Electrode potentials, Ecell and predicting reactions

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

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

Predicting reactions

Will sulfur precipitate when H2S gas is bubbled through NiSO4 solution? E°(S/H2S) = 0.17 V, E°(Ni2+/Ni) = -0.23 V1 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 VE° = –0.23 V

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 – EOxRemember ‘Red – ox’

So Ecell = 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.

Will sulfur precipitate when H2S gas is bubbled through NiSO4 solution? E°(S/H2S) = 0.17 V, E°(Ni2+/Ni) = -0.23 V ox red

-0.23 V0.17 V

For cells written using the IUPAC cell convention,

E°cell = E°RHE – E°LHE

What is the Ecell for the cell below?

C(s) / C2O42–(aq)/CO2(g) // Cr2O7

2–(aq) , Cr3+(aq) /C(s)

E°(CO2/C2O42–) = –0.20 V E°(Cr2O7

2–,Cr3+) = 1.33 V

E°cell = E°RHE – E°LHE

= 1.33 V – -0.20 V

= 1.53 V

RHE = right hand electrode,

LHE = left hand electrode

Do not reverse the polarity of the left hand (oxidation) electrode – that’s done when we subtract it.

Calculating Ecell from a cell diagram