Equilibrium Reactions

An Equilibrium Reaction Goes In The Forward and
Reverse Directions (Reversible Reaction)
Reactants react to form products:
Products react to re-form reactants:
A + B ---> C + D
C + D ---> A + B
At Equilibrium, reactants and products will be present.
At Equilibrium, reactants and products will be present.
The ratio of reactants and products is quantified with an
Π [products]n
___________
Equilibrium Constant: Keq =
Π [reactants]m
Example: aA + bB ⇔cC + dD
c [D]d
[C]
Keq = ___________
[A]a [B]b
More reactants means Keq is:
(i) > 1
(ii) = 1
(iii) < 1
More products means Keq is:
(i) > 1
(ii) = 1
(iii) < 1
Example: Acetic acid dissociates into its ions.
CH3COOH + H2O --> CH3COO- + H3O+
Keq = 1.8x10-5
a. Which picture represents this mixture at equilibrium?
A
B
C
= CH3COOH
= H 2O
= CH3COO-
= H3O+
b. At equilibrium, are more reactants present or more products?
c. Draw a concentration vs. time graph.
Objective: Use LeChatelier s Principle To Control The
Direction (Forward/Backward) Of A Reaction
LeChatelier s Principle: When a stress is placed on a reaction,
the reaction shifts in a direction that relieves the stress (and reestablishes equilibrium).
Stress
Stress Relief
Upset stomach
Rolaids
Add reactant
Rxn shifts toward products
Remove reactant
Rxn shifts toward reactants
Add product
Rxn shifts toward reactants
Remove product
Rxn shifts toward products
One minute left on the exam and two
pages to go
panic
Add/remove heat
Increase/decrease pressure
Someone looking over your shoulder
Anxiety
Aspirin Synthesis (Chem 1A Lab 4 ) is an Equilibrium Reaction.
Salicylic acid + acetic anhydride -- H3PO4 catalyst --> aspirin + acetic acid
C7H6O3 + C4H6O3 ----> C9H8O4 + CH3COOH + heat
Reaction conditions affect Reaction Rate and Yield:
T = 75oC, time = 20 minutes
Use excess C4H6O3 --> reaction shifts to products --> higher
yield. Reaction occurs:
(i) faster
(ii) slower
Use higher T --> reaction shifts to reactants --> lower yield.
Reaction occurs:
(i) faster
(ii) slower
Use catalyst --> no effect on equilibrium or yield
Reaction occurs:
(i) faster
(ii) slower
You made Esters in Lab 1 in Chem 1B lab.
Acid + Alcohol ---> Ester + Water
a. Is the reaction exothermic or endothermic?
b. What is the effect of temperature in this reaction?
c. Should an excess of one reactant be used? If so, which
reactant should be the excess reactant?
d. How can the yield be increased?
e. What does the sulfuric acid catalyst do?
Lab 4: How can you get more salt to dissolve in water?
NaCl dissolution is endothermic.
Write a chemical equation that represents this reaction.
What is the effect of T?
What is the effect of adding more water?
Apply LeChatelier s Principle to Make Drano Work Better
Drano drain cleaner consists of Al filings and NaOH pellets.
When water is added, the heat produced melts and saponifies
the fat that clogs a drain.
2 Al (s) + 2 NaOH (s) + 6 H2O (l) --> 2 NaAl(OH)4 (aq) + 3 H2 (g)
a.  Would you expect this reaction to have a large K or small K?
Explain.
b. In terms of reactants and products, explain how this reaction can
be shifted to the right (toward products).
c. If an excess amount of water is added, how will the reaction be
affected?
d. If hot water was added instead of cold water, would Drano work
better? In other words, which direction would the reaction shift?
e. If the drain was plugged immediately after Drano was added to
the clog, would Drano work better? In other words, which direction
would the reaction shift?
Determine Reaction Conditions by Applying LC Principle
Under the conditions of a car engine, nitrogen and oxygen
reacts to form NOx, which is a component of smog:
N2+ O2 --> 2 NO
(1)
N2+ 2 O2 --> 2 NO2
(2)
Under what temperature (low T or high T) and pressure (low P
or high P) conditions does each reaction occur? Give reasons
based on LeChatelier s principle.
ΔHf of NO (g) = 90.4 kJ/mole ==> Hess s law ==> Rxn (1) is endothermic
Increase in T shifts reaction toward products ==> Use High T
ΔHf of NO2 (g) = 33.9 kJ/mole ==> Hess s law ==> Rxn (2) is endothermic
Increase in T shifts reaction toward products ==> Use High T
Rxn (1): 2 moles of gas reactants --> 2 moles of gas products
Changing P does not shift reaction ==> Use Any P
Rxn (2): 3 moles of gas reactants --> 2 moles of gas products
Increase in P shifts reaction toward products ==> Use High P
500 million tons of Nitrogen Fertilizer is Produced Annually
(as NH3, NH4NO3, and urea) and Uses 1-2% of World s Energy Supply
Plants Make NH3 from N2 (Nitrogen Fixation)
The Haber process is the industrial process for the synthesis
of ammonia from N2 and H2. In this process, the reaction
conditions are high pressure and low temperature. A catalyst is
used and product is removed from the reaction vessel during
the reaction.
a. Write a chemical equation that represents the Haber
process.
b. Explain why these reaction conditions are used. If a catalyst
was not used, could a higher reaction temperature be used to
optimize the yield? Give reasons.
Does Salt Melt Ice?
ΔHsolution of NaCl is close to 0. So q ≈ 0. Melting?
Melting: H2O (s) <==> H2O (l)
As soon as some salt dissolves in the ice's surface film, the
solute ions interfere with the H2O (l) molecules' (leftward)
ability to join the solid lattice, and the equilibrium therefore
shifts to the right: toward less solid and more solution. That is,
the ice dissolves in the solution.
Pardon my vernacular, but that ain't melting, and it ain't no
freezing point depression phenomenon, neither.
Yes, the solution's freezing point is lower than that of pure
water, but so what? Our practical concern here is with ice
dissolving into the solution, not the solution freezing into ice.
CEN, 3/31/08, Newscripts, p. 56
Objective: Determine the amounts of reactants and products
at Equilibrium
Haber process: Chang, 6th ed., p. 539, Problem 15.72.
N2 (g) + 3 H2 (g) <==> 2 NH3 (g)
Kp = 4.31 x 10-4 at 375oC
In a certain experiment, a student starts with 0.862 atm of N2 and
0.373 atm of H2 in a constant volume vessel at 375oC. Calculate the
partial pressures of all species when equilibrium is reached.
Solution:
Initial p
Amount reacts
Amount at equilibrium
Kp = 4.31 x 10-4 =
Solve for x!
N2 (g) + 3 H2 (g) <==> 2 NH3 (g)
0.862
0.373
0
x
3x
2x
0.862-x 0.373-3x
2x
(2x)2
____________
(0.862-x)(0.373-3x)3
Objective: Determine the amounts of reactants and products
at Equilibrium
1 mole each of H2 and I2 are placed in a 1 l box at 490oC.
What are the concentrations of reactants and products at
equilibrium?
H2 + I2 <==> 2 HI
K = 45.9 at 490oC
Method: balance chemical equation
set up initial amount ,
amount reacts ,
amount at equilibrium
plug equilibrium amount into K
solve for x
Water behaves like an Acid or a Base
Water dissociates into H+ and OH-: (equilibrium reaction)
H 2O +
H2O<===> H3O+ + OH-
acid
base
SHORTCUT:
H2O <===> H+ (aq) + OH-
Kw = [H+] [OH-] = 1 x 10-14
H+ (aq) = H3O+
Why does water have a pH of 7? (do an equilibrium calculation)
Only 1 H2O molecule in 10 million dissociates into H+ and OH-!
Why is pH = 7 considered neutral? (compare [H+] to [OH-])
Water behaves like an Acid or a Base
Water dissociates into H+ and OH-: (equilibrium reaction)
H2O <===> H+ + OH-
Kw = [H+] [OH-] = 1 x 10-14
Why does water have a pH of 7? (do an equilibrium calculation)
[H+] = 1 x 10-7 " pH = -log [H+]
Why is pH = 7 considered neutral? (compare [H+] to [OH-])
[H+] = 1 x 10-7 = [OH-]
What is the pOH of water?
pOH = -log [OH-]
pH + pOH = 14
Every Acid has a Partner (conjugate) Base (and vice versa).
HA + H2O <===> H3O+
acid
base
gives up
its H+
conjugate acid
can give up
its H+
SHORTCUT:
HA <===> H+ + Aacid
gives up
its H+
+
conjugate base
can accept a H+
conjugate base
can accept a H+
Ka = (See Chang,
Table 16.3, 4)
What is the conjugate base of sulfuric acid?
What is the conjugate acid of NaOH?
A-
Acids can be Strong or Weak
Strong acids have large Ka. (easily donates H+, many H+ in soln)
Weak acids have Ka < 1. (wants to keep its H+, low [H+])
Which picture best represents HCl?
H+ Cl- H+
H+
ClCl-
Cl-
A
Cl-
H+
HCl
HCl
Cl-
H+
H+
HCl
HCl
B
Acids can be Strong or Weak
Strong acids have large Ka. (easily donates H+, many H+ in soln)
Weak acids have Ka < 1. (wants to keep its H+, low [H+])
Which picture best represents CH3COOH (HA)?
H+ A- H+
H+
AA-
A-
C
A-
H+
HA
HA
A-
H+
H+
HA
HA
D
Acids can be Strong or Weak
Strong acids have large Ka.
(easily donates H+, many H+
in soln)
H+ Cl- H+
ClH+
ClCl
A
Cl-
Weak acids have Ka < 1.
(wants to keep its H+, low [H+])
H+
HA
HA
A-
H+
H+
HA
HA
D
Which statement is true?
(i) 0.1 M HCl has a higher [H+] than 0.1 M acetic acid.
(ii) 0.1 M HCl has a higher pH than 0.1 M acetic acid.
Objective: Calculate the pH of a weak acid
0.1 M HCl (strong acid) has a pH of 1. Why?
However, 0.1 M acetic acid (weak acid) has a pH greater than
1. Calculate the pH of 0.1 M acetic acid.
CH3COOH <==> H+ + CH3COOAnswer?
(a) pH = 1
(b) pH = 2.9
(iii) pH = 7
Ka = 1.8 x 10-5
Bases undergo hydrolysis
A- + H2O <===> HA + OH-
Kb = Kw/Ka
Bases can be strong or weak.
Which statement is true?
(i) Weak bases accept H+ more easily than strong bases.
(ii) Weak bases have a lower Kb than strong bases.
(iii) NaOH is a weaker base than NaC2H3O2.
(iv) 0.1 M NaOH has the same pH as 0.1 M NaC2H3O2.
(v) 0.1 M NaOH has a lower [OH-] than 0.1 M NaC2H3O2.
Objective: Calculate the pH of a weak base
What is the pH of a 0.1 M NaCH3COO solution?
To calculate the pH of a weak base, e.g., NaC2H3O2:
write base hydrolysis reaction
calculate Kb. Look up Ka of acid.
do an equilibrium calculation to calculate [OH-]
calculate pOH (= - log [OH-]
calculate pH (using pH + pOH = 14)
http://www.tums.com/products.html
1. Decomposition of CaCO3:
CaCO3 <==> CaO + CO2.
At 1000oC, K = 9.75
At 848oC, K = 1.
CaCO3 is the active ingredient in
Tums and Rolaids. Is it safe to
store Tums at room temperature?
Calculate K at room temperature to
confirm your answer.
2. Sulfuric acid is the acid found
in car batteries. What are the
major sulfur containing
substance(s) in this solution?
http://www.propertiesofmatter.si.edu/carbattery.html
Chemical reactions produce the COLOR and SMELL of
Grilled Meat, Roasted Coffee, Dark Beer, Toasted Bread
http://www.starkinsider.com/2009/07/steakmarinade-recipe-tender-juicy-bb.html
http://www.suite101.com/
view_image.cfm/1464543
http://www.shutterstock.com/
pic-9044662/stock-photo-sideview-of-a-blank-white-plate-with-ainch-caramelized-sugar-cageused-as-an-edible-prop-foran.html
4 types of browning reactions in foods:
Maillard: sugar + amino cpd (protein) ---> aroma, flavor, color
Caramelization: sugar + heat ---> caramel flavor, color
ascorbic acid oxidation: Vitamin C
enzymatic browning (Lab 5): phenolics --- enzyme --> color, flavor
Enzyme is a protein --> amino acid --> acid/base
http://cen.acs.org/articles/90/i11/Deciphering-Caramels-Deliciousness.html
3/12/12, CEN, p. 52 Caramel Composition determined
http://italiandish.squarespace.com/
imported-20090913150324/2008/11/29/
caramel-cake-daring-bakers-novemberchallenge.html
http://www.thekitchn.com/good-questionhow-do-i-make-ch-45160
glucose, fructose, and saccharose ---heat---> caramel
Caramel consists of several thousand compounds.
These include oligomers with up to 12 carbohydrate units formed through
unselective glycosidic bonding, dehydration products of oligomers that
can lose up to eight water molecules, hydroxyfurfural derivatives, and
colored aromatic products.
Enzymatic Browning occurs
when fruit tissue is cut or peeled.
Polyphenol Oxidase (PPO) enzyme
is released, which oxidizes Phenolic
compounds.
Fruit turns Brown.
Chlorogenic acid, and other phenols are converted into
melanin precursors with polyphenol oxidase (PPO), .
Fruit can be prevented from browning with
preservatives or buffers
a. What is the pH of an apple? Banana?
http://www.engineeringtoolbox.com/food-ph-d_403.html
b. At what pH is the fruit browning enzyme active?
http://www.worthington-biochem.com/TY/default.html
Polyphenol oxidase in mushroom
c. If the fruit browning enzyme is active at the same pH as the
apple, what color will the apple be?
d. Do fruits/vegetables that contain Vitamin C turn brown? Give
reasons. (see https://appliedphlogiston.wordpress.com/2010/07/06/the-science-of-cooking-why-guacamole-turns-brown/ )
References:
http://www.landfood.ubc.ca/courses/fnh/301/brown/brown_prin.htm
http://www.food-info.net/uk/colour/enzymaticbrowning.htm
http://www.5min.com/Video/Learn-about-the-Maillard-Reaction-83227082
http://www.cfs.purdue.edu/fn/fn453/ld_amino.html
http://www.math.unl.edu/~jump/Center1/BioChemLabs.html
http://www.exploratorium.edu/cooking/meat/INT-what-makes-flavor.html
http://class.fst.ohio-state.edu/fst605/605p/Maillard.pdf
http://cen.acs.org/articles/91/i14/Engineered-Apples-Near-Approval.html
4/8/13, CEN, p. 31 Engineered Apples Near Approval
Genetically engineered apples
won t turn brown. Okanagan
Specialty Fruits (British Columbia)
triggers a selective gene-silencing
pathway and inserts a selection
gene that is broadly recognized as
harmless to humans.
Agrobacterium tumefaciens injects this
plasmid, modified by Okanagan
scientists, into infected cells. Only the
genes between the border sequences of
this plasmid are incorporated into the
apple genome. The rest of the plasmid
contains genes used by the bacterium
(yellow).
in the U.S., 88% of corn, 93% of soybeans, and 94% of cotton is genetically
engineered (Source: U.S. Department of Agriculture, 2012)
Buffers are used in food preservatives, blood, …
A Buffer Resists Change in pH
A Buffer contains a Weak Acid and its Conjugate Base
(or a weak base and its conjugate acid).
H+
HA
HA
A-
HA
HA
Why doesn’t a buffer contain a strong acid?
Remember the Titration!
http://analytical.wikia.com/wiki/Burette!
http://www.monsterprelaunch.com/surf2b/1
Acids React With Bases!
Acid + Base !
A Titration Curve Tells us about a Buffer
endpoint
pH
Buffer region
1/2 way point
Volume of base added
-]
[A
_______
Henderson-Hasselback equation: pH = pKa + log
[HA]
At starting point:
At 1/2 way point:
At end point:
only HA present ==> use Ka
[A-] = [HA] ==> pH = pKa
only A- present ==> use Kb
Use Henderson-Hasselbach equation to make a Buffer
1.  Find acid with pKa near pH of buffer you want to make.
2. ID conjugate base of acid.
3. Use H-H equation to calculate [base]/[acid] ratio.
4. Calculate % acid and % base to use.
pH
endpoint
Buffer region
1/2 way point
Volume of base added
Before 1/2 way point:
(i) [A-] < [HA] (ii) [A-] = [HA]
After 1/2 way point:
(i) [A-] < [HA] (ii) [A-] = [HA]
(iii) [A-] > [HA]
(iii) [A-] > [HA]
On the Titration Curve, show where the buffer capacity is exceeded.
Objective: draw titration curve and determine pHs
A Titration Curve Gives Us A Lot Of Information About
Acids, Bases, Buffers, and Charge
20 ml of 0.1 M aspirin (monoprotic acid) solution is titrated with
0.1 M NaOH.
Calculate the pH at the start, half-way point, and end point.
Draw a titration curve.
At what pH can aspirin be used as a buffer?
At starting point:
At 1/2 way point:
At end point:
only HA present ==> use Ka
[A-] = [HA] ==> pH = pKa
only A- present ==> use Kb
Lab Report
Objective: Communicate What You Did and What It Means
Table 1. (Descriptive title)
Include your data and results.
Apple time to Banana time
turn brown,
to turn brown,
min
min
Apple
reaction
ranking
Banana
reaction
ranking
Control
pH x
pH y
Vitamin C
NaCl
Discussion: Describe your Data. Convert Data to Results.
Explain (interpret) what your Results mean.
With ______, the time for the apple to turn brown was ___ min
compared to ____ min for the control. ∴ The effect of _____ on
the fruit browning reaction is ____.
Fruit Fresh Keeps Fruit from Turning Brown
“Protect from browning up to 6 hours. All natural antioxidant.”
Ingredients:
Dextrose,
Ascorbic Acid (Vitamin C),
Citric Acid,
Silicon Dioxide (Anti-caking).
Contains No Sulfites.
http://www.amazon.com/Jarden-24100-Fruit-Fresh-Produce-Protector/dp/
B000KOSP6M/ref=pd_sbs_k_2/176-2250616-3954955?
ie=UTF8&refRID=1XF2KHQ8K5ED71QC141G
How Else to Keep your Fruit Fresh?
http://en.wikipedia.org/wiki/Lemon
http://www.drvita.com/
product/eat-cleaner-fruitamp-vegetable-wash-8-oz/
9331
Ingredients:
citric acid
Sodium citrate
Calcium ascorbate
Sea salt
glycerin
Milk Does a Body Good
87.3% water
3.9 % fat
8.8% solids-not-fat
protein 3.25% (3/4 casein)
lactose 4.6%
minerals 0.65% - Ca, P, Mg, K, Na, Zn, Cl, Fe,
Cu, .
vitamins - A, C, D, thiamine, riboflavin, others
http://www.npr.org/blogs/money/
2012/12/27/168147765/understanding-themilk-cliff
Milk pH = 6.6
What substance in milk makes it an acid?
Is milk a buffer?
Protein precipitates at pH 4.6
How to make cheese from milk?
http://blog.fooducate.com/
2011/06/26/if-milk-iswhite-why-is-cheeseyellow/
Coagulation and precipitation
Concentration of curd
Ripening
http://pubs.acs.org/isubscribe/journals/cen/82/i42/html/8242sci1.html CEN, 10/18/04, p. 47)
CHEESE WHIZZES: Food scientists plumb the depths of this ancient food,
which overflows with chemistry
≈ 10 lb of milk to make 1 lb
of cheese
≈ 400 flavor compounds in
cheddar (from metabolism of
bacteria, mold, and yeast, as well
as from chemical reactions
involving other ingredients as
cheese develops and ages)
http://pubs.acs.org/isubscribe/journals/cen/82/i42/html/8242sci1.html CEN, 10/18/04, p. 47)
CHEESE WHIZZES: Food scientists plumb the depths of this ancient food,
which overflows with chemistry
≈ 10 lb of milk to make 1 lb of cheese
Method:
1. Starter culture of bacteria:
- converts lactose to lactic acid, citric acid, (and other metabolic
products)
-  reduces pH from 6.7 to 5.3
2. Rennet added to break down casein and coagulate curds.
3. Heat, stir, drain whey, salt and season, curing.
Medium cheddar: 60 days
XSharp cheddar: 15 months
http://cen.acs.org/articles/90/i52/Shooting-Fish-Barrel-Extreme-Cheese.html
12/24/12, CEN, p. 48 Extreme Cheese Making
Bacteria, e.g., Lactobacillus, is used to make typical cheese.
Bacteria ferments the lactose --> lactic acid --> curds (protein)
Researchers swabbed their armpits, hands, noses, and feet to
collect starter bacteria cultures for cheese fermentation.
Armpit cheese is especially pleasant smelling and tastes like a
fresh farmer s cheese.
Avoid the foot cheese.
A Titration Curve Gives Us A Lot Of Information
About Acids, Bases, Buffers, and Charge
20 ml of 0.1 M H2C2O4 (diprotic acid) is titrated with 0.1 M
NaOH.
Calculate pH at start, 1/2 way points, and end points.
At 1st endpoint: pH = 0.5 (pKa1 + pKa2)
Draw titration curve and show buffer region(s).
Show the carbon compound and charge at each point.
At what pH is oxalate present?
Spinach contains oxalate
Oxalate ion + Ca2+ --> kidney stones
http://www.apartmenttherapy.com/-2910
Objective: draw titration curve and determine pHs
A Titration Curve Gives Us A Lot Of Information
About Acids, Bases, Buffers, and Charge
20 ml of 0.1 M H3PO4 (triprotic acid) is titrated with 0.1 M
NaOH.
Calculate pH at start, 1/2 way points, and end points.
At 1st and 2nd endpoints: pH = 0.5 (pKai + pKa(i+1))
Draw titration curve and show buffer region(s).
Show the phosphorus compound and charge at each point.
Glycine is the simplest amino acid.
a.  Circle the H s that are acidic in glycine. Which H is the
strongest acid? Give reasons.
b.  Draw a titration curve of glycine. Label the pH at the starting
point, each half way point, and each end point. Assume a 0.1
M solution of glycine is titrated with 0.1 M base.
c. What is the isoelectric point of glycine? Show the pI on your
titration curve.
d. Could glycine be used for a pH 5 buffer? Give reasons.
Bring MSG to Lab on Thursday (MSG = Accent)
What s That Stuff? MSG – monosodium glutamate. MSG is a
flavor enhancer made from glutamic acid (amino acid).!
a.  Circle the H s that are acidic in glutamic acid. Which H is
the strongest acid? Give reasons.
b.  Draw a titration curve of glutamic acid. Label the pH at
each end and half way point. Assume a 0.1 M solution.
c. At what pH is MSG prepared? Draw the structure of MSG
at this pH.
Post-Spring Break Review
1.  PABA is an active ingredient in sunscreen. It ____ in water.
a. washes off
b. does not wash off
c. is a buffer
2. The b.p. of coffee is _____ than the b.p. of water.
a. higher
b. lower
c. same
3. Ethanol ______ to acetic acid.
a. donates a H+
b. oxidized
c. reduced
4. 0.9 M acetic acid is a ____ acid with a ____ Ka, _____ pKa,
and has a pH of _____.
a. strong/large/high/1
b. strong/large/low/0.05
c. weak/large/low/2.9
d. weak/small/high/2.4
5. Acetic acid is titrated with NaOH. At pH 5, there is ___ acid
than base.
a. more
b. less
c. same
pH
start
Use Ka1
1st 1/2
way pt
pKa1
1st end
point
0.5(pKa1+pKa2)
2nd 1/2
way pt
pKa2
2nd end 0.5(pKa2+pKa3)
point
http://chemwiki.ucdavis.edu/Analytical_Chemistry/
Quantitative_Analysis/Titration/
Titration_Of_A_Weak_Polyprotic_Acid
3rd 1/2
way pt
pKa3
3rd end
point
Use Kb3
Weak Acids and Bases Are Found in Biology!
Electrophoresis is a lab technique used to separate and purify
substances based on charge. This technique is used to
separate and purify amino acids and proteins.
Glutamic acid and glycine are amino acids.
a.  Draw the structure of each amino acid.
b.  How many protons are donated by each amino acid?
Amino acid pKa values:
http://homepage.smc.edu/kline_peggy/Organic/Amino_Acid_pKa/Amino_Acid_pKa.htm
c. At low pH, what is the charge?
d. At what pH does the AA have a charge = 0 (isoelectric
point)?
e. How would you use electrophoresis to separate a mixture of
these two amino acids?
What pH would you use to separate a glutamic acid and
glycine mixture using electrophoresis?
Lab Objective: Determine the pKa s and pI (isoelectric point)
of Milk
Milk pH = 6.6
protein = calcium caseinate
What is the charge on the
caseinate ion at pH 6.6?
Protein precipitates at pH 4.6
protein = casein
What is the charge on casein at
pH 4.6?
http://www.milkfacts.info/Milk
%20Composition/Protein.htm
How would you use the titration curve of milk to find pI?
Application: Baking Soda Buffers Our Blood
One Blood Buffer Consists of HCO3-/CO2
A buffer is effective only in the region of its pKa. Blood has a normal
pH of 7.35-7.45 and contains two major buffer systems. It is
important that the pH of blood remains relatively constant because at
pH below 6.8 or greater than 8.0, cells cannot function properly and
death may result.
The HCO3-/CO2 (aq) blood buffer in vivo is an open system in which
the concentration of dissolved CO2 is maintained constant. Any
excess CO2 produced by the reaction
H+ + HCO3- ---> H2O + CO2
is expelled by the lungs. Note that a typical laboratory buffer is a
closed system. The concentration of conjugate acid increases when
H+ reacts with the conjugate base.
In the HCO3-/CO2 blood buffer, what is the acid?
What is the base?
Baking Soda Buffers our Blood: The HCO3-/CO2 Blood Buffer
a. Calculate the Keq and pK of Reaction (4) from the following
reactions and K values.
CO2 (g) <==> CO2 (aq)
K1 = 3 x 10-5 at 37oC.
CO2 (aq) + H2O (l) <==> H2CO3 (aq)
K2 = 5 x 10-3 at 37oC
H2CO3 (aq) <==> H+ (aq) + HCO3- (aq)
pKa = 3.8 at 37oC
CO2 (aq) + H2O (l) <==> H+ (aq) + HCO3- (aq)
K4 = ?
(i) K4 = K1 + K2 + Ka = 5.21 x 10-3
(ii) K4 = K1 K2 Ka = 2.38 x 10-11
(iii) K4 = K2 + Ka = 5.16 x 10-3
(iv) K4 = K2 Ka = 7.92 x 10-7
b. The [HCO3-] = 0.024 M in blood at pH 7.4. Calculate the [CO2 (aq)]
in blood at this pH.
(i) Use K1 for Rxn (1); [CO2 (aq)] = 5.21 x 10-3 M
(ii) Use H-H equation for Rxn (2); [CO2 (aq)] = 5.16 x 10-3 M
(iii) Use H-H equation for Rxn (4); [CO2 (aq)] = 1.2 x 10-3 M
Baking Soda Buffers our Blood: The HCO3-/CO2 Blood Buffer
c. 0.01 M H+ is added to blood. Calculate the pH of blood under
conditions such that the increased [CO2 (aq)] can not be released as
CO2 (g). In other words, assume that the blood buffer is a closed
system. (Hint: do stoichiometry calculation, then use H-H equation)
d. 0.01 M H+ is added to blood. Calculate the pH of blood under
conditions such that the increased [CO2 (aq)] can be released as CO2
(g). In other words, assume that the blood buffer is an open system.
Remember that the [CO2 (aq)] remains constant in this open buffer
system.
e. Your pH calculations should show a large decrease in pH in part c
and a small decrease in pH in part d. Based on these calculations, it
would appear that HCO3- should be quickly depleted in an open
system when acid is added. How is HCO3- replenished in blood?
Hemoglobin Gives Blood its Red Color and is a Blood Buffer
The diagram below represents a simplified version of the buffering action of
hemoglobin as a buffer and the uptake and release of oxygen (Reference: I.H. Segel,
Biochemical Calculations , 2nd ed., Wiley, 1976, p. 88). Hemoglobin is the oxygen carrier in
blood that transports oxygen from our lungs to tissues. Diffusion due to partial
pressure differences is one mechanism by which oxygen transport occurs.
H HgbH Hgb O2HgbHgb O2+ O2+ O2+++H+foodH+ + HCO3-H2O + CO2H2O + CO2Replenishes HCO3-K02 = 1K02 = 0.
KO2 = 1
H Hgb
H Hgb O2
+ O2
pKa = 7.7
pKa' = 6.2
K'O2 = 0.032
Hgb
+ O2
+
+
H+
Hgb O 2
+
H + + HCO3-
food
CO2 + H2O
replenishes HCO3-
H 2O +
CO2 (g)
Hemoglobin is a Blood Buffer
a. Does the conjugate acid of hemoglobin (H Hgb) have a
higher, lower, or same affinity for oxygen than its conjugate
base (Hgb)? Give reasons.
b. In the lungs, is the conjugate acid or conjugate base of
hemoglobin formed? Give reasons.
c. In the tissues, is the conjugate acid or conjugate base of
hemoglobin formed? Give reasons.
d. When O2 is released, which equilibrium reaction is affected?
In which direction does this reaction shift?
e. How does eating (metabolic pathway) replenish HCO3- in
blood? Give reasons. What other effect does eating have on
the buffering action of hemoglobin?
Quiz 2
Our blood pH is around 7.4. CO2 (aq) and HCO3- (aq) act as
buffers.
CO2 (aq) + H2O (l)
HCO3- (aq)
H+ (aq) + HCO3- (aq)
H+ (aq) + CO32- (aq)
K1 = 7.92 x 10-7
pK2 = 10.3
1. 20 ml of 0.1 M CO2 (aq) is titrated with 0.1 M NaOH. Draw the
titration curve. Label the axes. Show the pH at the starting point,
each half-way point, and each end point. Show your pH
calculations for full credit.
2. At pH 7.4, what carbon-containing substance is present in the
highest amount?
3. Carbonate (CO32-) binds to Ca2+, which could cause muscles
to spasm. Does CaCO3 form at blood pH? Give reasons.
4. Acidosis causes an increase in [H+]. Why does the CO2/HCO3buffer keep the blood pH relatively constant as an open system?
5. Acids contain hydrogen. Why is CO2 considered an acid?
Eggshells are composed mostly of calcium carbonate
(CaCO3) formed by the reaction
Ca2+ (aq) + CO32- (aq) <==> CaCO3 (s)
The carbonate ions are supplied by CO2 produced as a
result of metabolism. Explain why eggshells are thinner in
the summer, when the rate of chicken panting is greater.
Suggest a remedy for this situation. (Chang, 6th ed., p. 539,
Problem 15.71)
If you have an upset stomach, should you chew on egg
shells or drink milk?
Chem 1A: Some Ionic Compounds are Soluble in water.
Other Ionic Compounds are Insoluble in Water.
See Solubility Rules for ionic compounds (Table 4.2, p. 95).
What chemical force determines solubility of ionic compounds?
However, Insoluble Compounds Are Very Slightly Soluble
in Water
Solubility Equilibria of Ionic Compounds:
AB (s) <===> A+ (aq) + B- (aq)
Ksp = [A+ (aq)][B- (aq)]
See Ksp Table in textbook.
Compound
Ksp
MgCO3
4.0 x 10-5
CaCO3
3.8 x 10-9
Mg(OH)2
1.5 x 10-11
Which of the three compounds is the least soluble? Why?
Objective: How to dissolve an insoluble solid
Insoluble Carbonates and Hydroxides Are Soluble In Acid
a. Most carbonate salts are insoluble in water. What substance
would you use to dissolve calcium carbonate? Write a chemical
equation that represents the solubility of CaCO3. Explain using
LeChatelier s principle.
b. Most hydroxide salts are insoluble in water. What substance
would you use to dissolve magnesium hydroxide? Write a
chemical equation that represents the solubility of Mg(OH)2.
Explain using LeChatelier s principle.
Objective: How to dissolve an insoluble solid
Insoluble Carbonates and Hydroxides Are Soluble In Acid
1 mole of Mg(OH)2 is dumped in 1 l of water. Calculate the
[Mg2+] in water. Calculate the pH of solution.
Calculate K for the following reaction. Would you expect K to be
big or small?
Mg(OH)2 + HCl -->
Salinas Has Hard Water, i.e., Our Water Has a
Lot of Ca2+ and Mg2+ In It
Hard water causes:
•  lime stains on utensils
and fixtures,
•  requires more soap for
cleaning,
•  and leaves clothes a
dingy white color after
washing.
How does water get hard?
http://www.rayneoffullerton.com/articles.php
Salinas Has Hard Water, i.e., Our Water Has a
Lot of Ca2+ and Mg2+ In It
b. 1 mole of calcium carbonate is dumped in 1 l of water. Calculate the
concentration of Ca2+ ion in this solution. (Hint: use the solubility
product constant for this reaction.)
c. You know that calcium carbonate dissolves in HCl. Write a balanced
chemical equation that represents this reaction. Calculate the
equilibrium constant for this reaction. (Hint: use Ksp and Ka of
carbonic acid and add a few equations together.) Calculate the mass
of calcium carbonate that dissolves in 1 l of pH 4 HCl.
d. Does calcium carbonate dissolve in acetic acid? Write a balanced
chemical equation that represents this reaction. Calculate the
equilibrium constant for this reaction. (Hint: use Ksp and Ka of acetic
acid and add a few equations together.) Calculate the mass of calcium
carbonate that dissolves in 1 l of pH 4 acetic acid.
Objective: How to dissolve an insoluble solid
Some Compounds Can Be Dissolved By Forming Complex Ions
From Lab 5: Al(OH)3 solid can be dissolved by adding acid or
base.
Al3+ (aq) + 3 OH- (aq) ! Al(OH)3 (s)
K = 1/Ksp
Al(OH)3 (s) + OH- (aq) ! Al(OH)4- (aq)
K
Al3+ (aq) + 4 OH- (aq) ! Al(OH)4- (aq)
Kf
Objective: How to dissolve an insoluble solid
Some Compounds Can Be Dissolved By Forming Complex Ions
In Gold mining, Gold (and
silver) ore are processed
by forming a complex
metal cyanide ion:
http://www.greenkarat.com/education/goldlabels/gold-mining.asp
4 Au + 8 NaCN + O2 + 2 H2O --> 4 Na[Au(CN) 2] + 4 NaOH
(Reference: http://en.wikipedia.org/wiki/Gold_cyanidation)
Au+ (aq) + 2 CN- (aq) ! Au(CN)2- (aq)
Ag+ (aq) + 2 CN- (aq) ! Ag(CN)2- (aq)
Kf
Kf
a. Does gold exist on the Earth s crust as a metal or ion?
b. Does silver exist on the Earth s crust as a metal or ion?
c. What oxidizes Au to Au+?
http://cen.acs.org/articles/90/i42/Mining-Microbes.html
10/15/12, CEN, p. 34 Mining with Microbes
Harnessing microbes to do mining work is called biomining,
or sometimes bioextraction or bioleaching. The strategy has
been most extensively studied for copper and gold: Colorado-based
mining consultant Corale L. Brierley estimates that 10 to 15% of copper
and 5% of gold worldwide are currently being harvested through
biomining. Mining takes 3-5% of energy produced globally.
The Escondida
copper mine in
Chile hosts a
biomining
operation.
Green acidloving algae
feast on copper
sulfide in black
rock.
Chemistry of Swimming Pools
Chlorine is added to swimming pools to sanitize and disinfect
the water from microorganisms.
Chlorine gas, calcium hypochlorite (Ca(OCl)2, and sodium hypochlorite
(NaOCl) are common forms of chlorine that are used in pools (Reference: B.
Selinger, Chemistry in the Marketplace , 4th ed., Harcourt, 1989, p. 188). For
example, when chlorine is bubbled through water, two acids are formed:
Cl2 + H2O -----> HOCl + HCl
(1).
When bleach (NaOCl) is added to water, several reactions occur:
NaOCl + H2O -----> Na+ + OCl- + H2O
(2)
OCl- + H2O -----> ____________ + _____________
(3)
HOCl -----> ____________ + _____________
(4).
In each case, hypochlorous acid (HOCl) and hypochlorite ions (OCl-) are
formed. HOCl is more effective in killing bacteria than the hypochlorite ion.
a. How does chlorine work in killing bacteria? Why is HOCl more effective
in killing bacteria than the hypochlorite ion?
b. Complete Reactions 3 and 4. Look up or calculate the equilibrium
constant for each reaction.
More Pool Chemistry
c. According to PoolCenter.com (http://www.poolcenter.com/chlor.htm), The
efficacy of chlorine, that is, the power of it to have an effect, is greatly
influenced by the care with which you manage your pH levels. As the pH of
your pool increases, the killing power of your chlorine decreases. At a pH of
6.0, we'll get 96% or so of the potential out of each lb of chlorine, but at what
cost? Such a low pH would wreck havoc on all of the surfaces the water
comes in contact with, including swimmers. It's just too corrosive. Move the
pH up to 7.0 and the efficacy of the chlorine drops to 73%, but raise it up to
8.0, where many a pool seems to drift to, and it drops dramatically...down to
21%! At a perfect pH level of 7.5, we can expect to have about 50% of our
chlorine in the molecular structure of hypochlorous acid, the active, killing
form. The remaining half is in the form of a hypochlorite ion, which is also an
active form of chlorine, but very weak and slow to kill.
Give one reason that the perfect pH for a swimming pool is 7.5. Based on
the information given in this paragraph, calculate the equilibrium constant for
Reaction 4.
HOCl -----> ____________ + _____________
(4).
Pool Chemistry for Spring Break!
d. Chlorine reacts with ammonia and ammonia-like compounds that are
formed from organic waste to form chloramines. Chloramines do not sanitize;
however, they block free chlorine molecules from oxidizing bacteria and
result in the chlorine smell. In addition, hypochlorite ion is destroyed by UV
light from the sun to make chloride ion and O2. Cyanuric acid is added to
pools as a stabilizer to prevent the loss of chlorine.
Write a chemical equation(s) that describes the formation of chloramines.
Use reaction rate concepts to explain how chloramines block chlorine from
oxidizing bacteria. Write a chemical equation that shows how cyanuric acid
prevents the loss of chlorine.
Moral of the Pool Story?
http://ipoolproducts.com/Pool_And-Spa-Signs-.html
Moral of the Pool Story?
Don’t
___ in
the
pool!
http://www.cartoonstock.com/directory/d/diaper.asp
Practice Equilibrium Problems Make Perfect!
1.  What chemical can you use to get rid of: a. soap scum (calcium or
magnesium tallowate due to hard water)
b. Red brown iron stains (Fe(OH)3)?
c. Grease?
2. Oxalate, C2O42-, is poisonous to animals because it precipitates Ca2+ to
form insoluble CaC2O4 (Ksp = 2.3x10-9). Calcium ions, which are needed
for proper muscle control, is then removed from blood and muscles go into
spasm. Calcium oxalate kidney stones form when the concentrations of
Ca2+ and C2O42- are sufficiently high. Kidney stones do not dissolve
appreciably in acetic acid but it does go into solution in dilute strong acids.
a. Write a chemical equation that represents the reaction between solid
CaC2O4 and a strong acid, e.g., HCl. Calculate the numerical value of the
equilibrium constant for this reaction.
b. Would you expect the numerical value of K for the reaction between
solid CaC2O4 and acetic acid to be larger or smaller than for the K in part
a? Give reasons.
c. Explain why kidney stones dissolve in strong acid but not weak acid.
d. Could you use H2O2 to get rid of kidney stones, CaC2O4? Give reasons.

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