Question

$E_{M^{2+}\left(aq\right)/M\left(s\right)}^0$has a positive value for which of the following elements of 3d transition series?

• A. Cu
• B. Zn
• C. Cr
• D. Co

Answer 1

The correct option is A Cu

According to the electrochemical series, elements present above $H$ have postive $E^0$value.

Usually across the first transition series, the negative values for standard electrode potential decrease except for Mn and Zn due to stable $d^5$ and $d^{10}$-configuration.
For ${M^{2+}}_{\left(aq\right)}/M_{\left(s\right),outofthegivenoption,}C{u^{2+}}_{\left(aq\right)}/C{u}_{\left(s\right)}willbetheanswer$

$C{u^{2+}}_{\left(aq\right)}+2e^{-}\to C{u}_{\left(s\right)};E^0=+0.34V$
$Z{n^{2+}}_{\left(aq\right)}+2e^{-}\to Z{n}_{\left(s\right)};E^0=-0.763V$
$C{o^{2+}}_{\left(aq\right)}+2e^{-}\to C{o}_{\left(s\right)};E^0=-0.28V$
$C{r^{2+}}_{\left(aq\right)}+2e^{-}\to C{r}_{\left(s\right)};E^0=-0.9V$