how to
make and characterize solutions.
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There is a right way to make Chocolate Milk.
add goo first! (SOLUTE)
then SOME milk (SOLVENT)
then MORE milk (MORE SOLVENT)
Here’s how it
looks when
chemists do it.
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Now we have to concentrate on concentration.
how much solute is in the solution, which also
contains solvent
• grams of solute / kilograms of solution
• grams of solute / Liters of solution
• moles of solute / (moles of water + moles of solute)
• moles of solute / kilograms of water
• moles of solute / Liters of solution
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M = Molarity =
Moles of Solute / Liters of Solution
Molarity is often written in brackets.
The following two lines say the same thing:
there are 0.3 moles of Barium ion per liter of
solution.
Use it in a sentence: 25
mL of 0.3 M BaCl2 will
+2
[Ba ] = 0.3
be added to 13.2 mL of
+2
0.3 M Ba
0.503 M HF.
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Example: yes, this sort of thing will be
on the final.
We have 0.05 Molar K2Cr2O7 as a “stock solution”
That’s written identically as 0.05 M K2Cr2O7 or
[K2Cr2O7] = 0.05.
We measure out 1.25 liters of the solution. How
many moles of K2Cr2O7 is this?
1.25 liter
0.05 moles K2Cr2O7
1 liter
= 0.0625 moles K2Cr2O7
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The number of something is
given by molarity times volume.
Moles = Molarity  Volume
n = MV
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What is the potassium (K)
concentration in that
0.05 M K2Cr2O7 solution?
What is the Chromium (Cr)
concentration in that 0.05 M
K2Cr2O7 solution?
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This example is harder.
What is the molar concentration (molarity) of F- if 5.2 g of MgF2 are dissolved
in water, the final volume being 500 mL?
You can assume that MgF2 falls apart (ionizes) totally:
MgF2

Mg+2 + 2F24.3 + 2 x 19.0 = 62.3 g/mol
Each MgF2 yields two fluoride ions, so the concentration is:
Moles of F- = (5.2 g MgF2)  (1 mol MgF2/ 62.3 g MgF2)
 (2 mole F-/1 mol MgF2) = 0.167 moles FVolume = 500 mL = 0.5 L
Molarity of F- = 0.167 moles F- / 0.5 L = 0.334 mole F- per liter.
Solution is 0.334 M in F- or we can say [F-] = 0.334
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Dilution: M1V1=M2V2
This equation says: molecules are not lost.
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What is the final concentration of
H2SO4 if 0.628 L of 18 M H2SO4
stock solution is diluted to a final
volume of 2.0 L?
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We need logarithms to talk about acid
concentrations (pH). p is a function.
Logs are intimately related to exponent math
Logs conveniently tame large numbers
annual U.S. budget = $1.6 x 1012
log (annual U.S. budget) = 12.2
To compute a log, hit log on calculator
log(6.02 x 1023) = 23.8
The opposite of log is 10x
1023.8 = 6.02 x 1023
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x
0
1
2
3
4
5
6
7
8
9
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Logarithms convert products to sums,
log10(x) quotients to subtractions.
-
0
.3
.5
.6
.7
.8
.85
.9
.95
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log10(a  b) = log10a + log10b
e.g., log10(200) = log102 + log10100  0.3+2 = 2.3
log10(a  b) = log10a - log10b
e.g., log10(0.8) = log108 - log1010  0.9 -1 = -0.1
Pretty easy to memorize!
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The "p" Function -- at last
Define p(X) = -log10(X).
Examples:
p(Annual national debt)  - log10(5 x 1012) = -12.7
p(Chance of winning lottery)  -log10(10-8) = 8
Things that are huge  fairly small negative number
Things that are small  fairly small positive number
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pH function -- at really long last
Define pH = -log10[H+]
where [H+] is given in moles proton per liter
pH lower than 7 = acidic
pH = 7 = neutral
pH greater than 7 = basic
Normal range: -1.3 to 15
Most people think: 0 to 14
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Example: what is pH of 6 M HCl?
Answer: since the HCl molecules completely
dissociate, [H+] = 6 moles/liter
pH = -log106 » -0.8.
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Combining pH and dilution:
Suppose we dilute 50 mL of 6 M
HCl to a new volume of 500 mL.
What is the new pH?
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If pH changes by +1, [H+] went down
by a factor of 10
If pH changes by -1, [H+] went up by
a factor of 10.
How does [H+] change if pH goes
from 5 to 3?
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Fun factoids about pH – Amaze your friends
The scale was invented by a beer maker
Comes from pouvoir hydrogene (“hydrogen power”)
pH values of common things
Orange juice: ~ 3
Tomatoes: ~ 4
Pure water: 7.0
Blood: 7.4
Sodium bicarbonate: ~ 8.2
Milk of magnesia: ~ 10
Clorox bleach: ~ 12.3
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pOH is related to pH
pOH = -log10[OH-] tells the concentration of hydroxide ion.
It turns out (for no particular reason) that the product of the
Concentrations of hydroxide and hydronium is about 10-14
[H3O+][OH-] = 10-14
If you know [H3O+] you also know [OH-] = 10-14/[H3O+]
The product rule for logarithms converts our relation to:
pH + pOH = 14
pOH = 14 - pH
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Proton Wars: Neutralization
often called titration
+
5 H3O+
=
4 OH -
1 H3O+ + 4 H2O
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Buffers
Naturally occurring weak acids and bases that "soak up" extra
acidity or alkalinity--like sponges.
The most important buffer is "the CO2 system"
1.
CO2 + H2O  H2CO3
2. H2CO3 + H2O  H3O+ + HCO33. HCO3- + H2O  H3O+ + CO3-2
Too much acid after big Italian lunch and soda? You’ll lose it as CO2.
Too much base after TUMS? You can use CO2 to soak it up.
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Stuff we forgot to say
about acids & bases
Oxides of nonmetals are generally acidic
Example: CO2 as we just saw
Oxides of metals tend to be basic
Example: CaO + H2O  Ca+2 + 2OH-
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Stop here?
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Today’s factoids exemplify the p function.
Population of China: 1,320,000,000
(20% of global population)
Population of USA: 300,000,000
Log10(China) = 9.1
Log10(USA) = 8.5
U.S.–awarded Ph.D.’s in science &
engineering: ~25,000/year