A student assembles a cell by immersing a zinc strip in 1.0 M ZnSO₄ solution and a…
CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry
Problem
A student assembles a cell by immersing a zinc strip in 1.0 M ZnSO₄ solution and a copper strip in 1.0 M CuSO₄ solution. The two half-cells are connected by a salt bridge containing KNO₃(aq), and the metals are joined externally through a voltmeter.
After allowing the cell to operate for 20 min, the student disconnects the voltmeter and instead connects the two metal strips to a DC power supply so that the copper strip is attached to the negative terminal and the zinc strip is attached to the positive terminal. The magnitude of the applied potential is adjusted until a steady current is observed.
Answer the following questions about the two operating modes of this apparatus.
(a) Identify the direction of electron flow through the external circuit when the apparatus functions as the voltaic cell described in the first paragraph. Justify your answer in terms of relative electrode potentials.
(b) Describe the observable change in mass, if any, of each metal strip after 20 min of voltaic operation. Explain the chemical processes responsible for these changes.
(c) Once the external power supply is connected, does the zinc strip undergo oxidation, reduction, or neither? Support your answer with a discussion of polarity and electron flow in the second operating mode.
(d) Compare the energy conversions occurring during the voltaic operation and the electrolytic operation of this apparatus. In your comparison, explicitly state the form of energy entering and leaving the cell in each mode.