INSERT
READ THIS BEFORE BEGINNING
(A periodic table in on the back.)
This final examination has two parts:
1) a one-page lab section with 4 equally weighted questions
2) a seven-page section with 7 questions that covers the lecture portion of the
course.
The lecture portion of the examination deals with the chemistry of acrylic acid and its
derivatives. The molecular structure of acrylic acid, C3H4O2, is given below.
H
H
O
H
O
H
Useful constants and data
NA = 6.022 x 1023 molecules/mole
F = 96485 C/mole
RTln(10)/F = 0.0592 V at 25C
1 e V = 96.485 kJ/mole
Kw = 1.00 x 10-14 at 25C
h = 6.626 x 10-34 J-s
c = 2.9979 x 108 m/s
R = 8.31451 J/mole-K = 0.0820578 L-atm/mole-K
ln(10) = 2.303
1 L-atm = 101.325 J
Student's t as a function of degrees of freedom, f
t 12.706 4.303 3.182 2.776 2.571 2.447
f
1
2
3
4
5
6
pKa's of Selected Acid-Base Indicators
methyl red
bromthymol blue
cresol red
5.00
7.30
8.46
thymol blue
9.20
phenolphthalein
9.50
NAME:____________________________________ FINAL, Chem 51, fall, 2004
LAB-1) (The following numbers were obtained from a calculator and tabulated without
regard to the principles of significant digits.) Four measurements of an enthalpy change
yield an average value of 47.125 kJ/mole and a standard deviation of 4.21 kJ/mol. Give
the average enthalpy change to the correct number of significant digits and calculate the
relative 95% confidence interval of the mean.
LAB-2) Suggest a method for determining the concentration of copper(II) in the presence
of sodium(I) and calcium(II). The anion in the aqueous solution is nitrate.
LAB-3) A forensic laboratory is presented with a sample that contains H2O, C2H5OH,
and CH3OH. Suggest a method for determining the mole fraction of CH3OH in the
sample.
LAB-4) A sample is prepared by dissolving HCl and acetic acid in water. Suggest a
method for determining the concentration of the acetic acid in the sample.
LEC-1 (33 points) Lippard and Lewis obtained a value of 4.260 for the pKa of acrylic
acid at 25C. Moreau and Boutaric obtained 4.26 at 18C.
a) Suppose you wish to assay a sample of acrylic acid. You dissolve some in
water and titrate it at 25C with 0.100 M NaOH. Calculate the pH at the end
point. Assume an initial molar concentration 0.10 M for the acrylic acid.
b) Select from the list on the insert an optimal indicator for the titration.
c) Calculate values of the standard enthalpy and entropy changes for the
dissociation of acrylic acid in water, i.e. for HA(aq)  H+(aq) + A-(aq).
LEC-2) (28 points) The value of the highest occupied molecular orbital (HOMO) can be
a useful measure of the reactivity of this compound. Given the utility of this quantity,
Katrib and Rabalais measured the ionization energy of acrylic acid. They irradiated
gaseous molecules of acrylic acid with radiation from excited helium with a wavelength
of 30.378 nm and found that the kinetic energy of the photoelectrons was 2899 kJ/mole.
a) Calculate from these data the ionization energy of acrolein. (Hint: first
calculate the energy/mole of the radiation from the helium that ionizes the acrylic
acid.)
b) Obtain the energy of the HOMO from your answer to part (a).
c) The light used in the spectrometer was produced by selectively excited He+
species which decay directly to the ground electronic state of He+. Calculate the n
(principal) quantum number of the He+ species that emits the radiation.
LEC-3) (21 points) When acrylic acid is mildly heated in the presence of a trace of
hydrogen peroxide, it is converted into an amorphous substance that has the same
empirical formula, C3H4O2, but very low solubility in all solvents and a very high melting
point. In fact, upon heating, the product decomposes before it melts. The reaction does
not occur with propionic acid, CH3CH2COOH.
a) What is the chemical nature of the product? What is the molecular basis for its
physical properties?
b) Using your knowledge of bonding theory, suggest why hydrogen peroxide
reacts with acrylic acid. (Hint: The O-O bond in hydrogen peroxide is weak.)
c) A column can be packed with the powdered product of the reaction discussed
in (a). This column can be used to treat wastes from nuclear reactors. That is, if
one passes an aqueous solution of the wastes through the column, the actinideelement components of the waste are retained on the column. What is the
chemical basis for this retention?
LEC-4) (20 points) The structure of acrylic acid was determined via X-ray
crystallography and is shown below. Use your knowledge of molecular bonding to
provide an explanation for two striking features of the structure. 1) The molecule is
planar and 2) the molecule is present as a dimer.
LEC-5) (18 points) D. Acharya and M. N. Das at Jadavpur University studied the kinetics
of the bromination of acrylic acid as a function of temperature.
H2C=CH-COOH(aq) + Br2(aq)  H2BrC-CHBr-COOH(aq)
They measured the concentration of Br2 electrochemically as a function of time by setting
up an electrochemical cell with the half reaction 2 Br-(aq)  Br2(aq) + 2 e- at the anode.
In a typical run, the initial concentrations of acrylic acid and Br2 were 0.1 and 0.001 M,
respectively. Bromide was present in large excess. They wrote "The emf (Voltage)
plotted against time gave good straight lines over a range of 60 mV, corresponding to a
decrease in bromine concentration by a factor of 100."
a) Show that a change in the concentration of Br2 by a multiplicative factor of
100 leads to a voltage change of 60 mV.
b) The authors observed that the Voltage of the cell varied linearly with time and
inferred from their fact a first order dependence on the concentration of Br2. How
does a linear V versus t plot demonstrate first order kinetics?
c) The authors also report that the slope of the Voltage versus time plots is
directly proportional to the initial concentration of acrylic acid. From this result
and the information provided above, what is the empirical rate law for the
reaction?
LEC-6) (15) The following 2 mechanisms have been proposed for the bromination
reaction studied by Acharya and Das. I+ and II+ are intermediates.
Mechanism A
Br2(aq) + H2C=CH-COOH(aq)  I+(aq) + Br-(aq) slow formation of I+
Br-(aq) + I+(aq)  H2BrC-CHBr-COOH(aq) rapid step
Mechanism B
Br2(aq)  Br+(aq) + Br-(aq) slow formation of reactive Br+
Br+(aq) H2C=CH-COOH(aq)  II+(aq) rapid formation of II+
I+(aq) + Br-(aq)  H2BrC-CHBr-COOH(aq) rapid formation of product
a) Derive an empirical rate law in each case.
b) Which rate law is consistent with the experimental data?
LEC-7) (15 points) Acharya and Das also report that the rate constant for the
bromination reaction is 0.040 and 0.35 at 20C and 40C, respectively.
a) Calculate the activation energy for the reaction from these data.
b) Their rate data also yield an entropy of activation of -3 J/K-mole. Is this result
consistent with your choice of mechanism for the reaction? A discussion is
expected. The structures of the intermediates I+ and II+ and one activated
complex are provided below.
Br
Br
+
H
H
Br
H
O
O
H
intermediate
I+ and II+
H
H
H
O
O
H
activated complex
for first step of
mechanism A