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Electrode potentials, E cell and predicting reactions

Electrode potentials, E cell and predicting reactions. Electrode potentials. Standard E° values are written as reduction potentials: E° (Mg 2+ /Mg) = –2.36 V relates to the equation Mg 2+ (aq) + 2e – → Mg(s)

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Electrode potentials, E cell and predicting reactions

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  1. Electrode potentials, Ecell and predicting reactions

  2. Electrode potentials Standard E° values are written as reduction potentials: E°(Mg2+/Mg) = –2.36 V relates to the equation Mg2+(aq) + 2e–→ Mg(s) — in other words the reaction goes from the species on the left (Mg2+) to the species on the right (Mg). If the reaction occurring in this half-cell is oxidation, then the sign of the electrode potential must be reversed: E°(Mg/Mg2+) = +2.36 V

  3. E°(Au3+, Au+) = 1.41 V E°(U4+, U3+) = –0.61 V E°(PbO2/Pb2+) = 1.46 V Au+(aq) → Au3+(aq) + 2e–E° = PbO2(s) + 4H+(aq) + 2e– → Pb2+(aq) E° = U3+(aq) → U4+(aq) + e– E° = U4+(aq) + e– → U3+(aq) E° = –1.41 V 1.46 V 0.61 V –0.61 V

  4. Predicting reactions Will sulfur precipitate when H2S gas is bubbled through NiSO4 solution? E°(S/H2S) = 0.17 V, E°(Ni2+/Ni) = -0.23 V 1 Write equations for the proposed reaction: H2S(g) → S(s) + 2H+(aq) + 2e– Ni2+(aq) + 2e– → Ni(s) 2 Add the E° values, reversing the sign for the cell where oxidation occurs. 3 Add up the E° values to find the E°(cell) –0.17 V + –0.23 V = –0.40 V 4 Use the sign of the E°(cell) to determine whether the reaction occurs. Since E°(cell) is negative, the reaction does not occur. E° = –0.17 V E° = –0.23 V

  5. While it is safer to write out the complete half-equations, it isn’t strictly necessary just to find Ecell. Simply put the reduction potentials into this equation: Ecell = ERed – EOx Remember ‘Red – ox’ Will sulfur precipitate when H2S gas is bubbled through NiSO4 solution? E°(S/H2S) = 0.17 V, E°(Ni2+/Ni) = -0.23 V 0.17 V -0.23 V red ox SoEcell = ERed – EOx Ecell = -0.23 V – 0.17 V = -0.40 V Since the Ecell is negative, the reaction is not spontaneous: sulfur is not precipitated.

  6. Calculating Ecell from a cell diagram For cells written using the IUPAC cell convention, E°cell = E°RHE – E°LHE RHE = right hand electrode, LHE = left hand electrode What is the Ecell for the cell below? C(s) / C2O42–(aq)/CO2(g) // Cr2O72–(aq) , Cr3+(aq) /C(s) E°(CO2/C2O42–) = –0.20 V E°(Cr2O72–,Cr3+) = 1.33 V E°cell = E°RHE– E°LHE = 1.33 V – -0.20 V = 1.53 V Do not reverse the polarity of the left hand (oxidation) electrode – that’s done when we subtract it.

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