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Electrode Potentials and Predicting Redox Reactions in Electrochemical Cells

This guide explores standard electrode potentials (E°) and their role in predicting electrochemical reactions. By analyzing the reduction potentials of half-reactions (e.g., Mg2+/Mg, Au3+/Au, Ni2+/Ni), we can determine the spontaneity of reactions in cells. The methodology includes writing half-reactions, reversing electrode potentials for oxidation, and calculating Ecell to assess whether reactions occur. Using specific examples, such as the precipitation of sulfur from H2S and interaction with NiSO4, we demonstrate how to apply these principles for accurate predictions.

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Electrode Potentials and Predicting Redox Reactions in Electrochemical Cells

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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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