Exam # 2
11/07/05, Chem431
What is cooking?
Last Name _______________________
Question 1. (75 pts)
a. ___________ (10 pts)
b. ___________ (5 pts)
c. ___________ (15 pts)
d. ___________ (15 pts)
e. ___________ (5 pts)
f. ___________ (10 pts)
g. ___________ (5 pts)
h. ___________ (10 pts)
i. ___________ (bonus 15 pts)
Question 2. (25 pts)
a. ___________ (5 pts)
b. ___________ (5 pts)
c. ___________ (15 pts)
d. ___________ (bonus 5 pts)
Total:__________ (100pts)
SSN _______________________
Whatever portion of the exam is collected after 1 hour in class will carry 10% higher grade
from its nominal value. You can submit your answers online before midnight 11/07/05 or
leave it in my mailbox before 10 am 11/08/05 (no delays will be accepted).
1. What is cooking? The first question is dedicated to one reaction that is typically used in cooking
(not only, of course) - the neutralization reaction between soda and vinegar:
NaHCO3 (aq) + CH3COOH(aq) Æ CH3COONa(aq) + H2O(l) + CO2(g)
a) (10 pts) Calculate the equilibrium constant for the reaction at 25oC and 1 bar (use the Table at
the end of your textbook)
b) (5 pts) Write the expression for the quotient for this reaction (Hint: which activity can be
omitted?).
c) (15 pts) Calculate the partial pressure of CO2(g) in a close 20 mL container where 5 mL of 10
mM NaHCO3 and 5 mL of 10 mM CH3COOH(aq) were mixed at constant T = 298 K. Presume
that liquids are incompressible. (Hint: remember that the standard condition for a gas is
pressure Po = 1 bar = 105 Pa).
5mL
5mL
20 mL
flask
d) (15 pts) Calculate pH of the initial individual solutions (10 mM NaHCO3 and 10 mM
CH3COOH(aq)) and the product of their mixture after container was opened and all CO2 left
the solution. Why is pH not neutral at the end? (Hint: for H2CO3: pKa1 = 6.38, pKa2 = 10.32;
pKa (acetic acid) = 4.76)
e) (5 pts) Will the equilibrium shift to the left or to the right with temperature? What do you need
to know for that? (Hint: you probably need to use the same Table again)
f) (10 pts) Write the expression for the rate of CO2 production presuming that the reaction
mechanism consists of quasi equilibrium of acid dissociation (step I) followed by the diffusion
controlled neutralization step II
k1
I) CH3COOH(aq) Ù CH3COO−(aq) + H+(aq)
k-1
kD
−
+
II) H (aq) + HCO3 (aq) Æ H2O(l) + CO2(g)
Where we presume that k-1 = kD
g) (5 pts) If kD = is kD = 1010 M-1s-1 (a typical diffusion controlled rate constant), what is the value
of k1? (Hint: you know the pKa of acetic acid)
h) (10 pts) Calculate the rate when the concentration of acetic acid drops to ½ of its original value
given for the above mentioned mixture.
i) (extra 15 pts) Derive the expression for the time dependence of [CH3COOH]
2. (25 pts) For winemakers in the Roman Empire, nothing but lead would do. When boiling
crushed grapes, Roman vintners insisted on using lead pots or lead-lined copper kettles. "For, in
the boiling," wrote Roman winemaker Columella, "brazen vessels throw off copper rust which has
a disagreeable flavor." Lead’s sweet overtones, by contrast, were thought to add complementary
flavors to wine and to food as well. The metal enhanced one-fifth of the 450 recipes in the Roman
Apician Cookbook, a collection of first through fifth century recipes attributed to gastrophiles
associated with Apicius, the famous Roman gourmet. From the Middle Ages on, people put lead
acetate or "sugar of lead" into wine and other foods to make them sweeter. A form of lead
intoxication known as saturnine gout takes its name from ancient Rome. Saturn was a demonic
god, a gloomy and sluggish figure who ate his own children.
a) (5 pts) Write the expression for electrochemical reaction that accompanies formation of lead
acetate from acetic acid and a lead pot where only the oxidizing and the reducing reagents are
left in the reactants. Make sure to balance it.
b) (5 pts) Identify the oxidizing and the reducing reagents among the reactants.
c) (15 pts) Write the expression for Ecell that accompanies the reaction, i.e. how Ecell depends on the
concentration of species involved. (Hint: when calculating the potential with hydrogen
evolution, 2H+(aq) + 2e− Æ H2(g), remember that it corresponds to [H+(aq)] =1.0 M but a
weak acid produces H+(aq) in accordance with its dissociation equilibrium constant, [H+] =
Ka[acid] /[base]) Is reaction spontaneous? Comment on why the recipes insisted on mixing the
thickening must upon boiling it down.
d) (extra 5 pts) Calculate the potential difference for a geometry shown below when one pot has
0.1 M acetic acid half reacted and the other pot has 1.0 M acetic acid also half reacted? Note that
they share gaseous atmosphere.
Salt bridge
Pb pot
Pb pot