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JEE Main Last 10 Years Question Papers with Solutions PDF 2024 to 2014

Electrochemical Series - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

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  • 18 Questions around this concept.

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Given below are the half reactions with their standard reduction potential at a temperature of 298 \mathrm{~K}. Compare those and select the correct option representing the decreasing order of reducing strength.
\mathrm{\begin{array}{ll} 2 \mathrm{H}^{+}(\text {aq })+2 e^{-} \rightleftharpoons \mathrm{H}_2(\mathrm{~g}) & 0.000 \\ \mathrm{~Fe}^{3+}(\mathrm{aq})+\mathrm{e}^{-} \rightleftharpoons \mathrm{Fe}^{2+}(\mathrm{aq}) & 0.770 \\ \mathrm{Cr}^{3+}(\mathrm{aq})+3 e^{-} \rightleftharpoons \mathrm{Cr}(\mathrm{s}) & -0.740 \\ \mathrm{Zn}^{2+}(\mathrm{aq})+2 e^{-} \rightleftharpoons \mathrm{Zn}(\mathrm{s}) & -0.762 \end{array}}

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{Al}^{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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