Unformatted text preview: Name: BLANK Chem 230 Exam #3
November 24, 2008 Discussion Section #: Use only the space provided to answer the questions. If you need more work space, start your
answer on the back of a page and then write only your final answer in the space provided
Anything written in the margins and on the back of pages will not be graded Make sure your final answer is clearly marked The exam totals to 150 points. Where relevant, use F : 105 C mol’ Multiple Choice:
Mark all multiple choice clearly, by both circling the correct choice and writing the letter. Each
multiple choice question is worth 3 points. 1) Which characteristic of an electrochemical cell is affected by the number of electrons that
appears while balancing a redox reaction? a) The voltage b) The current c) The work (1) B & C c) All of the above l) None of the above 2) For the galvanic electrochemical cell Zn | Zn2+ || Cu2+ | Cu, which of the following is a poor
negative ion choice for the salt bridge? E°(Zn2+ | Zn) = -0.8, E"(Cu2+ | Cu) = 0.3. a) $2033-, EO(S4062— I 82033.) = 0.1 b) Sez‘, E°(Se | Sez‘) = -0.7 C) SO42} E°(SO42‘ I SO32-) = -0.9 d) All of the above are good choices e) None of the above are good choices 3) Which of the statements is false for a rechargeable battery? 21) The electrochemical and galvanic products should remain near the electrodes
b) Charge must be balanced without using a salt bridge c) Ions appearing in the balanced reaction are undesirable d) All of the above
e) None of the above Name: BLANK 4) For the electrochemical cell ZrO(OH)2(s) | Zr(s) || SOz(g) | SO42‘(aq), which of the following
is a correctly balanced reaction in base? a) ZrO(OH)2(s) -- SOz(g) —> Zr(s) + SO42‘(aq) + H20(l) b) ZrO(OH)2(s) -- 2 SOz(g) + 4 OH‘(aq) —> Zr(s) + 2 SO42‘(aq) + 3 H+(aq) c) ZrO(OH)2(s) + 2 SOz(g) + 4 OH‘(aq) —> Zr(s) + 2 SO42‘(aq) + 3 H20(l) d) 2 ZrO(OH)2(s) + SOz(g) —> 2 Zr(s) + SO42‘(aq) + 4 OH‘(aq) e) None of the above 5) Is HClO4 most likely to be an oxidizing agent or a reducing agent?
a) Oxidizing agent b) Reducing agent c) Both (1) Neither 6) What type of reaction is Fe203 + 3 H20 —> 2 Fe(OH)3?
a) Redox b) Acid-base c) Both (1) Neither 7) If the overall reaction 2 A + 2 B —> C + D has a rate law of k[A][B]2 and the reaction
mechanism list below, which step is the rate determining step? szaE mAnEac+2F c) A + F —> G (1) G -- F —> D e) None of the above 8) Which of the following rate laws for elementary reactions would most likely be the fastest,
considering the molecularity? 21) k[A][B] b) k[A]2[B] c) k[A]2[B]2 d) k[A]3 e) All of the above would be the same Name: BLANK 9) The overall reaction: 2 N3(g) —> 3 N2(g)
Has the reaction mechanism:
C1'(g) + N3(g) -> C1N'(g) -- N2(g)
C1N'(g) + Ns(g) -> 2 Ms) -- C1'(g) What is Cl-?
21) Heterogeneous Catalyst
b) Homogeneous Catalyst
(1) A and C
e) None of the above 10) The overall reaction A + 2 B + C —> D + E has the reaction mechanism listed below. During the reaction, there is a measurable increase in concentration of species F that remains for most of
the reaction, and disappears as the reaction comes to completion. Which is the rate determining step?
d) All steps happen quickly
e) More than one step is the rate determining step 11) An overall reaction is made up of 20 elementary reactions. Halfway through the reaction
mechanism, the concentration of one particular species has decreased. What is the most likely
role of this species in the reaction? a) Product b) Reactant c) Intermediate (1) Catalyst e) A or C f) B or D 12) During a second order reaction, a reactant decreases from 1 mol L‘1 to 0.5 mol L‘1 in time T1.
The same reactant then decreases from 0.5 mol L‘1 to 0.25 mol L‘1 in time T2. Is T1 longer,
shorter, or the same as T2? 21) T1 is longer than T2 b) T1 and T2 are the same c) T1 is shorter than T2 (1) Insufﬁcient information to say Name: BLANK 13) In the reaction 147N + X —> “sC‘ + l1p+, what is X?
a) 0—16- b) 016+ C) 1011 d) l-lp- e) None of the above 14) If the rate constant for a radioactive decay process is 0.07 s‘l, what is the half-life of the
process? a) 10 s b) l s c) 0.1 s d) 0.01 s e) None of the above 15) An atom spontaneously decays and emits radiation equivalent to 0.9 MeV. The mass of an
electron is equivalent to 0.5 MeV. Which process may have occurred? a) Neutron emission b) Positron emission c) B emission d) Proton emission e) All of the above 1) None of the above 16) “6C has a background rate of 10 disintegrations s‘l per g of carbon, and a half-life of 5,700 years. A fragment of pottery from an archaeological site is supposedly 8,000-12,000 years old. If the food residue found on this fragment of pottery has an activity of 5 disintegrations s‘l per g
of carbon, what is a reasonable estimate for the age of the pottery fragment? 21) Less than 8,000 years old b) 8,000-12,000 years old c) More than 12,000 years old (1) There is insufﬁcient information to say Name: Short Answer BLANK Most questions can be answered solely with numbers, equations/expressions, or short phrases.
When asked to “explain” or ’justiﬁz, " use a maximum of m sentences or an annotated equation.
Provide units for all final numerical answers, if any. Showing clear work within the space
provided is recommended for calculations to allow for partial credit. Clearly indicate your final answer by boxing or underlining, if showing work or changing your answer. 17) (10pts) Consider the galvanic cell Al | Al3+ || Ag+ | Ag, diagrammed below. Using the
choices l-6 from the list of choices below, mark each of (a) - (j) (in the table of answers, not on
the diagram itself) with the appropriate numerical choice. You will have to use some of the
choices l-6 more than once. Note that (c) - (j) refer to the neighboring arrows. List of Choices:
(2) cathode (3) direction of movement of positive ions
(4) direction of movement of negative ions (5) direction of electron ﬂow (6) none of the above Table of Answers (numbers only) (a) = (b) = (C) = (d) = (e) (f) = (g) = (h) = (i) = (j) 18) ( 6 pts) An inventor claims to have a created a single-cell (1 reaction) battery that yields a
power of 100 W from a current of 2 A. Your textbook's table of standard reduction potentials has values from ~ 3 V to -3 V. Is the inventor's claim reasonable? Brieﬂy justify your answer. Name: BLANK 19) (24 pls) Answer the following questions using the following electrochemical cell in acid:
Mn(s) | Mn(OH)2(s) || Br03‘(aq) | Br‘(aq).
21) Write a correct, fully balanced half-reaction for the anode. b) Write a correct, fully balanced half-reaction for the cathode. c) Write a correct, fully balanced overall reaction. (1) If E°(anode) = -l .5, and E°(cathode) = 0.5, calculate AGO for the cell. e) Write the expression (not numerical value) for Q that would appear in the Nernst equation for
this system. f) If all aqueous products and reactants are at twice their standard concentrations but otherwise
under standard conditions, would the cell produce more, less, or equal work compared to the
work under completely standard conditions? Brieﬂy justify your answer. Name: BLANK 20) (20 pls) K = 10‘2 for the reaction N204(aq) —> 2 N02(aq). This reaction is elementary in both
directions, so the rate law for the forward reaction is 1([N204], where k = 10‘2 s‘l.
a) Ignoring the reverse reaction, how long will the ﬁrst half-life of this reaction be if the starting concentration of N204 is 0.5 mol L‘l? (Leaving your answer as a fraction is acceptable.) b) Using the concentration of product after one half-life of the forward reaction, what will be the
rate of formation of N204? c) After the ﬁrst half-life, will the overall reaction be running forward, in reverse, or have
reached equilibrium? Brieﬂy justify your answer. 21) (10 pls) Calculate the rate law of the reaction (including the numerical value and units of k),
given the experimental data below. Initial |A| gmol L‘l) Initial |B| gmol Li) Rate gmol L‘l s‘lz
0 2 . 0.25 0.0625
0.2 0.5 0.25
0.4 0.5 0.25 22) ( 6 pls) Brieﬂy explain how is steady state different from equilibrium. Name: BLANK 23) (8 pls) Consider the Arrhenius equation, where the activation energy is a Gibb's free energy:
k=Ae‘AGa/RT. Give two ways that temperature would increase rate. Brieﬂy justify your answers. a) b) 24) (10 pls) Both 146C and 4019K decay by emitting B-particles, and are commonly incorporated
into living organisms at concentrations proportional to their concentration in the environment.
a) What is the atom produced from 4019K decay? Use the full atomic format for your answer. b) What can you say about the mass of the atom produced compared to 4019K? c) Consider this relationship: total radiation energy = (particle energy) X (reaction rate). 4019K emits a B-particle that is 10 times more energetic than the B-particle from 146C, has a half-
life that is 2x105 times longer, and has a 106 greater concentration in the environment. Estimate
the relative radiation dose from 4°ng compared to 146C. 131 25) (8 pls) Give two reasons why 531, which decays to 13154Xe with a half-life of 8 days, is a
better choice for medical imaging than 108531, which decays to 104518b with a half-life of 0.04 s.
Brieﬂy justify your answers. a) b) ...
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