The standard reduction potential for the half-reaction Sn4+ + 2e- Sn2+ is +0.15

Question

The standard reduction potential for the half-reaction Sn4+ + 2e- Sn2+ is +0.15 V. Consider data from the table of standard reduction potentials for common half-reactions, in your text. For a galvanic cell under standard conditions, which of the following cathodic half reactions would produce, at the anode, a spontaneous oxidation of Sn to Sn2+ but not Sn2+ to Sn4+. Yes or No? 2H2O + 2e- H2 + 2OH- Yes or No? PbSO4 + 2e- Pb + SO42- Yes or No? Cu2+ + e- Cu+ Yes or No? Sn2+ + 2e- Sn Yes or No? Fe2+ + 2e- Fe Yes or No? 2H+ + 2e- H2

Explanation / Answer

In order for it to be spontaneous reduction Ecell < 0, so the number must be negative. in this question, you only want the reaction that they give you to be spont. reduc. with Sn4--->Sn2 but NOT with Sn2----->Sn. The potential reduction values are: for Sn4----->Sn2, -0.15 for Sn2----->Sn, +0.14 (3) H2 + 2OH- --> 2H2O + 2e-, NO 0.83 +(-0.15) = + 0.83 +(0.14) = + {if you look up the reduc. potential for the above reaction it will be -0.83, but our reaction is reversed, so we must change the sign to +} Therefore, no its not spont. red. with Sn4 because it produces a positive value with when we calculate Ecell (1)Sn --> Sn2+ + 2e-, YES 0.14 +(-0.15) = - 0.14 +(0.14) = + Yes, b/c only the Sn4 has a negative Ecell (2)Sn2+ --> Sn4+ + 2e-, I'M ABOUT 98% SURE ITS NO, BUT I'M NOT 100% 0.15 +(-0.15) = + 0.15 +(0.14) = + therefore no b/c they both have a + Ecell (4)Pb + SO42- --> PbSO4 + 2e-, NO 0.35 +(-0.15) = + 0.35 +(0.14) = + Therefore no b/c they are both positive. (5) Pb----->Pb2 + 2e 0.13 - 0.15= - 0.13 + 0.14= + Yes, because this rxn produces a negative Ecell with only Sn4 (6) H2-----> 2H + 2e 0 - 0.15 = - 0 +0.14= + yes, for same reason as above

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