Electochemical Series

7/28/2019 Electochemical Series

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By measuring the potentials of various electrodes versusstandard hydrogen electrode (SHE), a series of standardelectrode potentials has been established. When theelectrodes (metals and non-metals) in contact with their ionsare arranged on the basis of the values of their standardreduction potentials or standard oxidation potentials, theresulting series is called the electrochemical

or electromotive or activity series of the elements.


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Element

Electrode Reaction(Reduction)

Standard ElectrodeReduction potential

Eo, volt

LiKCaNaMgAlZnCrFeCdNiSnH2

CuI2AgHgBr2Cl2AuF

2

Li+ + e- = LiK+ + e- = K

Ca2+ + 2e- = CaNa+ + e- = Na

Mg2+ + 2e- = MgAl3+ + 3e- = AlZn2+ + 2e- = ZnCr3+ + 3e- = CrFe2+ + 2e- = FeCd2+ + 2e- = CdNi2+ + 2e- = NiSn2+ + 2e- = Sn2H+ + 2e- = H2

Cu2+ + 2e- = CuI2 + 2e

- = 2I-Ag+ + e- = Ag

Hg2+ + 2e- = HgBr2 + 2e

- = 2Br-Cl2 + 2e

- = 2Cl-Au3+ + 3e- = AuF

2

+ 2e- = 2F-

-3.05-2.925-2.87-2.714-2.37-1.66

-0.7628-0.74-0.44-0.403-0.25-0.140.00

+0.337+0.535+0.799+0.885+1.08+1.36+1.50+2.87


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The negative sign of standard reduction potential indicates that anelectrode when joined with SHE acts as anode and oxidation occurs on thiselectrode. For example, standard reduction potential of zinc is -0.76 volt.When zinc electrode is joined with SHE, it acts as anode (-ve electrode) i.e.,

oxidation occurs on this electrode. Similarly, the +ve sign of standardreduction potential indicates that the electrode when joined with SHE acts ascathode and reduction occurs on this electrode.

Fe

CdNiSnH2CuI2AgHg

Fe2+ + 2e- = Fe

Cd2+

+ 2e-

= CdNi2+ + 2e- = NiSn2+ + 2e- = Sn2H+ + 2e- = H2Cu2+ + 2e- = Cu

I2 + 2e- = 2I-

Ag+ + e- = AgHg2+ + 2e- = Hg

-0.44

-0.403-0.25-0.140.00

+0.337+0.535+0.799+0.885


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The substances which are stronger reducing agents than hydrogenare placed above hydrogen in the series and have negative values ofstandard reduction potentials. All those substances which have positivevalues of reduction potentials and placed below hydrogen in the series areweaker reducing agents than hydrogen.

ElementElectrode Reaction

(Reduction)Standard Electrode Reduction

potentialEo, volt

LiKCaNaMgAlZn

CrFeCdNiSnH2

Li+ + e- = LiK+ + e- = K

Ca2+ + 2e- = CaNa+ + e- = Na

Mg2+ + 2e- = MgAl3+ + 3e- = AlZn2+ + 2e- = Zn

Cr3+ + 3e- = CrFe2+ + 2e- = FeCd2+ + 2e- = CdNi2+ + 2e- = NiSn2+ + 2e- = Sn2H+ + 2e- = H2

-3.05-2.925-2.87-2.714-2.37-1.66

-0.7628

-0.74-0.44-0.403-0.25-0.140.00


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The metals on the top (having high negative values of standardreduction potentials) have the tendency to lose electrons readily. These are

active metals. The activity of metals decreases from top to bottom. The non-metals on the bottom (having high positive values of standard reductionpotentials) have the tendency to accept electrons readily. These are activenon-metals. The activity of non-metals increases from top to bottom.


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The activity of the metal depends on its tendency to lose electron orelectrons, i.e., tendency to form cat ion (M"+). This tendency dependson the magnitude of standard reduction potential. The metal whichhas high negative value of standard reduction potential readily losesthe electron or electrons and is converted into cat-ion. Such a metalis said to be chemically active.

The chemical reactivity of metals decreases from top to bottom inthe series. The metal higher in the series is more active than themetal lower in the series. For example,

(a) Alkali metals and alkaline earth metals having high negativevalues of standard reduction potentials are chemically active. Thesereact with cold water and evolve hydrogen. These readily dissolve inacids forming corresponding salts and combine with those substanceswhich accept electrons.


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(b) Metals like Fe, Pb, Sn, Ni, Co, etc., which lie a little down in the series donot react with cold water but react with steam to evolve hydrogen.

(c) Metals like Cu, Ag and Au which lie below hydrogen are less reactive anddo not evolve hydrogen from water.


(Reduction)Standard ElectrodeReduction potential

Eo, volt

LiK

CaNaMgAlZnCrFeCd

NiSnH2CuI2AgHg

Li+ + e- = LiK+ + e- = K

Ca2+

+ 2e-

= CaNa+ + e- = Na

Mg2+ + 2e- = MgAl3+ + 3e- = AlZn2+ + 2e- = ZnCr3+ + 3e- = CrFe2+ + 2e- = FeCd2+ + 2e- = Cd

Ni2+ + 2e- = NiSn2+ + 2e- = Sn2H+ + 2e- = H2Cu2+ + 2e- = Cu

I2 + 2e- = 2I-

Ag+ + e- = AgHg2+ + 2e- = Hg

-3.05-2.925

-2.87-2.714-2.37-1.66

-0.7628-0.74-0.44-0.403

-0.25-0.140.00

+0.337+0.535+0.799+0.885


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The electropositive character also depends on the tendency to loseelectron or electrons. Like reactivity, the electropositive character ofmetals decreases from top to bottom in the electrochemical series.On the basis of standard reduction potential values, metals are

divided into three groups:(a) Strongly electropositive metals: Metals having standard

reduction potential near about -2.0 volt or more negative like alkalimetals, alkaline earth metals are strongly electropositive in nature.(b) Moderately Electropositive Metals: Metals having values

of reduction potentials between 0.0 and about -2.0 volt aremoderately electropositive. Al, Zn, Fe, Ni, Co, etc., belong to thisgroup.(c) Weakly Electropositive Metals: The metals which arebelow hydrogen and possess positive values of reduction potentialsare weakly electropositive metals. Cu, Hg, Ag, etc., belong to thisgroup.


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(a) To predict whether a given metal will displace another,from its salt solution. A metal higher in the series will displace themetal from its solution which is lower in the series, i.e., the metalhaving low standard reduction potential will displace the metal fromits salt's solution which has higher value of standard reductionpotential. A metal higher in the series has greater tendency to

provide electrons to the cat-ions of the metal to be precipitated.


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Displacement of one nonmetal from its salt solution by anothernonmetal: A nonmetal higher in the series (towards bottom side), i.e.,having high value of reduction potential will displace another nonmetal withlower reduction potential i.e., occupying position above in the series. Thenonmetal's which possess high positive reduction potentials have thetendency to accept electrons readily.

These electrons are provided by the ions of the nonmetal having lowvalue of reduction potential. Thus, Cl2 can displace bromine and iodine from

bromides and iodides.

Cl2 + 2KI --> 2KC1 + I221- --> I2 + 2e

- (Oxidation)Cl2 + 2e- --> 2C1

- (Reduction)

[The activity or electronegative character or oxidizing nature of the nonmetalincreases as the value of reduction potential increases.]


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The metal which can provide electrons to H+ ions present in dilute acidsfor reduction, evolve hydrogen from dilute acids.

Mn --> Mn"+ + ne- (Oxidation)2H+ + 2e- --> H2 (Reduction)

The metal having negative values of reduction potential possess the propertyof losing electron or electrons.

Thus, the metals occupying top positions in the electrochemicalseries readily liberate hydrogen from dilute acids and on descendingin the series tendency to liberate hydrogen gas from dilute acidsdecreases.

The metals which are below hydrogen in electrochemical series like Cu, Hg,Au, Pt, etc., do not evolve hydrogen from dilute acids.


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Iron and the metals above iron are capable of liberating hydrogen fromwater. The tendency decreases from top to bottom in electrochemical series.

Alkali and alkaline earth metals liberate hydrogen from cold water but Mg, Zn

and Fe liberate hydrogen from hot water or steam.


(Reduction)Standard ElectrodeReduction potential

Eo, volt

LiKCaNaMgAlZnCrFe

CdNiSnH2CuI2AgHg

Li+ + e- = LiK+ + e- = K

Ca2+ + 2e- = CaNa+ + e- = Na

Mg2+ + 2e- = MgAl3+ + 3e- = AlZn2+ + 2e- = ZnCr3+ + 3e- = CrFe2+ + 2e- = Fe

Cd2+ + 2e- = CdNi2+ + 2e- = NiSn2+ + 2e- = Sn2H+ + 2e- = H2Cu2+ + 2e- = Cu

I2 + 2e- = 2I-

Ag+ + e- = AgHg2+ + 2e- = Hg

-3.05-2.925-2.87-2.714-2.37-1.66

-0.7628-0.74-0.44

-0.403-0.25-0.140.00

+0.337+0.535+0.799+0.885


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Reducing nature depends on the tendency of losing electron orelectrons. More the negative reduction potential, more is the tendency tolose electron or electrons. Thus, reducing nature decreases from top tobottom in the electrochemical series. The power of the reducing agentincreases as the standard reduction potential becomes more and more

negative.

Sodium is a stronger reducing agent than zinc and zinc is a stronger reducingagent than iron.

Element Na Zn FeReduction potential -2.71 -0.76 -0.44

-------------------------------------------->Reducing nature decreases

Alkali and alkaline earth metals are strong reducing agents.


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Oxidizing nature depends on the tendency to accept electron orelectrons. More the value of reduction potential, higher is the tendency toaccept electron or electrons. Thus, oxidizing nature increases from topto bottom in the electrochemical series. The strength of an oxidizingagent increases as the value of reduction potential becomes more and more

positive.

F2 (Fluorine) is a stronger oxidant than Cl2, Br2 and I2.Cl2 (Chlorine) is a stronger oxidant than Br2 and I2.

Element I2 Br2 Cl2 F2Reduction potential +0.53 +1.06 +1.36 +2.85

-------------------------------->Oxidizing nature increases


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Thus in electrochemical series


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The thermal stability of the metal oxide depends on its electropositivenature. As the electro-positivity decreases from top to bottom, the thermalstability of the oxide also decreases from top to bottom. The oxides of metalshaving high positive reduction potentials are not stable towards heat. Themetals which come below copper form unstable oxides, i.e., these are

decomposed on heating.

HeatAg2O ---------> 1/2 O2

2 Ag

Heat2HgO ------------> 1/2 O2

2 Hg


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In case two or more types of positive and negative ions are present insolution, during electrolysis certain ions are discharged or liberated at theelectrodes in preference to others.

In general, in such competition the ion which is strongeroxidizing agent (high value of standard reduction potential) isdischarged first at the cathode. The increasing order of deposition of

few cat-ions is:

K+, Ca2+, Na+, Mg2+, Al3+, Zn2+, Fe2+, H+, Cu2+, Ag+, Au3+----------------------------------------------------------->

Increasing order of deposition

Similarly, the anion which is stronger reducing agent (low value of

standard reduction potential) is liberated first at the anode.The increasing order of discharge of few anions is:

SO42-, NO3

-, OH-, Cl-, Br-, I-------------------------------------->Increasing order of discharge


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Thus, when an aqueous solution of NaCl containing Na+, Cl-, H+ and OH"

ions is electrolyzed, H+ ions are discharged at cathode and CF ions at theanode, i.e., H2 is liberated at cathode and chlorine at anode.

When an aqueous solution of CuS04 containing Cu2+

, , H+

and OH-

ions iselectrolyzed, Cu2+ ions are discharged at cathode and OH- ions at the anode.

Cu2+ + 2e- --> Cu (Cathode reaction)4OH- --> O2 + 2H2O + 4e

- (Anode reaction)

Cu is deposited on cathode while 02 is liberated at anode.


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Corrosion is defined as the deterioration of a substance because of itsreaction with its environment. This is also defined as the process by whichmetals have the tendency to go back to their combined state, i.e., reverse ofextraction of metals.

Ordinary corrosion is a redox reaction by which metals are oxidized byoxygen in presence of moisture. Oxidation of metals occurs more readily at

points of strain. Thus, a steel nail first corrodes at the tip and head. The endof a steel nail acts as an anode where iron is oxidized to Fe2+ ions.

Fe --> Fe2 + 2e- (Anode reaction)


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The electrons flow along the nail to areas containing impurities which act ascathodes where oxygen is reduced to hydroxyl ions.

O2 + 2H2O + 4e- --> 4OH- (Cathode reaction)

The overall reaction is2Fe + Oz + 2H2O = 2Fe(OH)2

Fe(OH)2 may be dehydrated to iron oxide, FeO, or further oxidised to Fe(OH)3

and then dehydrated to iron rust, Fe203.


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A more electropositive metal can displace a less electropositive metalfrom its salt's solution. This principle is applied for the extraction of Ag andAu by cyanide process. Silver from the solution containing sodium argentocyanide, NaAg(CN)2, can be obtained by the addition of zinc as it is moreelectro-positive than Ag.

2NaAg (CN)2 + Zn --> Na2Zn(CN)4 + 2Ag

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Electochemical Series