Tuesday, 25 August 2015

electrochemistry - Deriving a reduction potential from two other reduction potentials



Given:


\begin{align} \ce{Co^{3+}(aq) + e- &-> Co^{2+}(aq)} & E° &= \pu{+1.82 V} \\ \ce{Co^{2+}(aq) + 2e- &-> Co(s)} & E° &= \pu{-0.28 V} \end{align}


what is the standard reduction potential for $\ce{Co^{3+}(aq) +3e- -> Co(s)}$?




If I add up these two half-equations to get the desired half-equation, I get $E^\circ = \pu{+1.54 V}$. But the correct answer is $\pu{+ 0.42 V}$!


Am I missing something?




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