CHM3120_Exam 2 A

1 Name: CHM 3120 Exam 2 (2023) Form Code A Instructions: When complete, return the exam, scrap paper, and scantron. The exam with solutions will be posted prior to make-up exams. Bubbling errors of any kind will count as an incorrect response. 1 . Balance the following redox reaction in an ACIDIC solution using the smallest possible whole numbers: CN - (aq) + MnO 4 - (aq) → MnO 2 (s) + CNO - (aq) Which of the following statements is TRUE for the balanced reaction? (A) 2 moles of OH - (aq) are produced. (B) 2 moles of H + (aq) are consumed. (C) 2 moles of CNO - (aq) are produced. (D) 4 moles of CN - (aq) react with 1 mole of MnO 4 - (aq). (E) 3 moles of MnO 4 - (aq) are consumed. 2. The current from an unknown sample of Mg 2+ was 1.48 μA in an amperometry measurement. Then, 1.500 mL of a 10.5 mM Mg 2+ solution was added to 45.0 mL of the unknown solution. The current increased to 3.96 μA. Find the concentration of Mg 2+ in the unknown. (A) 0.1 mM (B) 1 mM (C) 0.2 mM (D) 2 mM (E) 0.02 M 3 . Which statements are TRUE about electrochemical cells? (1) In a galvanic cell the cell reaction is non-spontaneous. (2) In a galvanic cell oxidation occurs at the anode. (3) A battery is an electrolytic cell. (4) Electrons flow from cathode to anode. (5) None of the above are true. (A) (5) (B) (2) (C) (3) (D) (1)(4) (E) (1)(2) 4 . An Ag(s) electrode (area = 0.15 cm 2 ) is immersed into a solution that is 10 -3 M in Ag + . The E o for the reduction of Ag + is 0.799 vs. SHE. Assume a voltage of 0.699 vs. SHE is applied. If the flux of Ag + to the electrode is 5x10 -4 mole/(s cm 2 ), what is the current? (A) 7.24 A (B) 0.777 nA (C) 48.3 A (D) 7.77 mA (E) 52.1  A 5 . Which statements are TRUE about redox reactions? (1) Only one type of redox reaction (either an oxidation or a reduction) occurs in an electrochemical cell. (2) Redox reactions involve proton transfer across the electrode/solution interface. (3) Redox reactions cause ions to move through an electrochemical cell. (4) The agreed-upon convention is to write all half-reactions as reductions. (5) Multiplying a half-reaction by any number does not change the standard reduction potential. (A) (4)(5) (B) (1)(3) (C) (2)(3)(5) (D) (4) (E) (3)(4)(5) 6 . A voltaic cell involves the following half-reactions: Cd 2+ ( aq ) + 2e - Cd( s ) (E° = − 0.402 V) Ag + ( aq ) + e - Ag( s ) (E° = 0.7993 V) Assuming there is a salt bridge, what is the correct shorthand notation for this cell? (A) Ag( s ) | Ag + ( aq ) || Cd 2+ ( aq ) | Cd( s ) (B) Ag + ( aq ) | Ag( s ) || Cd( s ) | Cd 2+ ( aq ) (C) Ag( s ) | Ag + ( aq ) || Cd( s ) | Cd 2+ ( aq ) (D) Cd 2+ ( aq ) | Ag + ( aq ) || Ag( s ) | Cd( s ) (E) Cd( s ) | Cd 2+ ( aq ) || Ag + ( aq ) | Ag( s ) 7 . Which statements are TRUE about the Clark oxygen sensor? (1) It requires no enzymes. (2) It can be used in whole blood. (3) It only measures free oxygen in blood. (4) The membrane coating the Pt electrode is highly impermeable to O 2 . (5) It requires no external reference electrode. (A) (1)(2)(3)(5) (B) (2)(3)(4) (C) (1)(3)(4)(5) (D) (1)(2)(3) (E) All of the above 8 . An electrochemical cell used the following redox reactions: Pb 2+ ( aq ) + 2e - Pb( s ) (E° = − 0.126 V) PbSO 4 ( s ) + 2e - Pb( s )+ SO 4 2- ( aq ) (E° = − 0.355 V) If the measured cell voltage is 0.105 V and [SO 4 2- ] = 0.16 M, what is the molar concentration of Pb 2+ ? (A) 2.6 x 10 -2 (B) 8.4 × 10 −5 (C) 4.0 x 10 -4 (D) 1.4 x 10 -3 (E) 3.8 x 10 -3