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Two half cell reactions are given below.
$Co^{3+}+e^-\rightarrow Co^{2+}$, $E^\circ_{Co^{2+}/Co^{3+}}=-1.81$ V
$2Al^{3+}+6e^-\rightarrow 2Al(s)$, $E^\circ_{Al/Al^{3+}}=+1.66$ V
The standard EMF of a cell with feasible redox reaction will be:
Detailed Solution
The given values are oxidation potentials. Reduction potentials: $E^\circ_{Co^{3+}/Co^{2+}}=+1.81$ V and $E^\circ_{Al^{3+}/Al}=-1.66$ V. $E^\circ_{cell}=E^\circ_{cathode}-E^\circ_{anode}=1.81-(-1.66)=+3.47$ V.