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Question

$$E^{\ominus}\, for\, Cr^{3+}\, +\, 3e^-\, \rightarrow\, Cr\,\space and \space Cr^{3+}\, +\, e^-\, \rightarrow\, Cr^{2+}$$ are $$-0.74 V$$ and $$-0.40 V$$, respectively. $$E^{\ominus}$$ for the reaction is: $$Cr^{+2}\, +\, 2e^-\, \rightarrow\, Cr$$


A
0.91V
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B
+0.91V
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C
-1.14V
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D
+0.34V
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Solution

The correct option is A $$-0.91 V$$
$$Cr^{3+}\, +\, 3e^-\, \rightarrow\, Cr;$$
$$Cr^{3+}\, +\, e^-\, \rightarrow\, Cr^{2+};$$
$$Cr^{2+}\, +\, 2e^-\, \rightarrow\, Cr;$$
$$\displaystyle E^{\ominus}_{3}\, =\, \dfrac{n_1 E^{\ominus}_1\, -\, n_2 E^{\ominus}_2}{n_3}$$
$$\displaystyle =\, \dfrac{[3\, \times\, (-0.74)]\, -\, [(-1)\, \times\, (-40)]}{2}\, =\, -0.91\, V$$

Chemistry
NCERT
Standard XII

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