Explain the trends in $E_{{M^{3 + }}/{M^{2 + }}}^ o $ electrode potential.
The scandium has low value of $\mathrm{M}^{3+} / \mathrm{M}^{2+}$ potentials because of its noble gas configuration.
The highest value of $\mathrm{M}^{3+} / \mathrm{M}^{2+}$ is for zinc because $\mathrm{Zn}^{2+}$ has $\mathrm{d}^{10}$ configuration which is extra stable. High value of $\mathrm{Mn}^{3+} \mid \mathrm{Mn}^{2+}$ is due to half-filled $\left(d^{5}\right)$ orbitals in $\mathrm{Mn}^{2+}$ while low values of $\mathrm{Fe}^{3+} \mid \mathrm{Fe}^{2+}$ shows extra stability of $\mathrm{Fe}^{3+}\left(d^{5}\right)$. The vanadium has comparatively less negative values because of higher stability of $\mathrm{V}^{2+}$ that has half filled configuration $\left(\mathrm{t}_{2 \mathrm{~g}}^{3}\right)$.
The high value of $\mathrm{Co}^{3+} / \mathrm{Co}^{2+}$ is because of high values of $\mathrm{CFSE}$ of $\mathrm{t}_{2 \mathrm{~g}}^{5} \mathrm{e}^{2}\left[\mathrm{Co}^{2+}\right]$ system in a aqueous medium.
The high values of $\mathrm{M}^{3+} / \mathrm{M}^{2+}$ of $\mathrm{Mn}^{3+}$ and $\mathrm{Co}^{3+}$ indicates that they are good oxidizing agents while $\mathrm{Ti}^{2+}, \mathrm{V}^{2+} \mathrm{Cr}^{2+}$ etc. are good reducing agents and thus these liberates $\mathrm{H}_{2}$ from acids.
Ex. : $2 \mathrm{Cr}_{\text {(aq) }}^{2+}+2 \mathrm{H}_{\text {(aq) }}^{+} \rightarrow 2 \mathrm{Cr}_{\text {(aq) }}^{3+}+\mathrm{H}_{2 \text { (aq) }} \uparrow$
Elements | Electrode potentials$\left(\mathrm{M}^{3+} / \mathrm{M}^{2+}\right)$ |
$Ti$ | $-0.37$ |
$V$ | $-0.26$ |
$\mathrm{Cr}$ | $-0.41$ |
$Mn$ | $+1.57$ |
$\mathrm{Fe}$ | $+0.77$ |
$Co$ | $+1.97$ |
In presence of oxidizing agents such as $\mathrm{HNO}_{3}$, the chromium forms layer of oxide $\mathrm{Cr}_{2} \mathrm{O}_{3}$ on its surface and hence it is passive towards oxidation.
Which one of the following does not correctly represent the order of property indicated against it
Which of the following will be coloured ion :
Which of the following pairs of elements might form an alloy
Among the following, the compound that is both are diamagnetic and coloured is
Stainless steel contains ........... $\%$ $Cr$