2012-07-23

Redox reactions


We looked at a few redox reactions today in class, some of you wanted the solutions posted. Enjoy.

For each pair of half cells, write the balanced spontaneous (standard) reaction and calculate the spontaneous (standard) cell voltage. {Actually, you'd do the secondpart of that process before you could do the first part}
Cu+1|Cu (E0red= +0.521V) and Fe3+|Fe2+(E0red= +0.771V)
Cu half must be reversed to be the oxidation half rxn
E0cell = E0red+ E0ox = 0.771V + (-0.521V) = +0.250V
Cu(s) ⇄ Cu+1(aq) + 1e(aq)
1e + Fe3+(aq) ⇄ Fe2+(aq)
------------------------------------------------------------------
Cu(s) + Fe3+(aq) ⇄ Fe2+(aq) + Cu+1(aq)

BrO3-1|Br2(E0red= +1.478V) and Pt2+|Pt (E0red= +1.188V)
Pt half must be reverse to be the oxidation half rxn
E0cell = E0red+ E0ox = 1.478V + (-1.188V) = +0.290V
5 { Pt(s) ⇄ Pt+2(aq) + 2e(aq) }
12 H(aq) + 10e(aq) + 2 BrO3-1(aq) ⇄ Br2(l) + 6 H2O(l)
------------------------------------------------------------------
12 H(aq) + 5 Pt(s) + 2 BrO3-1(aq) ⇄ Br2(l) + 5 Pt+2(aq) + 6 H2O(l)

Cr+2|Cr (E0red= -0.403V) and Pb+4|Pb+2(E0red= +1.69V)
Cr half must be reverse to be the oxidation half rxn
E0cell = E0red+ E0ox = 1.69V + (0.403V) = +2.09V
Cr(s) ⇄ Cr+2(aq) + 2e(aq)
2e + Pb+4(aq) ⇄ Pb+2(aq)
------------------------------------------------------------------
Cr(s) + Pb+4(aq) ⇄ Pb2+(aq) + Cr+2(aq)

TeO2|Te (E0red= +0.604V) and Se|H2Se (E0red= -0.115V)
Se half must be reverse to be the oxidation half rxn
E0cell = E0red+ E0ox = 0.604V + (0.115V) = +0.719V
2 { H2Se(aq) ⇄ Se(s) + 2e(aq) + 2 H(aq) }
4 H(aq) + 4e + TeO2(s) ⇄ Te(s) + 2 H2O(l)
------------------------------------------------------------------
4 H(aq) + 2 H2Se(aq) + TeO2(s) ⇄ Te(s) + 2 Se(s) + 4 H(aq) + 2 H2O(l)
2 H2Se(aq) + TeO2(s) ⇄ Te(s) + 2 Se(s) + 2 H2O(l)

MnO4-1|Mn+2(E0red= +1.23V) and ClO4-1|ClO3-1(E0red= +1.201V)
Cl half must be reverse to be the oxidation half rxn
E0cell = E0red+ E0ox = 1.23V + (-1.201V) = +0.03V
5 { H2O(l) + ClO3-1(aq) ⇄ ClO4-1(aq) + 2e(aq) + 2 H(aq) }
2 { 8 H(aq) + 5e + MnO4-1(aq) ⇄ Mn2+(aq) + 4 H2O(l) }
------------------------------------------------------------------
616H(aq) + 5 H2O(l) + 2 MnO4-1(aq) + 5 ClO3-1(aq) ⇄ 5 ClO4-1(aq) + 2 Mn2+(aq) + 38H2O(l) + 10 H(aq)
6 H(aq) + 5 H2O(l) + 2 MnO4-1(aq) + 5 ClO3-1(aq) ⇄ 5 ClO4-1(aq) + 2 Mn2+(aq) + 3 H2O(l)

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