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Electrochemistry Free-Response Questions for AP Chemistry

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By — McGraw-Hill Professional
Updated on Feb 2, 2011

Review the following concepts if necessary:

Questions

First Free-Response Question

Answer the following question in 15 minutes. You may not use a calculator. You may use the tables at the back of the book.

Free-Response Questions

The above galvanic cell is constructed with a cobalt electrode in a 1.0 M Co(NO3)2 solution in the left compartment, and a silver electrode in a 1.0 M AgNO3 solution in the right compartment. The salt bridge contains a KNO3 solution. The cell voltage is positive.

  1. What is the balanced net ionic equation for the reaction, and what is the cell potential?
  2. Which electrode is the anode? Justify your answer.
  3. Could KCl be substituted for the KNO3 in the salt bridge? Justify your answer.
  4. If some solid Co(NO3)2 is added to the cobalt compartment, what will happen to the voltage? Justify your answer.
  5. If the cell is allowed to operate until equilibrium is established, what will the potential be? Justify your answer.

Second Free-Response Question

You have 15 minutes to answer the following questions. You may not use a calculator. You may use the tables in the back of the book.

Free-Response Questions

The above galvanic cell is constructed with a cadmium electrode in a 1.0 M Cd(NO3)2 solution in the left compartment, and a silver electrode in a 1.0 M AgNO3 solution in the right compartment. The salt bridge contains a KNO3 solution. The cell voltage is positive.

  1. What is the balanced net ionic equation for the reaction, and what is the cell potential?
  2. Show how to calculate the equilibrium constant for the cell.
  3. Write the expression for Q that would be needed in the Nernst equation. Explain why any substances from the net ionic equation do not appear in Q.
  4. Show how to calculate the free energy for the reaction.
  5. Identify the anode, the cathode, the oxidizing agent, and the reducing agent.

Answers And Explanations

First Free-Response Question

  1. The cell reaction is:
  2. Give yourself 1 point if you got this correct. The physical states are not necessary.

    The calculation of the cell potential may be done in different ways. Here is one method:

    Give yourself 1 point for the correct answer regardless of the method used. The most common mistake is to multiply the silver voltage by two. You do not get the point for an answer of 1 V.

    The half-reactions and their standard reductions potentials are supplied on the exam, not in the problem as given here. You will be expected to find the appropriate half-reactions in a table.

  3. The cobalt is the anode. You get 1 point for this statement.
  4. The reason Co is the anode is because the Co is oxidized. You get 1 point for this statement, or if you say the Co loses electrons.

  5. KCl cannot be substituted for KNO3 in this case. This is worth 1 point.
  6. The Cl ion from the KCl would react with the silver ion to give insoluble AgCl. Give yourself 1 point for this explanation.

  7. The voltage would decrease. Give yourself 1 point for this answer.
  8. The excess Co2+, from the Co(NO3)2, would make the cell nonstandard. The Nernst equation should be used to calculate the new voltage. The concentration of the cobalt ions appears in the numerator of the logarithm term of the Nernst equation. This makes the logarithm term more negative, yielding a lower voltage. Give yourself 1 point for any answer relating to this. It might be helpful if you wrote out the Nernst equation for this cell.

  9. At equilibrium the cell voltage would be 0 V. This is worth 1 point. At equilibrium no work is done, so the potential must be zero. Give yourself 1 point for this answer.
  10. There are a total of 10 points possible on these questions.

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