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Electrochemistry: Basic Concepts

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    283158
  • A REDOX Example

    Equation_Redox.png

    1. One would predict a spontaneous reaction between….
    2. What is the coefficient in front of Fe2+ in the balanced reaction?
    3. What is E° for the total reaction?

    ΔG, E and K Example

    Eo for the following reaction (as written) is 0.736 volts. Calculate ΔG and K for the reaction.

    \[\ce{MnO4- + 8H+ + 5Fe^2+  ⇌  Mn^2+ + 4H2O + 5Fe^3+}\nonumber\]

    Calculating Potential of a Cell (Nernst equations)

    Calculate the potential for the following cell:

    \[\mathrm{Pt \mid Sn^{2+}(0.400\: M),\: Sn^{4+}(0.500\: M) \mid\mid NO_3^-(0.100\: M),\: H^+(pH=2.00),\: HNO_2(0.400\: M) \mid Pt}\nonumber\]

    Potentiometry – Cells (line notation with REF)

    Calculate the cell potential of the following cell:

    Equation_Potentiometry.png

    Bringing in Ksp

    Think about the previous problem…

    Calculate the cell potential of the following cell:

    \[\mathrm{Ag \mid AgCl_{(s)} \mid AgCl_{(aq)}(sat’d),\: Cl^- (1.00\: M) \mid\mid \textrm{reduction half cell} \mid Pt}\nonumber\]

    Couldn’t one use the Ag+ + e- ⇌ Ag(s) half cell at Eo = 0.799?

    “Adding” Reactions

    When you “add” chemical reactions, you multiply the K values and/or add the E values.

    Given the following Eo values:

    \[\ce{Ag+ + e- ⇌ Ag_{(s)}} \textrm{ (0.799 V vs SHE) and}\nonumber\]

    \[\ce{AgCl_{(s)} + e- ⇌ Ag_{(s)} + Cl-} \textrm{ (0.222 V vs SHE)}\nonumber\]

    Calculate the Ksp for AgCl.

     

    “Mixing” Problems

    30.00 mL of 0.120 F NaNO3 is mixed with 20.00 mL of 0.150 F SnCl2 and the solutions is buffered at pH=2.00. (NOTE: no gas or solid is observed)

    1. Calculate Eo and K for the spontaneous reaction that will occur.
    2. Calculate the solution potential of the resulting solution.

    Potentiometry Problems (mixing, then measuring)

    Calculate the cell potential, as measured at a Pt/SCE electrode pair, for a solution prepared by mixing 30.00 mL of 0.120 F NaNO3 and 20.00 mL of 0.150 F SnCl2. The solution is buffered at pH=2.00. (except for SCE, this is the same problem as the previous exercise)

    Potentiometry Problems, II (mixing, then measuring, EP)

    Calculate the cell potential, as measured at a Pt/SCE electrode pair, for a solution prepared by mixing 25.00 mL of 0.120 F NaNO3 and 20.00 mL of 0.150 F SnCl2. The solution is buffered at pH=2.00.

    KEY: Reactant conc’ns are “Not really zero, just really small….AND in a defined ratio” …as indicated by coefficients in balanced eqn!

    Formal Potential (Eo’)

    Derive an equation for the pH-dependent formal potential for the dichromate/ chromium III half reaction, and calculate the Eo’ at pH 7.00.

    \[\ce{Cr2O7^2- + 14H+ + 6e- ⇌ 2Cr^3+ + 7H2O} \hspace{30px} \mathrm{E^o} = \textrm{1.33 V vs SHE}\nonumber\]

    \[E^{0'}_{Cr_2O_7^{2-}/Cr^{3+}} = E^0_{Cr_2O_7^{2-}/Cr^{3+}} - \dfrac{0.0591}{6} \log\left(\dfrac{1}{[H^+]^{14}}\right)\nonumber\]

    \[E^{0'}_{Cr_2O_7^{2-}/Cr^{3+}} = E^0_{Cr_2O_7^{2-}/Cr^{3+}} - \dfrac{0.0591(14)}{6} pH\nonumber\]

     

    \[\mathrm{E^{o’}}\textrm{ at pH 7.00 = 0.365 V vs SHE}\nonumber\]

    Graph_FormalPotential.png

    Redox Titration

    Consider the titration of 50.00 mL of 0.100 M Cr2O72- with 0.300 M Fe2+ as monitored with a Pt/SCE electrode pair...calculate the measured cell potential and construct the expected titration plot. The pH is maintained at 0.00.

    \[\ce{Cr2O7^2- + 14H+ + 6e- ⇌ 2Cr^3+ + 7H2O} \hspace{30px} \mathrm{E^o} = \textrm{1.33 V vs SHE}\nonumber\]

    \[\ce{Fe^3+  + e- ⇌ Fe^2+} \hspace{30px} \mathrm{E^o} = \textrm{0.771 V vs SHE}\nonumber\]

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