Cliffs AP Chemistry, 3rd Edition

Samples: Free-Response Questions

1. A student places a copper electrode in a 1 M solution of CuSO 4 and in another beaker
   places a silver electrode in a 1 M solution of AgNO 3. A salt bridge composed of Na 2 SO 4
   connects the two beakers. The voltage measured across the electrodes is found to be
   +0.42 volt.

(a) Draw a diagram of this cell.

(b) Describe what is happening at the cathode. (Include any equations that may be

useful.)

(c) Describe what is happening at the anode. (Include any equations that may be useful.)

(d) Write the balanced overall cell equation.

(e) Write the standard cell notation.

(f) The student adds 4 M ammonia to the copper sulfate solution, producing the complex

ion Cu NH^ 3 h 42 +_aqi. The student remeasures the cell potential and discovers the

voltage to be 0.88 volt at 25°C. What is the Cu2+(aq)concentration in the cell after the

ammonia has been added?

Answer

1. Given: Cu electrode in 1 M CuSO 4.
   Ag electrode in 1 M AgNO 3.
   Voltage = 0.42 volt.
   (a) Restatement: Diagram the cell.

(b) Restatement: What happens at the cathode?

Reduction always occurs at the cathode. Note that E°redfor silver is +0.7991 volt,

according to the Table of Standard Reduction Potentials. E°redfor copper is +0.337. This

means that the copper metal is higher in the activity series than the silver metal, so copper

metal will reduce the silver ion. The equation that describes reduction (or the cathode

reaction) is therefore

red: Ag+(aq)+ e–→Ag(s)

copper Na+(aq) SO^42 – (aq)

electrode

(−)

e- .42v e-

CuSO 4

solution

silver

electrode

(+)

AgNO 3

solution

Reduction and Oxidation