EXAM 3 CHEMISTRY 224 April 8, Write your name and Purdue ID number on the answer sheet

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1 1. Read the following instructions carefully EXA 3 CHEISTRY 224 April 8, Write your name and Purdue ID number on the answer sheet 3. Write your Graduate Instructor s name on the line for Instructor on the answer sheet 4. Use a #2 pencil to code all information on the answer sheet. 5. Code your name and Purdue ID number on the answer sheet 6. Code 0224 as the Section number on the answer sheet. 7. Code the one best answer to each question on the answer sheet. THERE ARE 20 QUESTIONS. CHECK NOW TO BE SURE THAT YOUR EXA INCLUDES ALL 20 QUESTIONS. EACH QUESTION IS WORTH TEN (10) POINTS (200 POINTS TOTAL) YOU ARE SUPPOSED TO KNOW (REEBER) A NUBER OF BASIC EQUATIONS. ADDITIONAL FORULAS AND REFERENCE TABLES ARE INCLUDED AT THE END OF THE EXA ANY PROBLES CAN BE SOLVED ONLY USING THE REFERENCE ATERIALS IN EACH CASE IT IS YOUR RESPONSIBILITY TO CONSULT THE REFERENCE SECTION AT THE END OF THE EXA. IF IT SEES THAT YOU NEED A CERTAIN COPLICATED FORULA WHICH IS NOT GIVEN IN THE TEXT OF THE EXA WE SUGGEST THAT YOU HAVE A CLOSER LOOK AT THE PROBLE PERHAPS, IT CAN BE SOLVED WITHOUT USING THE FORULA!

2 1. The figure below shows the energy diagram for conformational transitions in cyclohexane. The energetically favorable conformation is chair (shown on the right side of the plot). The alternative conformation is twisted boat (left side of the plot). At room temperature (25 C), what is the proportion of the twisted boat conformation relative to the chair conformation? a % b. 0.1 % c. 1 % d. 10 % e. 100 % 2. Considering question 1, what happens with the proportion of the twisted boat when the temperature is raised from 25 C to 30 C? a. the proportion of the twisted boat drops to zero b. the proportion of the twisted boat remains unchanged c. the proportion of the twisted boat increases by 5 % d. the proportion of the twisted boat increases by 17 % e. the proportion of the twisted boat increases by 41 %

3 3. Considering question 1, what can be said of the transition rates in the following equilibrium: a. f is smaller than r b. f is equal to r c. f is equal to r "twisted boat" f r "chair" d. f is greater than r e. either (a), or (b), or (d), depending on temperature 4. Consider a system showing standard ichaelis-enten inetics: 1 2 E+S ES E+P 1 Things get more complicated when one encounters a scenario termed competitive inhibition : EI+S E+S+I ES+I E+P+I 3 1 Please, inspect this scheme carefully to see what is going on. Then determine which of the following statements about the inhibitor (I) is true: a. (I) catalyzes the transformation of the enzyme (E) into the substrate (S) b. (I) increases the catalytic rate 2 c. (I) can occupy the substrate-binding site of the enzyme (E) d. (I) prevents binding of the substrate (S) to the product (P) e. (I) slows down the conversion of (P) into (E)

4 5. Consider the competitive inhibition scheme in question 4. Written below are five inetic equations for this system. Only one of them is incorrect. Which one? a. d [ P ] = 2[ ES ] dt b. d [ S ] = 1[ E ][ S ] + 1[ ES ] dt c. d [ ES] = 1[ ES] 2[ ES] + 1[ E][ S] dt d. d [ EI ] = 3[ E ][ I ] + 3[ E ][ I ] dt e. d [ I ] = 3[ E ][ I ] + 3[ EI ] dt 6. For the simple unimolecular reaction A B+ C the loss of the reactant A follows the simple exponential function, [ A] = [ A] 0 exp( t). Describe the build-up of the product B. a. [ B] = [ B] 0 exp( t ) b. [ B] = [ B] 0 exp( ( /2) t ) c. [ B] = [ A] 0 exp( t ) d. [ B] = [ A] 0(1 exp( t )) e. [ B] = 1 [ A] 0 exp( t) 7. Consider the solution reaction SnCl + 2FeCl SnCl + 2FeCl. The starting conditions at t = 0 are [SnCl 2] = 1 m, [FeCl 3] = 1 m. The magnitude of is assumed to be 35-2 s -1. Please, write the inetic equation and determine the initial rate of buildup for tin tetrachloride (i.e. the value of d[sncl ]/ dt at t = 0). 4 2 a /s b /s c /s d /s e. 35 /s

5 8. Considering the reaction from question 7, SnCl2 + 2FeCl3 SnCl4 + 2FeCl2, select one true statement: a. In this reaction, both iron and tin become oxidized b. In this reaction, both iron and tin become reduced c. In this reaction, iron becomes reduced and tin becomes oxidized d. In this reaction, iron becomes oxidized and tin becomes reduced e. This is not a reduction/oxidation reaction 9. The Standard Hydrogen Electrode (SHE) has a cell potential of 0.00 by definition. Using this electrode would simplify the Nerst equation. Why do we not have you use the SHE in lab? I) Gas electrodes are hard to wor with II) Hydrogen gas is explosive III) The electrode is expensive to operate IV) It requires the use of 1 strong acid a. I & II b. III & IV c. II & IV d. I, III & IV e. all of the above 10. In a battery made with Cu/CuSO 4 and Zn/ZnSO 4, what is the cathode? a. Zn b. Cu c. CuSO 4 d. ZnSO 4 e. The salt bridge

6 11. The familiar electrolytic cell is initially assembled as: (Cu Cu 2+ (10m) Ag + (20m) Ag) and the voltage in the cell is measured to be E( a ) (certain numeric value). Then a new cell is assembled, where the concentration of Cu 2+ is increased 10-fold: and the voltage is measured again, E( b ). (Cu Cu 2+ (100m) Ag + (20m) Ag) What is the change in voltage, E = Eb ( ) Ea ( )? Reference information: assume room temperature, the number of transferred electrons n=2; the equilibrium constant for this cell is K = [Cu ]/[Ag ] a V b V c V d V d V 12. Consider a galvanic cell, where one of the electrodes is the Standard Hydrogen Electrode (SHE), and the other is a platinum electrode immersed in a 250 ml solution of: of potassium ferrocyanide (iron(ii) salt, K 4 Fe(CN) 6 ) and of potassium ferricyanide (iron(iii) salt, K 3 Fe(CN) 6 ). For this cell, what is the relationship between the potential of the cell (E) and the standard electrode potential (E o ) of the K 4 Fe(CN) 6 / K 3 Fe(CN) 6 half-cell. a. E >> E o b. E = E o c. E/E o =.0592 d. E << E o e. None of the above.

7 13. By means of protein engineering, the native sequence of the enzyme alanine racemase has been altered. As it turned out, the modified alanine racemase binds its substrate, L- alanine, more tightly than before. What are the consequences for ichaelis constant K and the efficiency of catalysis? a. K becomes higher; efficiency of catalysis remains unchanged b. K becomes higher; efficiency of catalysis drops c. K becomes higher; efficiency of catalysis is improved d. K becomes lower; efficiency of catalysis drops e. K becomes lower; efficiency of catalysis is improved 14. In laboratory 8 (potentiometric determination of H 2 O 2 ) the measured voltage is related to concentration of triiodide according to E = α + βln[ I 3 ]. In other words, there is a linear dependence between E and ln[ I 3 ] with the intercept α and slope β. Question: how does the slope β change with an increase in temperature T? a. decreases exponentially b. decreases linearly c. remains constant d. increases linearly e. increases exponentially 15. Consider the enzymatic reaction that is described by ichaelis-enten equation. In the beginning, the concentration of the substrate is [ S] 0 and the rate of the product buildup is V 0. What happens with the rate of the product build-up, dp [ ]/ dt, as time goes on and the reaction runs its course? a. the rate increases infinitely b. the rate increases and eventually reaches the plateau value Vmax c. the rate remains constant d. the rate decreases and eventually reaches the plateau value 0 e. the rate first increases and reaches the value V max, then decreases toward zero

8 16. Consider the following redox reaction: V + x V(OH) + y H 3VO + z H O where x, y, and z are unnown stoichiometric coefficients. Please find the proper values of x, y, and z to ensure that the reaction is balanced: a) x = 2, y=0, z=4 b) x = 1, y=4, z=4 c) x= 2, y=16, z= 12 d) x = 1, y=2, z=2 e) x = 2, y=2, z=5 17. Shown in this picture is the Cu/Ag electrochemical cell that has been discussed in class. How do the electrons travel in this system? a. along the wire, left to right b. along the wire, right to left c. through the salt bridge, left to right d. through the salt bridge, right to left e. by quantum tunneling between the beaers

9 18. The operation of electrochemical cell is described by two half-reactions: 2+ Zn(s) Zn (aq) + 2e 2+ Cu and (aq)+2e The net reaction is: Cu(s) Zn(s)+Cu (aq) Cu(s)+Zn (aq) Please, determine the equilibrium constant reaction (assume room temperature). K = eq [Zn (aq)]/[cu (aq)] for this net a b c d e The plot on the right (borrowed from itra and Holz, Journal of Biological Chemistry vol. 282, p /2007/) illustrates the efficiency of a certain enzyme as a function of ph. Why can the quantity cat / K ( = 2 / K ) be used to characterize the efficiency of enzyme? a. The quantity / 2 K is temperature independent when the system is brought to the standard state (concentrations of all components 1 ) b. According to ichaelis-enten eq uation, in the limit [ S] K the production rate ( dp [ ]/ dt ) is proportional to / 2 K c. According to ichaelis-enten equation, in the limit [ S] K the production rate ( dp [ ]/ dt ) is proportional to / 2 K d. According to ichaelis-enten equation, the maximum production rate ( dp [ ]/ dt ) max is proportional to / 2 K e. The quantity / 2 K has maximum sensitivity to ph at around neutral ph

10 20. Considering the plot in question 19, it turns out that the shape of the graph is fairly typical. any human enzymes perform at their best at around ph 7 8 and temperature ca C. Speculate as to the origin of this effect. a. These are the only conditions conducive to reduction-oxidation reactions b. These are ambient conditions normally found in warm climes c. Heating and abundance of H + ions are necessary to ensure the access of glucose to the substrate-binding pocet of the enzyme d. These are the conditions ensuring the validity of ichaelis-enten equation e. Nature optimized human enzymes to function at ph and temperature conditions found in the cells of human body.

11 Reference Section Constants: Universal gas constant J R = K mol Faraday constant F = C/mol Room temperature expressed in Kelvins T = 298 K

AP CHEMISTRY NOTES 12-1 ELECTROCHEMISTRY: ELECTROCHEMICAL CELLS

AP CHEMISTRY NOTES 12-1 ELECTROCHEMISTRY: ELECTROCHEMICAL CELLS Review: OXIDATION-REDUCTION REACTIONS the changes that occur when electrons are transferred between reactants (also known as a redox reaction)