Electrochemical Series MCQ - Practice Questions & Answers

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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.
$\mathrm{\begin{array}{ll} 2 \mathrm{H}^{+}(\text {aq })+2 e^{-} \rightleftharpoons \mathrm{H}_2(\mathrm{~g}) & 0.000 \\ \mathrm{~Fe}^{3+}(\mathrm{aq})+\mathrm{e}^{-} \rightleftharpoons \mathrm{Fe}^{2+}(\mathrm{aq}) & 0.770 \\ \mathrm{Cr}^{3+}(\mathrm{aq})+3 e^{-} \rightleftharpoons \mathrm{Cr}(\mathrm{s}) & -0.740 \\ \mathrm{Zn}^{2+}(\mathrm{aq})+2 e^{-} \rightleftharpoons \mathrm{Zn}(\mathrm{s}) & -0.762 \end{array}}$

$\mathrm{Fe}^{3+}>\mathrm{H}^{+}>\mathrm{Cr}^{3+}+\mathrm{Zn}^{2+}$

$\mathrm{Z n^{2+}>\mathrm{Cr}^{3+}>\mathrm{H}^{+}>\mathrm{Fe}^{3+}}$

$\mathrm{Zn}^{2+}>\mathrm{Cr}^{3+}>\mathrm{Fe}^{3+}>\mathrm{H}^{+}$

$\mathrm{H}^{+}>\mathrm{Fe}^{3+}>\mathrm{Cr}^{3+}>\mathrm{Zn}^{2+}$

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Concepts Covered - 1

Electrochemical Series

$\mathrm{Li}^{+} / \mathrm{Li}$	$\mathrm{Li}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Li}(\mathrm{s})$	$-3.04$
$\mathrm{K}^{+} / \mathrm{K}$	$\mathrm{K}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{K}(\mathrm{s})$	$-2.93$
$\mathrm{Ca}^{2+} / \mathrm{Ca}$	$\mathrm{Ca}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Ca}(\mathrm{s})$	$-2.87$
$\mathrm{Na}^{+} / \mathrm{Na}$	$\mathrm{Na}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Na}(\mathrm{s})$	$-2.71$
$\mathrm{Mg}^{2+} / \mathrm{Mg}$	$\mathrm{Mg}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Mg}(\mathrm{s})$	$-2.37$
$\mathrm{Pt}, \mathrm{H}_{2} / \mathrm{H}^{-}$	$\mathrm{H}_{2}(\mathrm{g})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}^{-}(\mathrm{aq} .)$	$-2.25$
$\mathrm{Al}^{3+} / \mathrm{Al}$	$\mathrm{Al}^{3+}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Al}(\mathrm{s})$	$-1.66$
$\mathrm{Mn}^{2+} / \mathrm{Mn}$	$\mathrm{Mn}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Mn}(\mathrm{s})$	$-0.91$
$\mathrm{OH}^{-} / \mathrm{H}_{2}, \mathrm{Pt}$	$2 \mathrm{H}_{2} \mathrm{O}(\ell)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}(\mathrm{g})+2 \mathrm{OH}^{-}(\mathrm{aq} .)$	$-0.83$
$\mathrm{Zn}^{2+} / \mathrm{Zn}$	$\mathrm{Zn}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Zn}(\mathrm{s})$	$-0.76$
$\mathrm{Cr}^{3+} / \mathrm{Cr}$	$\mathrm{Cr}^{3+}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Cr}(\mathrm{s})$	$-0.74$
$\mathrm{Fe}^{2+} / \mathrm{Fe}$	$\mathrm{Fe}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Fe}(\mathrm{s})$	$-0.44$
$\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}, \mathrm{Pt}$	$\mathrm{Cr}^{3+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Cr}^{2+}(\mathrm{aq} .)$	$-0.41$
$\mathrm{Cd}^{2+} / \mathrm{Cd}$	$\mathrm{Cd}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cd}(\mathrm{s})$	$-0.40$
$\mathrm{Co}^{2+} / \mathrm{Co}$	$\mathrm{Co}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Co}(\mathrm{s})$	$-0.28$
$\mathrm{Ni}^{2+} / \mathrm{Ni}$	$\mathrm{Ni}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Ni}(\mathrm{s})$	$-0.25$
$\mathrm{I^{-} / \mathrm{AgI} / \mathrm{Ag}}$	$\operatorname{AgI}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s})+\mathrm{I}^{-}(\mathrm{aq} .)$	$-0.15$
$\mathrm{Sn}^{2+} / \mathrm{Sn}$	$\mathrm{Sn}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Sn}(\mathrm{s})$	$-0.14$
$\mathrm{Pb}^{2+} / \mathrm{Pb}$	$\mathrm{Pb}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pb}(\mathrm{s})$	$-0.13$
$\mathrm{Fe}^{3+} / \mathrm{Fe}$	$\mathrm{Fe}^{3+}(\mathrm{aq.})+3 \mathrm{e}^{-} \longrightarrow \mathrm{Fe}(\mathrm{s})$	$-0.04$
$\mathrm{H}^{+} / \mathrm{H}_{2}, \mathrm{Pt}$	$2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}(\mathrm{g})$	$0.00$
$\mathrm{Br}^{-} / \mathrm{AgBr} / \mathrm{Ag}$	$\operatorname{AgBr}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s})+\mathrm{Br}^{-}(\mathrm{aq} .)$	$0.10$
$\mathrm{Cu}^{2+} / \mathrm{Cu}^{+}, \mathrm{Pt}$	$\mathrm{Cu}^{2+}(\mathrm{aq.})+\mathrm{e}^{-} \longrightarrow \mathrm{Cu}^{+}(\mathrm{aq} .)$	$0.16$
$\mathrm{Sn}^{4+} / \mathrm{Sn}^{2+}, \mathrm{Pt}$	$\mathrm{Sn}^{4+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Sn}^{2+}(\mathrm{aq} .)$	$0.15$
$\mathrm{SO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{SO}_{3}$	$\mathrm{SO}_{4}^{2-}(\mathrm{aq.})+4 \mathrm{H}^{+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{SO}_{3}(\mathrm{aq.})+\mathrm{H}_{2} \mathrm{O}(\ell)$	$0.17$
$\mathrm{Cl}^{-} / \mathrm{AgCl} / \mathrm{Ag}$	$\mathrm{AgCl}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s})+\mathrm{Cl}^{-}(\mathrm{aq} .)$	$0.22$
$\mathrm{Cl}^{-} / \mathrm{Hg}_{2} \mathrm{Cl}_{2} / \mathrm{Hg}(\mathrm{Pt})$	$\mathrm{Hg}_{2} \mathrm{Cl}_{2}(\mathrm{s})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Hg}(\ell)+2 \mathrm{Cl}^{-}(\mathrm{aq} .)$	$0.27$
$\mathrm{Cu}^{2+} / \mathrm{Cu}$	$\mathrm{Cu}^{2+}(\mathrm{aq.})+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})$	$0.34$
$\mathrm{Pt}, \mathrm{O}_{2} / \mathrm{OH}^{-}$	$\mathrm{O}_{2}(\mathrm{g})+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq} .)$	$0.40$
$\mathrm{Cu}^{+} / \mathrm{Cu}$	$\mathrm{Cu}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Cu}(\mathrm{s})$	$0.52$
$\mathrm{I}_{2} / \mathrm{I}^{-}, \mathrm{Pt}$	$1 / 2 \mathrm{I}_{2}(\mathrm{s})+\mathrm{e}^{-} \longrightarrow \mathrm{I}^{-}(\mathrm{aq} .)$	$0.54$
$\mathrm{Pt}, \mathrm{O}_{2} / \mathrm{H}_{2} \mathrm{O}_{2}$	$\mathrm{O}_{2}(\mathrm{g})+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq} .)$	$0.68$
$\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}, \mathrm{Pt}$	$\mathrm{Fe}^{3+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Fe}^{2+}(\mathrm{aq} .)$	$0.77$
$\mathrm{Hg}_{2}^{2+} / \mathrm{Hg}(\mathrm{Pt})$	$1 / 2 \mathrm{Hg}_{2}^{2+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Hg}(\mathrm{s})$	$0.79$
$\mathrm{Ag}^{+} / \mathrm{Ag}$	$\mathrm{Ag}^{+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Ag}(\mathrm{s})$	$0.80$
$\mathrm{Hg}^{2+} / \mathrm{Hg}_{2}^{2+}$	$2 \mathrm{Hg}^{2+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Hg}_{2}^{2+}(\mathrm{aq} .)$	$0.92$
$\mathrm{NO}_{3}^{-} / \mathrm{NO}, \mathrm{Pt}$	$\mathrm{NO}_{3}^{-}+4 \mathrm{H}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{NO}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\ell)$	$0.97$
$\mathrm{Pt}, \mathrm{Br}_{2} / \mathrm{Br}^{-}$	$\mathrm{Br}_{2}(\ell)+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Br}^{-}(\mathrm{aq} .)$	$1.09$
$\mathrm{MnO}_{2} / \mathrm{Mn}^{2+}$	$\mathrm{MnO}_{2}(\mathrm{s})+4 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Mn}^{2+}(\mathrm{aq} .)+2 \mathrm{H}_{2} \mathrm{O}(\ell)$	$1.23$
$\mathrm{H}^{+} / \mathrm{O}_{2} / \mathrm{Pt}$	$\mathrm{O}_{2}(\mathrm{g})+4 \mathrm{H}^{+}(\mathrm{aq} .)+4 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(\ell)$	$1.23$
$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} / \mathrm{Cr}^{3+}$	$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(\mathrm{aq} .)+14 \mathrm{H}^{+}(\mathrm{aq} .)+6 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Cr}^{3+}(\mathrm{aq} .)+7 \mathrm{H}_{2} \mathrm{O}(\ell)$	$1.33$
$\mathrm{Cl}_{2} / \mathrm{Cl}^{-}$	$1 / 2 \mathrm{Cl}_{2}(\mathrm{g})+\mathrm{e}^{-} \longrightarrow \mathrm{Cl}^{-}(\mathrm{aq} .)$	$1.36$
$\mathrm{Au}^{3+} / \mathrm{Au}$	$\mathrm{Au}^{3+}(\mathrm{aq} .)+3 \mathrm{e}^{-} \longrightarrow \mathrm{Au}(\mathrm{s})$	$1.40$
$\mathrm{MnO}_{4}^{-} / \mathrm{Mn}^{2+}, \mathrm{H}^{+} / \mathrm{Pt}$	$\mathrm{MnO}_{4}^{-}(\mathrm{aq.})+8 \mathrm{H}^{+}(\mathrm{aq} .)+5 \mathrm{e} \longrightarrow \mathrm{Mn}^{2+}(\mathrm{aq.})+4 \mathrm{H}_{2} \mathrm{O}(\ell)$	$1.51$
$\mathrm{Ce}^{4+} / \mathrm{Ce}^{3+}, \mathrm{Pt}$	$\mathrm{Ce}^{4+}+\mathrm{e}^{-} \longrightarrow \mathrm{Ce}^{3+}(\mathrm{aq} .)$	$1.72$
$\mathrm{H}_{2} \mathrm{O}_{2} / \mathrm{H}_{2} \mathrm{O}$	$\mathrm{H}_{2} \mathrm{O}_{2}(\ell)+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(\ell)$	$1.78$
$\mathrm{Co}^{3+} / \mathrm{Co}^{2+}, \mathrm{Pt}$	$\mathrm{Co}^{3+}(\mathrm{aq} .)+\mathrm{e}^{-} \longrightarrow \mathrm{Co}^{2+}(\mathrm{aq} .)$	$1.81$
$\mathrm{O}_{3} / \mathrm{O}_{2}$	$\mathrm{O}_{3}(\mathrm{g})+2 \mathrm{H}^{+}(\mathrm{aq} .)+2 \mathrm{e}^{-} \longrightarrow \mathrm{O}_{2}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(\ell)$	$2.07$
$\mathrm{Pt}, \mathrm{F}_{2} / \mathrm{F}$	$\mathrm{F}_{2}(\mathrm{g})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{F}^{-}(\mathrm{aq} .)$	$2.87$

Characteristics of Electrochemical Series
Metals with greater negative E^o (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 E^o_opor 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 E^o_op of H₂ is more than their E^o_op.
$\mathrm{Cu}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \text { no reaction }$
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 E_rp,they also decomposed on heating.
More E^o_OP means more ease or tendency to get oxidize that is, the act as better reducing agents while more E^o_RPmeans more ease to reduced that is, they act as better oxidizing agents. It means metal above hydrogen having positive E_op are reducing agents.
$\text { Reducing property } \propto \mathrm{E}_{\mathrm{OP}}^{\circ}$
For example, Li is the strongest reducing agent due to maximum E^o_OP
Metals placed lower in reactivity series (Cu Hg Ag Pt Au) having high E^o_RP 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 E^o_RPwill 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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