Homework Set #2
Due date: Monday, Sep. 25 at 11:59pm
ME/MSE/ChBE/CHEM 4759, Fall 2023
Instructor: Dr. Matthew McDowell
1. Develop an expression for the equilibrium voltage for the cell shown below.
𝐹𝑒 + 2𝑂𝐻! → 𝐹𝑒(𝑂𝐻)" + 2𝑒 !
𝑂" + 4𝑒 ! + 2𝐻" 𝑂 → 4𝑂𝐻!
The Gibbs energy of formation for Fe(OH)2 is -486.6 kJ/mol.
2. Consider the electrochemical cell below.
In this cell, the solid platinum regions are the electrodes, and the gases are bubbled within
the aqueous HCl to be in contact with the electrodes. The two reactions are:
𝐻" → 2𝐻# + 2𝑒 !
𝐶𝑙" + 2𝑒 ! → 2𝐶𝑙 !
Find an expression for the cell voltage U. If the pressure of hydrogen is 250 kPa and that of
chlorine is 150 kPa, what is the numerical value of U at 25 °C in 1 molal HCl?
3. LiPF6 (lithium hexafluorophosphate) is a common salt used in lithium batteries. For this salt
dissolved in propylene carbonate solvent, calculate the Debye length using Debye-Huckel
theory for LiPF6 at 30 °C. The concentration is 0.1 M. The dielectric constant, er = e/eo, for
the solvent is 64 and the density is 1.205 g/cm3. The density of the 0.1 M electrolyte is 1.286
g/cm3.
4. A solid-oxide fuel cell operates at 1000 °C. The overall reaction is:
0.5𝑂" + 𝐻" ↔ 𝐻" 𝑂
a) Calculate the standard cell voltage at 25 °C assuming the reactants and products are
gases.
b) Calculate the standard cell voltage at 1000 °C using the equation learned in class that
relates cell voltage to changes in temperature. State your assumptions.
5. Assume you are trying to design an electrochemical cell that contains a silver metal positive
electrode. Answer the following questions.
a) If you are using an aqueous electrolyte, which negative electrode is more appropriate to
pair with the silver electrode for a stable electrochemical cell: magnesium or zinc?
Explain your answer.
b) Now assume you use Al metal as the negative electrode in an aqueous electrolyte.
Based on thermodynamics alone, what do you expect to occur in the cell under open
circuit conditions?
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