PROBLEM SET 11

# PROBLEM SET 11 - NERNST EQUATION Electrochemical cells The...

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MSE 110 - University of Arizona Electrochemical cells The driving force of the redox reacion Δ G and therefore the cell potential Δ E o decrease has the cell discharges and the reactants are consumed. NERNST EQUATION ] [Cu ] [Zn ln RT G G 2 2 o + + + Δ = Δ ] [ ] [ ln RT E E 2 2 o + + Δ = Δ Cu Zn nF Δ G=-nF Δ E Cu 2+ + Zn ' Zn 2+ + Cu

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MSE 110 - University of Arizona Problem 1 What is the number of electrons “n” transferred in the reaction between the two couple [Cl 2 /Cl - ] and [Mg/Mg 2+ ]. Ans: Cl 2 + 2e - Æ 2Cl - Mg Æ Mg 2+ + 2e - --------------------- Mg + Cl 2 Æ 2Cl - + Mg 2+ 2 electrons are being transferred in the total redox reaction: n=2
MSE 110 - University of Arizona Problem 2 What is the number of electrons “n” transferred in the reaction between the two couples [Pb 2+ /Pb 4+ ] and [Fe 2+ /Fe 3+ ] Ans: Pb 4+ + 2 e - Æ Pb 2+ Fe 2+ Æ Fe 3+ + e - x2 ------------------------------------ Pb 4+ + 2 Fe 2+ Æ Pb 2+ + 2 Fe 3+ 2 electrons are being transferred in the total redox reaction: n=2 .

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MSE 110 - University of Arizona Problem 3 An electrochemical cell is built with the redox couple E o [Al/Al 3+ ] = -1.66 V and E o [La/La 3+ ] = -2.55 V. The concentration of ions in the cell is [Al 3+ ]= 2 moles and and [La 3+ ]= 0.5 moles. What is the initial voltage of the cell? [answer in Volts] Ans: The cell voltage is given by the Nernst equation The reaction is La + Al 3+ Æ Al + La 3+ and the number of electrons transferred is n=3 La/La 3+ is the reductant and Al/Al 3+ is the oxidant Hence ] [ ] [Re ln RT E E o Ox d nF Δ = Δ V Al La nF 9 . 0 2 5 . 0 ln 3 0257 . 0 ) 55 . 2 ( 66 . 1 ] [ ] [ ln RT E E 3 3 o = = Δ = Δ + +
- University of Arizona Problem 4 What is the voltage of the electrochemical cell of problem 3 with ions concentrations of [Al 3+ ]= 6 moles and and [La 3+ ]= 0.001 moles. Ans: The cell voltage is given by the Nernst equation The reaction is La + Al 3+ Æ Al + La 3+ and the number of electrons transferred is n=3 La/La 3+ is the reductant and Al/Al 3+ is the oxidant Hence ] [ ] [Re ln RT E E o Ox

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PROBLEM SET 11 - NERNST EQUATION Electrochemical cells The...

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