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Electrochemistry Review

1989 B

The electrolysis of an aqueous solution of potassium iodide, KI, results in the formation of hydrogen gas at the cathode and iodine at the anode. A sample of 80.0 millilitres of a 0.150 molar solution of KI was electrolyzed for 3.00 minutes, using a constant current. At the end of this time, the I2 produced was titrated against a 0.225 molar solution of sodium thiosulfate, which reacts with iodine according to the equation below. The end point of the titration was reached when 37.3 millilitres of the Na2S2O3 solution had been added.

I2 + 2 S2O32 2 I + S4O62

(a)How many moles of I2 was produced during the electrolysis?

(b)The hydrogen gas produced at the cathode during the electrolysis was collected over water at 25C at a total pressure of 752 millimetres of mercury. Determine the volume of hydrogen collected. (The vapor pressure of water at 25C is 24 millimetres of mercury.)

(c)Write the equation for the halfreaction that occurs at the anode during the electrolysis.

(d)Calculate the current used during the electrolysis.

1987 D

A dilute solution of sodium sulfate, Na2SO4, was electrolyzed using inert platinum electrodes. In a separate experiment, a concentrated solution of sodium chloride, NaCl, was electrolyzed also using inert platinum electrodes. In each experiment, gas formation was observed at both electrodes.

(a)Explain why metallic sodium is not formed in either experiment.

(b)Write balanced equations for the half-reactions that occur at the electrodes during electrolysis of the dilute sodium sulfate solution. Clearly indicate which half-reaction occurs at each electrode.

(c)Write balanced equations for the halfreactions that occur at the electrodes during electrolysis of the concentrated sodium chloride solution. Clearly indicate which halfreaction occurs at each electrode.

(d)Select two of the gases obtained in these experiments, and for each gas, indicate one experimental procedure that can be used to identify it.

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  1. Consider an electrochemical cell formed from a Cu(s) electrode submerged in an aqueous Cu(NO3)2 solution and a Cd(s) electrode submerged in a Cd(NO3)2 solution. The two electodes are connected by a wire and the two solutions are connected by a salt bridge containing NaNO3(aq). The following reaction takes place:

Cu2+ (aq) + Cd(s)  Cu(s) + Cd2+(aq)

Which of the following statements is true:

  1. electrons will flow from Cu(s) to Cd(s)
  2. electrons will flow from Cd(s) to Cu(s)
  3. nitrate will flow from the copper cell to thecadmium cell
  4. the cadmium electrode will lose mass
  5. The copper electrode will lose mass
  1. Consider an electrochemical cell where the following reaction takes place:

2Au3+ (aq) + 3Zn(s)  2Au(s) + 3An2+(aq)

Which of the following is the correct cell notation for this cell?

  1. Au│Au3+║Zn2+│Zn
  2. Au3+│Au║Zn│Zn2+
  3. Zn│Zn2+║ Au3+│Au
  4. Zn│ Au3+║Au│Zn2+
  1. Eo for the following redox reaction is 1.05 V.

2Ag+(aq) + Ni(s)  2Ag(s) + Ni2+(aq)

What is ∆Go for this reaction?

  1. +203 KJC. +101 KJ
  2. -203 KJD. -101 KJ
  1. The following redox reaction can be used to form iodine with ∆Go = -105 KJ.

Br2(aq) + 2I-(aq)  2Br-(aq) + I2(aq)

What is Eo for this reaction?

  1. +1.09 VC. +0.0011 V
  2. -1.09 VD. +0.545 V
  1. Given the two following half reactions:

Ni2+(aq) + 2e-  Ni(s)Eo = -0.25 V

Hg2+(aq) + 2e-  Hg(l)Eo = +0.86 V

Calculate Eo for the following reaction:

Hg2+(aq) + Ni(s)  Ni2+(aq) + Hg(l)

  1. -1.11 VC. +1.11 V
  2. +0.61 VD. +0.86 V
  1. An electrochemical cell of notation Pd│Pd2+║Cu2+│Cu has Eo = -0.65 V. If we know that the standard reduction potential of Cu2+/Cu is 0.34 V, what is the standard reduction potential for Pd2+/Pd?
  2. -0.99 VC. +0.31 V
  3. -0.31VD. +0.99 V
  4. Given the following two half reactions:

Ca2+(aq) + 2e- Ca(s)Eo = -2.87 V

V2+(aq) + 2e-  V(s)Eo = -1.21 V

Determine the Eo and the spontaneity of the following reaction.

Ca2+ + V  Ca + V2+

  1. -1.66 V, not spontaneous
  2. -1.66 V, spontaneous
  3. -4.08 V, not spontaneous
  4. +4.08 V, spontaneous
  1. Determine the cell potential for

Ni(s) + Fe2+(aq)  Ni2+(aq) + Fe(s)

When [Ni2+] = 0.60 M and [Fe2+] = 0.0030 M using the following standard reduction potentials: Ni2+/Ni Eo = -0.25 V Fe2+/Fe Eo = -0.44V

  1. +0.76 VC. -0.26 V
  2. -0.19 VD. -0.12 V
  1. Determine the equilibrium constant for the following reaction at 25oC:

Sn2+(aq) + V(s)  Sn(s) + V2+(aq), Eo = +1.07V

  1. 1.41 x 1036C. -1.07
  2. 7.10 x 10-27D. 1.19 x 1018
  1. How many coulombs of charge are required to deposit 1.00 g Ag from a solution of Ag+(aq)?
  2. 9.27 x 10-3C. 1790
  3. 894 D. 1.00
  4. If we wish to convert 1.00 g of Au3+(aq) into Au(s) in a “gold-plating” process, how long must we electrolyze a solution if the current passing through the circuit is 2.00 amps?
  5. 254 sC. 0.015 s
  6. 0.50 sD. 735 s
  7. If we electrolyze a solution of Ni2+(aq) to form Ni(s) and use a current of 0.15 amps for 10 minutes, how many grams of Ni(s) are produced?
  8. 0.027 gC. 4.56 x 10-4 g
  9. 0.054 gD. 5280 g
  10. Given the following half-reactions:

Ag+(aq) + e-  Ag(s)Eo = +0.799 V

AgCl(s) + e-  Ag(s) + Cl-(aq)Eo = +0.222 V

Calculate the molar solubility of AgCl

  1. 4.46 x 10-9 MC. 1.79 x 10-10 M
  2. 1.34 x 10-5 MD. -0.577 M
  1. An electrochemical cell is constructed with two Ni(s) electrodes, each in a cell containing Ni2+(aq). Electrons flow through the wire connecting the electrodes from the left hand cell to the right hand cell. What can be said about the overall cell?
  2. [Ni2+] right cell > [Ni2+] left cell
  3. [Ni2+] right cell < [Ni2+] left cell
  4. no magnetic fields penetrate the walls of the cell.
  5. Ni2+(aq) is more easily oxidized than Ni(s)

Consider the following half reactions for 15-20:

Half rxn / Eo(V)
A. / F2(g) + 2e-  2F-(aq) / +2.87
B. / I2(g) + 2e-  2I-(aq) / +0.56
C. / Cu2+(aq) + 2e-  Cu(s) / +0.34
D. / Al3+(aq) + 3e-  Al(s) / -1.66
  1. Which is the strongest oxidizing agent? A
  2. Which is the strongest reducing agent? D
  3. Will Al(s) reduce Cu2+(aq) to Cu(s)? yes
  4. Will F-(aq) reduce I2(aq) to I-(aq) no
  5. Name the elements or ions that can be reduced by Cu(s) I2 & F2
  6. If you build a battery using the I2/I- and Al3+/Al half reactions, what is the voltage of the battery (assuming standard conditions)?
  7. +1.66 VC. +2.20 V
  8. +0.56 VC. -2.20 V

For questions 21-25: A galvanic cell is setup based on the following half-reactions:

Ni2+(aq) + 2e-  Ni(s)Eo = -0.25 V

Cu2+(aq) + 2e-  Cu(s)Eo = +0.34 V

  1. Write a balanced equation depicting the reaction occurring in the cell and determine Eo for the cell.

Cu2+ + Ni  Ni2+ + Cu

  1. What is oxidized?Ni(s)
  2. What is the anode? Ni(s)
  3. What is the polarity (sign) of the Cu electrode? Pos.
  4. What is the direction of flow in the external wire?
  5. Cu to NiB. Ni to Cu