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Electrochemical Series - Practice Questions & MCQ

Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

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  • Electrochemical Series is considered one of the most asked concept.

  • 15 Questions around this concept.

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Zinc can be coated on iron to produce galvanized iron but the reverse is not possible. It is because

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Standard reduction potentials of the half-reactions are given below:

\mathrm{F _{2( g )}+2 e ^{-} \rightarrow 2 F _{\text {(aq) }}^{-} ; E ^0=+2.85 V}

\mathrm{Cl _{2( g )}+2 e ^{-} \rightarrow 2 Cl _{( aq )}^{-} ; E ^0=+1.36 V}

\mathrm{Br _{2(l)}+2 e ^{-} \rightarrow 2 Br _{( aq )}^{-} ; E ^0=+1.06 V}

\mathrm{I _{2( s )}+2 e ^{-} \rightarrow 2 I _{( aq )}^{-} ; E ^0=+0.53 V}

The strongest oxidizing and reducing agents respectively are:

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Among the following which is the strongest oxidizing agent?

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Four successive members of the first series of the tranisiton metals are listed below. For which one of them the standard potential \left ( E^{\circ}_{M^{2+}/M} \right ) value has a positive sign?

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Concepts Covered - 1

Electrochemical Series
\mathrm{Li}^{+} / \mathrm{Li} \mathrm{Li}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Li}(\mathrm{s}) -3.04
\mathrm{K}^{+} / \mathrm{K} \mathrm{K}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{K}(\mathrm{s}) -2.93
\mathrm{Ca}^{2+} / \mathrm{Ca} \mathrm{Ca}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Ca}(\mathrm{s}) -2.87
\mathrm{Na}^{+} / \mathrm{Na} \mathrm{Na}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Na}(\mathrm{s}) -2.71
\mathrm{Mg}^{2+} / \mathrm{Mg} \mathrm{Mg}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Mg}(\mathrm{s}) -2.37
\mathrm{Pt}, \mathrm{H}_{2} / \mathrm{H}^{-} \mathrm{H}_{2}(\mathrm{g})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}^{-}(\mathrm{aq} .) -2.25
\mathrm{Al}^{3+} / \mathrm{Al} \mathrm{A}^{3+}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Al}(\mathrm{s}) -1.66
\mathrm{Mn}^{2+} / \mathrm{Mn} \mathrm{Mn}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Mn}(\mathrm{s}) -0.91
\mathrm{OH}^{-} / \mathrm{H}_{2}, \mathrm{Pt} 2 \mathrm{H}_{2} \mathrm{O}(\ell)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}(\mathrm{g})+2 \mathrm{OH}^{-}(\mathrm{aq} .) -0.83
\mathrm{Zn}^{2+} / \mathrm{Zn} \mathrm{Zn}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Zn}(\mathrm{s}) -0.76
\mathrm{Cr}^{3+} / \mathrm{Cr} \mathrm{Cr}^{3+}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Cr}(\mathrm{s}) -0.74
\mathrm{Fe}^{2+} / \mathrm{Fe} \mathrm{Fe}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Fe}(\mathrm{s}) -0.44
\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}, \mathrm{Pt} \mathrm{Cr}^{3+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Cr}^{2+}(\mathrm{aq} .) -0.41
\mathrm{Cd}^{2+} / \mathrm{Cd} \mathrm{Cd}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cd}(\mathrm{s}) -0.40
\mathrm{Co}^{2+} / \mathrm{Co} \mathrm{Co}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Co}(\mathrm{s}) -0.28
\mathrm{Ni}^{2+} / \mathrm{Ni} \mathrm{Ni}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Ni}(\mathrm{s}) -0.25
\mathrm{I^{-} / \mathrm{AgI} / \mathrm{Ag}} \operatorname{AgI}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s})+\mathrm{I}^{-}(\mathrm{aq} .) -0.15
\mathrm{Sn}^{2+} / \mathrm{Sn} \mathrm{Sn}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Sn}(\mathrm{s}) -0.14
\mathrm{Pb}^{2+} / \mathrm{Pb} \mathrm{Pb}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pb}(\mathrm{s}) -0.13
\mathrm{Fe}^{3+} / \mathrm{Fe} \mathrm{Fe}^{3+}(\mathrm{aq.})+3 \mathrm{e}^{-} \longrightarrow \mathrm{Fe}(\mathrm{s})

-0.04
\mathrm{H}^{+} / \mathrm{H}_{2}, \mathrm{Pt} 2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}(\mathrm{g}) 0.00
\mathrm{Br}^{-} / \mathrm{AgBr} / \mathrm{Ag} \operatorname{AgBr}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s})+\mathrm{Br}^{-}(\mathrm{aq} .) 0.10
\mathrm{Cu}^{2+} / \mathrm{Cu}^{+}, \mathrm{Pt} \mathrm{Cu}^{2+}(\mathrm{aq.})+\mathrm{e}^{-} \longrightarrow \mathrm{Cu}^{+}(\mathrm{aq} .) 0.16
\mathrm{Sn}^{4+} / \mathrm{Sn}^{2+}, \mathrm{Pt} \mathrm{Sn}^{4+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Sn}^{2+}(\mathrm{aq} .) 0.15
\mathrm{SO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{SO}_{3} \mathrm{SO}_{4}^{2-}(\mathrm{aq.})+4 \mathrm{H}^{+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{SO}_{3}(\mathrm{aq.})+\mathrm{H}_{2} \mathrm{O}(\ell) 0.17
\mathrm{Cl}^{-} / \mathrm{AgCl} / \mathrm{Ag} \mathrm{AgCl}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s})+\mathrm{Cl}^{-}(\mathrm{aq} .) 0.22
\mathrm{Cl}^{-} / \mathrm{Hg}_{2} \mathrm{Cl}_{2} / \mathrm{Hg}(\mathrm{Pt}) \mathrm{Hg}_{2} \mathrm{Cl}_{2}(\mathrm{s})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Hg}(\ell)+2 \mathrm{Cl}^{-}(\mathrm{aq} .) 0.27
\mathrm{Cu}^{2+} / \mathrm{Cu} \mathrm{Cu}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s}) 0.34
\mathrm{Pt}, \mathrm{O}_{2} / \mathrm{OH}^{-} \mathrm{O}_{2}(\mathrm{g})+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq} .) 0.40
\mathrm{Cu}^{+} / \mathrm{Cu} \mathrm{Cu}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s}) 0.52
\mathrm{I}_{2} / \mathrm{I}^{-}, \mathrm{Pt} 1 / 2 \mathrm{I}_{2}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{I}^{-}(\mathrm{aq} .) 0.54
\mathrm{Pt}, \mathrm{O}_{2} / \mathrm{H}_{2} \mathrm{O}_{2} \mathrm{O}_{2}(\mathrm{g})+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq} .) 0.68
\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}, \mathrm{Pt} \mathrm{Fe}^{3+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Fe}^{2+}(\mathrm{aq} .) 0.77
\mathrm{Hg}_{2}^{2+} / \mathrm{Hg}(\mathrm{Pt}) 1 / 2 \mathrm{Hg}_{2}^{2+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Hg}(\mathrm{s}) 0.79
\mathrm{Ag}^{+} / \mathrm{Ag} \mathrm{Ag}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s}) 0.80
\mathrm{Hg}^{2+} / \mathrm{Hg}_{2}^{2+} 2 \mathrm{Hg}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Hg}_{2}^{2+}(\mathrm{aq} .) 0.92
\mathrm{NO}_{3}^{-} / \mathrm{NO}, \mathrm{Pt} \mathrm{NO}_{3}^{-}+4 \mathrm{H}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{NO}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\ell) 0.97
\mathrm{Pt}, \mathrm{Br}_{2} / \mathrm{Br}^{-} \mathrm{Br}_{2}(\ell)+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Br}^{-}(\mathrm{aq} .) 1.09
\mathrm{MnO}_{2} / \mathrm{Mn}^{2+} \mathrm{MnO}_{2}(\mathrm{s})+4 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Mn}^{2+}(\mathrm{aq} .)+2 \mathrm{H}_{2} \mathrm{O}(\ell) 1.23
\mathrm{H}^{+} / \mathrm{O}_{2} / \mathrm{Pt} \mathrm{O}_{2}(\mathrm{g})+4 \mathrm{H}^{+}(\mathrm{aq} .)+4 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(\ell) 1.23
\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} / \mathrm{Cr}^{3+} \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(\mathrm{aq} .)+14 \mathrm{H}^{+}(\mathrm{aq} .)+6 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Cr}^{3+}(\mathrm{aq} .)+7 \mathrm{H}_{2} \mathrm{O}(\ell) 1.33
\mathrm{Cl}_{2} / \mathrm{Cl}^{-} 1 / 2 \mathrm{Cl}_{2}(\mathrm{g})+\mathrm{e}^{-} \longrightarrow \mathrm{Cl}^{-}(\mathrm{aq} .) 1.36
\mathrm{Au}^{3+} / \mathrm{Au} \mathrm{Au}^{3+}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Au}(\mathrm{s}) 1.40
\mathrm{MnO}_{4}^{-} / \mathrm{Mn}^{2+}, \mathrm{H}^{+} / \mathrm{Pt} \mathrm{MnO}_{4}^{-}(\mathrm{aq.})+8 \mathrm{H}^{+}(\mathrm{aq} .)+5 \mathrm{e} \longrightarrow \mathrm{Mn}^{2+}(\mathrm{aq.})+4 \mathrm{H}_{2} \mathrm{O}(\ell) 1.51
\mathrm{Ce}^{4+} / \mathrm{Ce}^{3+}, \mathrm{Pt} \mathrm{Ce}^{4+}+\mathrm{e}^{-} \longrightarrow \mathrm{Ce}^{3+}(\mathrm{aq} .) 1.72
\mathrm{H}_{2} \mathrm{O}_{2} / \mathrm{H}_{2} \mathrm{O} \mathrm{H}_{2} \mathrm{O}_{2}(\ell)+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(\ell) 1.78
\mathrm{Co}^{3+} / \mathrm{Co}^{2+}, \mathrm{Pt} \mathrm{Co}^{3+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Co}^{2+}(\mathrm{aq} .) 1.81
\mathrm{O}_{3} / \mathrm{O}_{2} \mathrm{O}_{3}(\mathrm{g})+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{O}_{2}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(\ell) 2.07
\mathrm{Pt}, \mathrm{F}_{2} / \mathrm{F} \mathrm{F}_{2}(\mathrm{g})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{F}^{-}(\mathrm{aq} .) 2.87

Characteristics of Electrochemical Series
Metals with greater negative Eo (reduction) are strongly electropositive and have more reactivity. It means a lower placed element or metal is in the given series is less reactive is replaced by upper placed or higher element while higher element can be coated by lower metal.
\text { Example, (i) } \mathrm{Zn}+\mathrm{CuSO}_{4} \rightarrow \mathrm{ZnSO}_{4}+\mathrm{Cu}
Here Cu is replaced by Zn due to more oxidation potential or reactivity of Zn, while Zn is coated by Cu. Zn- Cu couple is also coated by Cu. Here, the solution turns from blue to colorless and the rod becomes Reddish-brown from Gray white.
\text { (ii) } \mathrm{Cu}+2 \mathrm{AgNO}_{3} \rightarrow \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}+2 \mathrm{Ag}
Here solution becomes colorless to blue and the rod becomes reddish-brown to white.

  • Metals above H2 can easily replace H2, from acid, bases, etc. due to their more positive Eoop or reactivity.
    For example,
    \mathrm{Mg}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{MgSO}_{4}+\mathrm{H}_{2}
    \begin{array}{l}{\mathrm{E}_{\mathrm{op}}^{\circ} \text { of } \mathrm{Mg}>\mathrm{E}_{\mathrm{OP}}^{\circ} \text { of } \mathrm{H}_{2}} \\ {\mathrm{R}-\mathrm{OH}+\mathrm{Na} \rightarrow \mathrm{R}-\mathrm{ONa}+\mathrm{H}^{+}}\end{array}
  • Lower placed metals (Cu Hg Ag Pt Au) to H2 can not do that as Eoop of H2 is more than their Eoop.
    \mathrm{Cu}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \text { no reaction }
  • Oxides of lower metals (Cu, Hg, Ag, Pt, Au) are easily reduced by H2 or carbon. As they are thermally more unstable due to positive Erp, they also decomposed on heating.
  • More EoOP means more ease or tendency to get oxidize that is, the act as better reducing agents while more EoRP means more ease to reduced that is, they act as better oxidizing agents. It means metal above hydrogen having positive Eop are reducing agents.
    \text { Reducing property } \propto \mathrm{E}_{\mathrm{OP}}^{\circ}
    For example, Li is the strongest reducing agent due to maximum EoOP
  • Metals placed lower in reactivity series (Cu Hg Ag Pt Au) having high EoRP are oxidizing agents and they have tendency to be reduced.

    \text { For example, Oxidizing power } \propto \: \mathrm{E}_{\mathrm{RP}}
    \mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}

    \begin{array}{l}{\text { Reducing power decreases }} \\ {\text { As } \mathrm{E}_{\text {op }}^{\circ} \text { of } \mathrm{I}^{-}>\mathrm{Br}^{-}>\mathrm{Cl}^{-}>\mathrm{F}^{-}}\end{array}
  • Elements with more positive EoRP will be discharged first at cathode i.e., discharging order increases as reduction potential increases.

            \text { Increasing ease of deposition of some cations }

            \mathrm{Li}^{+}, \mathrm{K}^{+}, \mathrm{Ca}^{+2}, \mathrm{Na}^{+}, \mathrm{Mg}^{+2}, \mathrm{Al}^{+3}, \mathrm{Zn}^{+2}, \mathrm{Fe}^{+2}, \mathrm{H}^{+}, \mathrm{Cu}^{+2}, \mathrm{Ag}^{+}, \mathrm{Au}^{+3}

  • In case of negative ions, anion with stronger reducing nature is discharged first at anode.


\text { Increasing ease of discharge of some anion }
\mathrm{SO}_{4}^{-2}<\mathrm{NO}_{3}^{-}<\mathrm{OH}^{-}<\mathrm{Cl}^{-}<\mathrm{Br}^{-}

  • Hydroxides of upper metals are strongly basic and their salts do not undergo hydrolysis while hydroxides of lower metals are weakly acidic and their salts undergo hydrolysis.

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Electrochemical Series

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