A) \[{{F}_{2}}+2{{e}^{-}}\xrightarrow{{}}2{{F}^{-}}\]
B) \[2{{H}^{+}}+\frac{1}{2}{{O}_{2}}+2{{e}^{-}}\xrightarrow{{}}{{H}_{2}}O\]
C) \[2C{{r}^{3+}}+7{{H}_{2}}O\xrightarrow{{}}C{{r}_{2}}O_{7}^{2-}+14{{H}^{+}}+6{{e}^{-}}\]
D) \[F{{e}^{2+}}\xrightarrow{{}}F{{e}^{3+}}+{{e}^{-}}\]
Correct Answer: B
Solution :
In a cell, oxidation occurs at anode while reduction takes place at cathode. \[{{F}_{2}}+2\overline{e}\xrightarrow{{}}2{{F}^{-}}\] There is gain of electron and hence it is reduction \[2{{H}^{+}}+\frac{1}{2}{{O}_{2}}+2{{e}^{-}}\xrightarrow{{}}{{H}_{2}}O\] In this case \[{{H}^{+}}\] ions take electrons to form \[{{H}_{2}}O\] So, it is also a reduction reaction. \[2\overset{+3}{\mathop{C}}\,{{r}^{3+}}+7{{H}_{2}}O\xrightarrow{{}}\overset{+6}{\mathop{C}}\,r2{{O}_{7}}^{-2}+14{{H}^{+}}+6{{e}^{-}}\] Here, oxidation state of chromium ion changes from \[+3\] to \[+6\]. So, it is an oxidation reaction but this reaction is not feasible under given condition: because chromium is not stable in \[+6\] oxidation I state in aqueous solution. \[F{{e}^{2+}}\xrightarrow{{}}F{{e}^{3+}}+{{e}^{-}}\] It is an oxidation reaction which occur at anode.
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