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.
Useful constants and data
NA = 6.022 x 1023 molecules/mole
F = 96485 C/mole
RTln(10)/F = 0.0592 V at 25C
1 e V = 96.485 kJ/mole
Kw = 1.00 x 10-14 at 25C
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 thymol blue phenolphthalein
5.00 7.30 8.46 9.20 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 25C. Moreau and Boutaric obtained 4.26 at 18C.
a) Suppose you wish to assay a sample of acrylic acid. You dissolve some in water and titrate it at 25C 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 actinide-element 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 JadavpurUniversity 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 Br2electrochemically 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 20C and 40C, 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.