152 Chem Practice Exam 2 (with answers) Exam 2 will cover Chapter 11 and Chapter 12 This practice test will be most useful if you attempt to take it before looking at the answers. Administrative Stuff 1. If your calculator has memory, it must be cleared. 2. You must sit in your assigned seat. 3. You will be permitted to bring 1 sheet of paper, no larger than 8.511 inches, upon which you may write anything you wish. You may use both sides, but you may not use magnifying devices to read it. The values of physical constants will be provided. 4. This is a 60 minute exam worth a total of 130 points. 5. Show all work for the long problems (Section II). No credit will awarded without work for the problems in Section II. Showing work for the short problems in Section I is not required. There is no partial credit in Section I. 6. Be sure to include the correct units with all numerical answers. 7. A subset of the examinations will be photocopied to guard against academic fraud. By taking the examination, you acknowledge that your exam may be photocopied and used as evidence of fraud should this become an issue. 8. It is impossible to grant extra time to people who are late to an examination in a manner which is fair to the other students. So assume it will not be done and plan accordingly. If you have any flexibility in your schedule plan on arriving early on the day of any exam (better several minutes early than a few minutes late). Section I: Multiple Choice and Short Answer Problems 1. Multiple Choice (4 pts. each): a) How many electrons are transferred in the following reaction? (balance it first) 2 SO3 (aq) + MnO4 ( aq) SO2 ( aq) + Mn 2 + ( aq) 4 A: 2 B: 4 C: 6 D: 8 E: 10
Explanation: The balanced half reactions are 2 H 2O + SO3 SO2 + 2H + + 2e 4
5e + 8H + + MnO4 Mn 2 + + 4H 2O
b) Using the table of half-cell potentials provided, which of the following species is the best reducing agent? A: Br2 B: H2 C: Mg2+ D: Mg E: Au3+ Explanation: Reducing agents are oxidized during reaction. Written as an oxidation, Mg has the largest positive E.
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c) The standard potential for the following reaction is 1.56 V: Zn( s ) + 2Ag + ( aq ) Zn 2 + ( aq ) + 2Ag( s ) What is the standard potential for the following reaction: 2Zn( s ) + 4Ag + ( aq ) 2Zn 2 + ( aq ) + 4Ag( s ) A: 3.12V B: 1.56V C: 0.00V D: 1.56V E: 3.12V
Explanation: You dont multiply the cell potentials by stoichometric coefficients. d) The equilibrium constant (K) for the following reaction under standard conditions 2Al(s) + 3Cu2+(aq)2Al3+ (aq) + 3Cu(s) is approximately: A: 10203 B: 1034 C: 1068 D: 1068 E: 10203
Explanation: For this reaction E=2.00 V, and n=6. At 25C,
0.0591 E =E log(Q ) n At equilibrium E = 0, and Q = K.
0.0591 E = log( K ) n nE (6)(2.00) = = 203 log( K ) = 0.0591 0.0591 K = 10203
e) An electrochemical cell has E = 1.00 V. If S for the reaction is positive, then as the temperature is increased, how does E change? A: E decreases B: E doesnt change C: E increases D: Not possible to tell from the information given. Explanation: If S > 0, then G = H TS becomes more negative as the temperature increases, so E becomes more positive.
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f) The following two half-reactions take place in a galvanic cell. At standard conditions, what species are produced at each electrode? Sn2+(aq) + 2e Sn(s) E = 0.14 V Cu2+(aq) + 2e Cu(s) E = 0.34 V 2+ A. Sn is produced at the anode and Cu is produced at the cathode. B. Sn is produced at the anode and Cu is produced at the cathode. C. Sn is produced at the cathode and Cu2+ is produced at the anode. D. Cu is produced at the cathode and Sn2+ is produced at the anode. E. Cu is produced at the anode and Sn2+ is produced at the cathode. Explanation: Reaction is spontaneous in the direction which results in a positive E. A positive E can be obtained by reversing the first reaction, which means that Sn is oxidized and Cu2+ is reduced. Reduction occurs at the cathode, so the answer is D, not E. g) Place the elements Se, Br, and Ar in order of increasing atomic radius (smallest first, largest last) A. Se, Br, Ar Explanation: Ar is n=3, while the other two are n=4, B. Ar, Br, Se so Ar is smallest. Atomic radius decreases left to right along C. Br, Ar, Se a row due to the increase in Z, so Br is the next smallest, and D. Se, Ar, Br Se is largest. E. None of these h) For which element are the d orbitals completely filled in the neutral atom? A: K B: V C: Fe D: Br E: Ni
i) What is the total number of electrons that can be accommodated in all of the orbitals corresponding to n = 3? A: 2 B: 8 C: 10 D: 18 E: 20 Explanation: 2 electrons in the 3s orbital, 6 in the 3p and 10 in the 3d 18 electrons j) The valence electron configuration of a neutral atom is ns2(n-1)d10np2. To which group does it belong? A:Group 3a B: Group 4a C: Group 5a D: Group 6a E: Group 7a k) The number of orbitals having a given value of orbital angular momentum ( ) is equal to: A: 2 + 1 B: 2n + 2 C: +m D: 3 , 0, E: n 1 , + . Add these up and
Explanation: The quantum number m can take on the values m = , there 2 + 1 possible values of m .
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l) In Bohrs atomic theory, movement of an electron from one energy level to another level more distant from the nucleus is accompanied by: A. Photon emission. Photon B. absorption. C. No photon absorption or emission D. None of the above. Explanation: If the electron is more distant, it is moving to an energy level with a larger energy. The atom must absorb energy to do this. (m-n) A) emission spectrum of hydrogen B) the photoelectric effect C) scattering of alpha particles by metal foil D) diffraction E) cathode rays From the preceding list of observations, choose the one that most clearly supports the following conclusion: m. Electrons have wave properties. Ans: D n. Electrons in atoms have quantized energies. Ans: A o) What is the de Broglie wavelength of a proton (mass = 1.671027 kg) moving at 200 m/s? A:1.651010 m B: 1.98109 m C: 3.97107 m D: 2.5210+6 m E: 5.0410+8 m
Ans: = h/(mv) = (6.6261034 J s)/[( 1.671027 kg)(200 m/s)] = 1.98109 m (p-r) Write down the expected electron configuration (using either shorthand or full notation) for the following species (grading is all or nothing, WRITE LEGIBLY, if we cant read it, its wrong.) p. Nb+2 q. P r. O2 Ans: [Kr]5s24d1 Ans: [Ne]3s23p3 Ans: [Ne]
Note: [Ne] = 1s22s22p6 [Kr] = [Ne]3s23p64s23d104p6 4
Section II: Long-Answer/Numerical Questions 2. (10 pts) A galvanic cell is constructed with copper electrodes and Cu2+ in each compartment. In one compartment, the [Cu2+] = 1.0 103 M and in the other compartment, the [Cu2+] = 2.0 M. Calculate the potential for this cell at 25C. The standard reduction potential for Cu2+ is +0.34 V. Be sure to indicate in which direction the reaction is proceeding (otherwise you will not get credit for getting the sign of E right). Ans: The reaction will proceed spontaneously if the cell is written as Cu( s ) | Cu 2 + ( aq, 0.001M) || Cu 2 + ( aq, 2.0M) | Cu( s ) so 0.0591 E =E log(Q ) n 0.0591 0.001 = 0V log 2 2 = (0.0296 V)( 3.30) = 0.098 V
3. (18 pts) Balance the following redox reaction and determine K for this reaction occurring under standard conditions. HClO2 ( aq) ClO3 ( aq) + HClO(aq ) You must show all steps in generating the balanced reaction to receive full credit. The following halfequations will prove useful in solving this problem:
ClO3 ( aq) + 2e
HClO2 ( aq) HClO( aq)
E1/ 2 = 1.21 V E1/ 2 = 1.65 V
HClO2 ( aq) + 2e
Ans: H 2O + HClO2 ( aq) HClO2 ( aq) + 2e + 2H + 2 HClO2 ( aq)
ClO3 ( aq) + 3H + + 2e
1.21 V +1.65 V +0.44 V
HClO( aq ) + H 2O
HClO( aq ) + ClO3 ( aq) + H +
nF Ecell = RT ln( K ) (2) 96, 485 C mole ln( K ) = 34.27 K = 7.6 1014
(
) ( 0.44 V ) = (8.314 J mole K ) (298K ) ln( K )
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4. (15 pts.) What is the maximum wavelength of light capable of removing an electron from He+ if the electron initially resides in the n = 1 state? Ans: 1 1 E = 2.178 1018J Z 2 2 2 n f ni 1 1 = 2.178 1018J 22 1
( ) ( )
= 8.7 1018J Ephoton = hc = hc Ephoton
(6.626 1034 J s)(3 108 m/s) = 2.28 108 m =
8.7 1018J
5a. (5 pts.) There has been much recent talk about fuel cells that use molecular hydrogen (H2) as a fuel source. Such a cell can be written schematically as: H 2 ( g ) | H 2O || OH | O2 ( g ) Write the balanced redox reaction for this cell with the reaction occurring in basic solution. Ans: Anode reaction is...