Ch. 17-21: Electrochemistry and Radiochemistry AP Review Questions Radioactivity: Zone of Stability All nuclides with 84 or more protons are unstable (radioactive). Light elements like the neutron to proton ratio to be equal to one. Heavier elements like the neutron to proton ratio to be greater than one. Even numbers of protons and neutrons are more stable than odd numbers of protons and neutrons. The magic (stable) numbers for protons and neutrons are 2, 8, 20, 28, 50, 82, 126. The magic (stable) numbers for electrons are 2, 10, 18, 36, 54, 86 (noble gas configurations.) 1) Which of the following radioactive isotopes is most likely to decay by emitting a beta particle? a) 235 U b) 40 Cl c) 35 Ar d) 246 Cf e) 21 Na 3) Radioactive decay often results in the emission of alpha particles, beta particles, positrons or neutrons. If in the radioactive decay of 210 Bi a single particle is observed to be emitted, which of the following is it most unlikely to be? a) 209 Bi b) 208 Au c) 210 Pb d) 206 Tl e) 210 Po 4) Which of the following choices represents 239Pu producing a positron? a) 239 Pu 235 Pu + 4 He 94 94 2 b) 239 Pu 0 e + 239 Np 94 +1 93 2) The nuclide 12 N is unstable. What type of radioactive decay would be expected? a) β- b) β+ c) Φ d) α e) 0 1 n c) 239 Pu + 0 e 239 Np 94-1 93 d) 239 Pu + 0 e 239 Np 94 +1 93 e) 239 Pu + 4 He 235 U 94 2 92 5) The nuclide produced after thorium-232 undergoes alpha emission is a) 224 Fr b) 228 Ra c) 228 Fr d) 228 Th e) 224 Ra
6). + n 7 Li + 4 He For the reaction shown above, what is the missing reactant? a) 9 Be b) 9 B c) 10 Be d) 10 B e) 11 B 10) A radioactive isotope has a half-life, t 1/2, of 4.5 hr. What expression should be used to calculate the rate constant for this process? a) k = ln t 1/2 b) k = (1 / t 1/2 ) ln2 c) k = 0.5 d) k = ln (t 1/2 / 2) e) k = rate / t 1/2 7) A 222 Rn nuclide decays through the emission of two beta particles and two alpha particles. The resulting nuclide is a) 214 Po b) 210 Po c) 214 Bi d) 210 Bi e) 214 Bb 11) Calculate the rate constant for the radioactive disintegration of an isotope that has a half-life of 6930 years. a) 1.00 X 10-5 yr -1 b) 1.00 X 10-4 yr -1 c) 1.00 X 10-3 yr 1 d) 1.00 X 10 3 yr 1 e) 1.00 X 10 4 yr 1 8) What happens to the nucleus of a radioactive nuclide when it emits an alpha particle? a) Atomic number decreases by one and the mass number increases by one. b) Atomic number increases by 2 and the mass number increases by 2. c) Atomic number increases by 2 and the mass number stays constant. d) Atomic number decreases by 2 and the mass number stays constant. e) Atomic number decreases by 2 and the mass number decreases by 4. 12) The half-life of a radioactive nuclide is two years. What fraction of the nuclide remains after 6 years? a) 1/6 b) 1/8 c) 1/4 d) 1/2 e) 1/16 9) What happens to the nucleus of a radioactive nuclide when it emits a beta particle? a) The atomic number increases by one and the mass number decreases by one. b) The atomic number increases by one and the mass number increases by one. c) The atomic number increases by one and the mass number stays constant. d) The atomic number decreases by one and the mass number stays constant. e) The atomic number decreases by one and the mass number increases by one.
13) 235 U has a half-life of 7.04 X10 8 years and can be used to determine the length of time since rocks have solidified. Which of the following rocks can be dated most precisely? a) lava from Mount St. Helens, less than 20 years old b) lava from the Mount Etna eruption of 700 B.C. c) lava from lunar craters, 1.5 billion years ago d) lava from Mount Vesuvius, 5 million years old e) lava from Mount Kenya, 20 million years old 15) The half-life of strontium-90 is twenty-nine years. A sample was found to have 2.25 mg of strontium-90. What mass of strontium-90 must have been present in the sample 116 years ago? a) 225 mg b) 36 mg c) 22.5 mg d) 1440 mg e) cannot be calculated from given information 14) A radioactive isotope has a half-life of 6.93 years and decays by beta emission. Determine the approximate fraction of the sample that is left undecayed at the end of 11.5 years. a) 1% b) 5% c) 30% d) 75% e) 99% 16) If 87.5 percent of a sample of pure 131 I decays in 24 days, what is the half-life of 131 I? a) 6 days b) 8 days c) 12 days d) 14 days e) 21 days
17) A 100 gram sample of pure 37 Ar decays be electron capture with a half-life of 35 days. How long will it take for 90 grams of 37 Cl to accumulate? a) 31 days b) 39 days c) 78 days d) 116 days e) 315 days 20) At the anode in a galvanic cell (voltaic cell), a) reduction takes place b) electrons are accepted c) electrons are produced d) oxidation takes place e) both (c) and (d) Questions 21-23 a) anode b) cathode c) salt bridge d) electrode e) saturated calomel electrode 21) Its symbol is a double line, // or, in a cell diagram. 22) Oxidation occurs here in a galvanic cell. 23) It is a common reference electrode. 18) 2 Fe 0 + 3 Cu 2+ 2 Fe 3+ + 3 Cu 0 This reaction was an alchemist s trick to convince greedy nobility that base metal (Fe) could be converted to gold (Cu freshly deposited does resemble gold.) If the reaction is spontaneous, the standard cell voltage (potential), E o /cell, and the standard free energy, G o, a) will both be positive b) will both be negative c) will have E o cell positive and G o negative d) will have E o cell negative and G o positive e) will both be zero 19) Which of the following cannot be used alone to predict if a reaction is spontaneous or not? a) G o b) K c c) E o cell d) H o e) S o and H o
Questions 24-28 a) Atomic mass unit, amu, or Dalton b) Frequency, ν c) Standard reduction potential, E o d) Change in the Gibbs Free Energy, G o e) Principal quantum number, n 24) Measured in volts 25) Is always negative for a spontaneous process 26) Can only have values equal to positive whole numbers 27) Defined in terms of the carbon-12 isotope 28) Measures the number of cycles per second for light waves 31) How many coulombs of charge are needed to reduce 1 mole of Ag 1+ ion to silver metal? 1 mole Ag = 108 g Ag a) 1 b) 6.02 X10 23 c) 108 d) 108 / 6.02 X10 23 e) 96500 32) An electrolysis cell operates for 45 minutes at a current of 0.15 amperes. How many coulombs are passed through the cell? a) (45)(0.15) b) (45)(60)(0.15) c) (45)(3600)(0.15) d) (45)(60) / (0.15) e) (3600)(0.15) / (45) 29) Copper is electroplated from CuSO 4 solution. A constant current of 10.00 amps is applied by an external power supply for exactly 75.0 minutes. How many grams of copper can be plated out during this time interval? The atomic mass of copper is 63.55. a) 3.67 X10-3 g b) 0.247 g c) 7.4 g d) 14.8 g e) 29.6 g 33) Molten NaCl is electrolyzed with a constant current of 1.00 ampere. What is the shortest amount of time, in seconds, that it would take to produce 1.00 mole of solid sodium? (1 faraday = 96,500 coulombs) a) 19,300 seconds b) 32,200 seconds c) 48,300 seconds d) 64,300 seconds e) 96,500 seconds 30) A large, old car bumper is going to be chrome plated. This can be accomplished using an electrolytic cell where the bumper will act as a cathode in an acidic dichromate, Cr 2 O 7 2-, solution. If the current is 20.0 amperes, how long will it take to deposit 100 grams of Cr (s) onto the bumper? The atomic mass of Cr is 52.00; 1 faraday = 96,500 coulombs. a) 15.5 h b) 1.29 days c) 309 min d) 76.3 s e) 5.3 days
34) When 1 Faraday of electrons is used in electrolysis to reduce copper (II) ion, the number of grams of elemental copper produced is approximately a) 1 b) 29 c) 63 d) 31 e) 96485 35) Which of the following is produced by the electrolysis of bauxite and cryolite? a) chlorine gas b) copper c) bromine d) aluminum e) sodium hydroxide 36) The reaction Cu + 2 Ag + 2 Ag + Cu 2+ is spontaneous. This means that a) G o for the reaction must be positive b) E o for copper must be positive c) E o for silver must be positive d) S o for the reaction must be positive e) K eq must be greater than 1.00 38) AlF 6 3- + 3 e - Al + 6 F - Which of the following occurs in the reaction? a) AlF 6 3- is reduced at the cathode. b) Al is oxidized at the anode. c) Aluminum is converted from the 3 oxidation state to the 0 oxidation state. d) F - acts as a reducing agent. e) F - is reduced at the cathode. 39) Which of the following is produced commercially by electrolysis? a) NaOH b) aluminum c) hydrogen d) chlorine e) all of these 37) Based on the following half reactions, Cl 2(g) + 2 e - 2Cl - (aq) E o = +1.36 V I 2(s) + 2 e - 2I - (aq) E o = +0.535 V Pb 2+ (aq) + 2 e - 2 Pb (s) E o = -0.126 V V 2+ (aq) + 2 e - 2 V (s) E o = -1.18 V The following is a true statement: a) The strongest oxidizing agent in the list is V 2+. b) Iodine will oxidize chloride. c) Vanadium will oxidize lead. d) Chlorine is the strongest oxidizing agent in the list. e) No galvanic cell can be constructed using these half reactions.
40) For the following reaction Zn (s) + 2 Ag + (aq) Zn 2+ (aq) + 2 Ag (s) the standard voltage E o cell has been calculated to be 1.56 volts. To decrease the voltage from the cell to 1.00 volts, one could a) increase the size of the zinc electrode b) reduce the coefficients of the reactions so that it reads ½ Zn (s) + Ag + (aq) ½ Zn 2+ (aq) + Ag (s) c) decrease the concentration of the silver ion in solution d) increase the concentration of the silver ion in solution e) decrease the concentration of the zinc ion in solution 42) In a voltaic cell, one half cell contains a cobalt electrode immersed in a 1.0 M Co 2+ solution. In the other half cell, a lead electrode is immersed in a 1.0 M Pb 2+ solution. The value for E o is 0.15 V in this cell. In a similar voltaic cell, a cobalt electrode is immersed in a 0.0010 M Co 2+ solution and a lead electrode is immersed in a 0.10 M Pb 2+ solution. Which statement below accurately describes the second voltaic cell? Co 2+ + 2 e - Co E o = -0.28 V Pb 2+ + 2 e - Pb E o = -0.13 V a) The cell will have a lower E. b) The cell will have a higher E. c) The cell will have an identical E, since E is unaffected by concentration. d) Cobalt will become the cathode. e) The cathode will dissolve. 41) Cu 2+ (aq) + Zn (s) Cu (s) + Zn 2+ (aq) A galvanic cell that uses the reaction shown above has a standard state electromotive force of 1.1 volts. Which of the following changes to the cell will increase the voltage? I. An increase in the mass of Zn (s) in the cell. II. An increase in the concentration of Cu 2+ (aq) III. in the cell. An increase in the concentration of Zn 2+ (aq) in the cell. a) I only b) II only c) III only d) I and III only e) I and III only
43) Ni (s) Ni 2+ (aq) Ag + (aq) Ag (s) Ni 2+ + 2 e - Ni (s) E o red = -0.25 volt Ag + + e - Ag (s) E o red = 0.80 volt Which of the following statements is true of the above reaction? a) The reaction is spontaneous, E o = 1.05 volts. b) The reaction is nonspontaneous, E o = -1.05 volts. c) The reaction is spontaneous, E o = -1.05 volts. d) The reaction is spontaneous, E o = 0.55 volts. e) The reaction is nonspontaneous, E o = -0.55 volts. 46) Co 2+ + 2 e - Co E o = -0.28 V Cr 3+ + e - Cr 2+ E o = -0.41 V According to the information shown above, which of the following statements is true regarding the reaction below? 2 Co + Cr 3+ 2 Co 2+ + Cr 2+ a) Cr 3+ acts as the reducing agent. b) The reaction would take place in a galvanic cell. c) K eq for the reaction is less than 1. d) Cobalt metal is reduced in the reaction. e) Chromium metal will plate out in the reaction. 44) Based on the following half reactions, I 2(s) + 2 e - 2I - (aq) E o = +0.535 V Pb 2+ (aq) + 2 e - 2 Pb (s) E o = -0.126 V V 2+ (aq) + 2 e - 2 V (s) E o = -1.18 V Cu 2+ (aq) + 2 e - Cu (s) E o = +0.340 V what is the standard voltage expected for the reaction 2 I - (aq) + Cu 2+ (aq) I 2(s) + Cu (s)? a) +0.875 V b) 0.875 V c) 0.195 V d) +0.195 V e) Standard voltage can not be determined from these values. 45) Which of the following reactions is possible at the anode of a galvanic cell? a) Zn Zn 2+ + 2 e - b) Zn 2+ + 2 e - Zn c) Ag + + e - Ag d) Mg + Ag + Mg 2+ + Ag e) Pb 2+ + 2 e - Pb 47) Mn (s) + Cu 2+ (aq) Mn 2+ (aq) + Cu (s) A voltaic cell based on the reaction above was constructed from manganese and zinc half-cells. The standard cell potential for this reaction was 1.52 volts, but the observed voltage at 25 o C was 1.66 volts. Which of the following could explain this observation? a) The Mn 2+ solution was more concentrated than the Cu 2+ solution. b) The Cu 2+ solution was more concentrated than the Mn 2+ solution. c) The manganese electrode was larger than the copper electrode. d) The copper electrode was larger than the manganese electrode. e) The atomic weight of copper is greater tan the atomic weight on manganese.
48) Ag + (aq) + e - Ag (s) E o = +0.80 V M (s) + 3 Ag + (aq) 3 Ag (s) + M 3+ (aq) E o = +2.46 V According to the information above, what is the standard reduction potential for the half-reaction M 3+ (aq) + 3 e - M (s)? a) 1.66 V b) 0.06 V c) 0.06 V d) 1.66 V e) 3.26 V
Written Questions: 1) In an electrolytic cell, a current of 0.250 ampere is passed through a solution of a chloride of iron, producing Fe (s) and Cl 2(g). a) Write the equation for the half-reaction that occurs at the anode. b) When the cell operates for 2.00 hours, 0.521 gram of iron is deposited at one electrode. Determine the formula of the chloride of iron in the original solution. c) Write the balanced equation for the overall reaction that occurs in the cell. d) How many liters of Cl 2(g), measured at 25 o C and 750 mm Hg, are produced when the cell operates as described in part (b)? e) Calculate the current that would produce chlorine gas from the solution at a rate of 3.00 grams per hour.
2) A strip of Ni metal is placed in a 1-molar solution of Ni(NO 3 ) 2 and a strip of Ag metal is placed in a 1-molar solution of AgNO 3. An electrochemical cell is created when the two solutions are connected by a salt bridge and the two metal strips are connected by wires to a voltmeter. a) Write the balanced chemical equation for the overall reaction that occurs in the cell and calculate the cell potential, E o. b) Calculate how many grams of metal will be deposited on the cathode if the cell is allowed to run at a constant current of 1.5 amperes for 8.00 minutes. c) Calculate the value of the standard free energy change, G o, for the cell reaction. d) Calculate the cell potential, E, at 25 o C for the cell shown above if the initial concentration of Ni(NO 3 ) 2 is 0.100-molar and the initial concentration of AgNO 3 is 1.20-molar.
3) Answer the following questions regarding the electrochemical cell shown below. a) Write the balanced net-ionic equation for the spontaneous reaction that occurs as the cell operates, and determine the cell voltage. b) In which direction do anions flow in the salt bridge as the cell operates? Justify your answer. c) If 10.0 ml of 3.0-molar AgNO 3 solution is added to the half-cell on the right, what will happen to (calculate) the cell voltage? Explain. d) If 1.0 gram of solid NaCl is added to each half-cell, what will happen to the cell voltage? Explain. e) If 20.0 ml of distilled water is added to both half-cells, the cell voltage decreases. Explain.
4) A galvanic cell is set up assuming that an iron electrode dipping into a Fe 2+ (aq) solution is the anode and that a platinum electrode dipping into a solution containing Co 2+ (aq) and Co 3+ (aq) ions is the cathode. a) Write the two balanced half-reactions and the complete balanced chemical reaction that is being studied using these assumptions. b) Under standard conditions show whether or not this reaction is spontaneous. c) What is the standard free energy change, G o, for this reaction at 20 o C? d) If the iron (II) concentration is 0.200 M, what is the [Co 3+ ] / [Co 2+ ] ratio when the cell voltage is 2.00 V in this galvanic cell?
5) Sr (s) + Mg 2+ Sr 2+ + Mg (s) Consider the reaction represented above that occurs at 25 o C. All reactants and products are in their standard states. The value of the equilibrium constant, K eq, for the reaction is 4.2 X10 17 at 25 o C. a) Predict the sign of the standard cell potential, E o, for a cell based on the reaction. Explain your prediction. b) Identify the oxidizing agent for the spontaneous reaction. c) If the reaction were carried out at 60 o C instead of 25 o C, how would the cell potential change? Justify your answer. d) How would the cell potential change, if the reaction were carried out at 25 o C with a 1.0-molar solution of Mg(NO 3 ) 2 and a 0.10-molar solution of Sr(NO 3 ) 2? Explain. e) When the cell reaction in (d) reaches equilibrium, what is the cell potential?
6) Neon-23 is an unstable nuclide with a half-life of 38 seconds. a) What form of decay would neon-23 be expected to undergo? Write the balanced nuclear reaction for that process. b) Explain why the nucleus of a neon-23 atom weighs less than the sum of its constituent protons and neutrons. c) Describe the change that a sample of neon-23 undergoes in 38 seconds. d) Describe alpha, beta (β-), and gamma radiation in terms of mass and charge.
7) Answer each of the following questions regarding radioactivity. a) Write the nuclear equation for decay of 239 Pu by alpha emission. b) Account for the fact that the total mass of the products of the reaction in part (a) is slightly less than that of the original 239 Pu. c) Describe how α, β, and γ rays each behave when they pass through an electric field. Use the diagram below to illustrate your answer. d) Why is it not possible to eliminate the hazard of nuclear waste by the process of incineration?