Best of Luck & Happy Holidays!

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1 Chemistry 4A, Final Exam Name December 8, 1999 Professor R.J. Saykally TA I. (40) II. (60) III. (50) IV. (50) TOTAL EXAM SCORE (200) Best of Luck & Happy Holidays! Rules: Work all problems to 2 significant figures No lecture notes or books permitted No word processing calculators Time: 3 hours Show all work to get partial credit Periodic Table, Tables of Physical Constants, Conversion Factors, T-distribution, Appendix D & E included

2 Chemistry 4A F 99, Final Exam Name 2 I. (5 points each) Farmers in the midwest produce a considerable amount of corn for fermentation to ethanol (C 2 H 5 OH). This is mixed with gasoline to produce oxygenated fuel. Use thermodynamic tables to help answer the following questions: Unknown Field Code Changed 1) Calculate the energy required to heat one mole of liquid ethanol from 25 C to 50 C at P = 1 atm. 2) Calculate the energy needed to vaporize one mole of liquid ethanol at its boiling point and P = 1 atm. 3) Calculate the normal boiling point (T b ) of liquid ethanol.

3 Chemistry 4A F 99, Final Exam Name 3 4) Calculate the energy equivalent (EE in kj/kg) of liquid ethanol.

4 Chemistry 4A F 99, Final Exam Name 4 5) Calculate the maximum total work that can be extracted from the combustion of 1 mole of gaseous ethanol at 2000K. 6) Calculate the equilibrium constant (K eq ) for the combustion of gaseous ethanol at 2000K. 7) What exhaust temperature is required for a heat engine to operate at 70% maximum efficiency if the cylinder temperature is 2000K? 8) Calculate the maximum electrical work that can be extracted from an ethanol fuel cell at 2000K.

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6 Chemistry 4A F 99, Final Exam Name 6 II. ( points) Oxalic acid is a diprotic acid (K a1 = 5.9 x 10 2, K a2 = 6.4 x 10 5 ). 1) Derive the Henderson-Hasselbach equation for the first ionization of oxalic acid. 2) 100 ml of a M solution of oxalic acid is titrated with M NaOH. What is the ph for 0.00 ml added titrant? 3) Use your equation in part 1 to determine the ph for 100 ml added titrant.

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8 Chemistry 4A F 99, Final Exam Name 8 4) Why is your answer in part 3 inaccurate? 5) Write the 5 equations necessary to solve exactly for the ph. For 100 ml added titrant, you measure (H 3 O + ) = ± M, (HC 2 O 4 ) = ± M, (H 2 C 2 O 4 ) = ± M (equilibrium concentrations). 6) What is the accuracy of the determination of K a1 from these data?

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10 Chemistry 4A F 99, Final Exam Name 10 7) What is the precision of your determination of K a1? Report your determined value of K a1 with the proper # of sig. figs. and with the precision. 8) Estimate (neglecting activity effects) the solubility of Pb (IO 3 ) 2, in a solution that is 1.00 x 10 2 M in lead nitrate [K sp = 2.6 x ]. 9) Define the activity of the Pb 2+ as a function of the equilibrium concentration and standard state.

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12 Chemistry 4A F 99, Final Exam Name 12 10) Calculate the ion pair concentration in the above solutions, given that γ Pb 2+= 0.5 and γ IO3 = 0.9 under the above conditions. III. ( points) Consider the lead-acid battery, used in essentially all cars today. anode Pb(s) + SO 4 2 (aq) PbSO 4 (s) + 2e cathode PbO(s) + SO 4 2 (aq) + 4H 3 O + (aq) + 2e PbSO 4 (g) + 6H 2 O(/) 1) Calculate the cell potential under standard conditions. 2) Calculate the cell potential for the fully charged condition ( C H2 SO 4 =10 M).

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14 Chemistry 4A F 99, Final Exam Name The gas-phase reaction between hydrogen and iodine k f H 2 (g) + I 2 (g) 2 HI(g) k r proceeds with a forward rate constant at 1000 K of k f = 240 L mol 1 s 1 and an activation energy of 165 kj mol 1. By using this information and thermodynamic, calculate the activation energy for the reverse reaction and the value of k r at 1000 K. Assume that H and S for the reaction are independent of temperature between 298 and 1000 K.

15 Chemistry 4A F 99, Final Exam Name In some reactions there is a competition between kinetic control and thermodynamic control over product yields. Suppose compound A can undergo two elementary reactions to stable products: k 1 k 2 A B or A C k -1 k -2 For simplicity we assume first-order kinetics for both forward and reverse reactions. We take the numerical values k 1 = 1 x 10 8 s 1, k -1 = 1 x 10 2 s 1, k 2 = 1 x 10 9 s 1, and k -2 = 1 x 10 4 s 1. (a) Calculate the equilibrium constant for the equilibrium A C an From this value, give the ratio of the concentration of C to that of B at equilibrium. This is example of thermodynamic control. (b) In the case of kinetic control, the products are isolated (or undergo additional reaction) before the back reactions can take place. Suppose the back reactions in the preceding example (k -1 and k -2 ) can be ignored. Calculate the concentration ratio of C to B reached in this case.

16 Chemistry 4A F 99, Final Exam Name Name the 1999 recipient of the Nobel Prize in Chemistry and describe the work for which the prize was given.

17 Chemistry 4A F 99, Final Exam Name 17 IV. (5 points each) A voltammetry experiment is performed on a solution of Cu + (aq), and the reduction e + Cu + (aq) Cu(s) is investigated. As the potential between the working electrode and reference electrode is scanned, the following voltammogram is recorded. GRAPH HERE 1) In section A of the voltammogram, why isn t any Faradaic current measured? 2) In section B, what is limiting the Faradaic current?

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19 Chemistry 4A F 99, Final Exam Name 19 3) In section C, what is limiting the Faradaic current? From the lecture on ph electrodes, you learned that the boundary potential (E b ), which the ph meter measures, is linearly dependent on the ph of the solution... E b = C ph + D where C & D are constants. To calibrate your ph electrode, you use the data: ph E b (V) ) Determine the constants C & D (with proper units).

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21 Chemistry 4A F 99, Final Exam Name 21 5) You place your ph electrode in solution A and measure E b to be 0.694V. What is the ph of solution A? 6) You then place your ph electrode in solution B, which is known to have a ph of about 9, but you get an E b value that deviates form the expected value. Explain what you must do to measure to ph of solution B accurately. A chemistry class of 28 students determined an average value for the K sp of AgCl as x10 10 with a standard deviation of 0.40 x ) Calculate the standard deviation of the mean.

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23 Chemistry 4A F 99, Final Exam Name 23 8) Calculate the probability that the true K sp lies within 1 standard deviation of the mean. 9) Sketch the phase diagram of water (labelling important features) and describe how it differs from normal (e.g. CO 2 ) behavior. 10) What fundamental properties of water and CO 2 produce the above behavior?

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i i ne. (1) i The potential difference, which is always defined to be the potential of the electrode minus the potential of the electrolyte, is ln( a

i i ne. (1) i The potential difference, which is always defined to be the potential of the electrode minus the potential of the electrolyte, is ln( a We re going to calculate the open circuit voltage of two types of electrochemical system: polymer electrolyte membrane (PEM) fuel cells and lead-acid batteries. To do this, we re going to make use of two