Hardik Darji Ppt

8/3/2019 Hardik Darji Ppt

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Electromotiveforce series

Name: Hardik b Darji

Roll No:903


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A serial arrangement of metallicelements or ions according to their

electrode potentials determinedunder specified conditions; the ordershows the tendency of one metal to

reduce the ions of any other metalbelow it in the series


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A series of metals and their ions arranged inorder of increasing standard electrodepotential in electrolytic solutions. Astandard hydrogen electrode is usuallychosen as the reference electrode, and theelectrode potential of hydrogen in theseries is given as zero. In the USSR andmany other European countries, the

electrode potential of a particular metal istaken to have the same sign as the chargeof the metals electrode relative to thestandard hydrogen electrode. In the USA,however, the opposite sign was used until

1975.


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The most negative potentials,approximately - 3 volts (V), arecharacteristic of the alkali metals,followed by the alkaline-earth metals. Themost positive potentials, approximately +1.5 V, are those of the noble metals. The

electromotive force series frequentlyincludes nonmetals, ions, and selectedchemical compounds. The order in whichthe most common metals are arranged in

the series is Li, K, Ca, Na, Mg, Al, Mn, Zn,Fe, Co, Ni, Sn, Pb, H2, Cu, Hg, Ag, and Au.


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The position of an element in theelectromotive force series is somewhatarbitrary inasmuch as the value of theelectrode potential depends on thetemperature and composition of thesolution in which the electrodes are

immersed and especially on the activity (orconcentration) of the ions of a givensubstance in the electrolyte. The conditionof the surface of the electrode (smoothor rough) is an important factor, in

particular the presence of a protectiveoxide layer.


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Table gives the standard electrodepotentials of metal with reference to

standard hydrogen electrode (SHE) whichis arbitrarily defined as zero. Potentialsbetween metals are determined by takingthe absolute difference between theirstandard potentials. The determination ofstandard electrode potential is shown fig.2.


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As shown above, the electrode potential oftwo different metals in an electrode can be

compared. Each metal in contact with anelectrolyte of its ion forms a half cell. Themost practical method of obtaining reliableand consistent the value of relative electrodepotential is to compare the value of each half

cell with a common reference electrode. This common electrode is the standardhydrogen electrode which consists of aplatinum wire platinized by electrolysissurrounded by a solution having a H+ ion

activity of(aH+=1)of one and enveloped in ahydrogen gas at1 atm pressure. The potentialof the hydrogen electrode is given by:

Pt(s)/H2 (1atm)/H+ (aH+=1), E=0V


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The electrode potential of all metals iscompared with the STD hydrogen electrode

and it is called the STD electrode potential(E0). Between two metals, such as zinc and

aluminum, aluminum is more active than zinc.A metal with a more negative potential has ahigher tendency to corrode (dissolve) than ametal with a less negative potential,although kinetic factors may intervene. Ifthe potential, reduction rather thanoxidation takes place (electrons are gained),M+ + e M.


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Metals which correspond to relatively lowerstd potentials E0 are called active metal

metals and metals which corresponds torelatively higher standard potential or lessnegative potentials are called noble metals.Cu, Ag, Au are example of noble metalswhereas K, Li, Mg are between iron and zinc;

Zinc is more actives than iron. Between Cuand Ag, Ag is more noble than Cu. Ifelectrodes reaction occurs on differentmetals at room temp are arranged inaccordance with the value of STDpotentials, an electrochemical series at250C is obtained.


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A less electropositive metal would displace amore electropositive metal from one of itssalts in aqueous solution. For instance, if a rodof zinc is placed in a solution of copper

sulfate, zinc would dissolve in the solution andcopper would be dischargedZn+CuSO4 ZnSO4+Cu

Consequently, copper will deposit on the zincrod.

Electrode potential also indicates thetendency of cations in aqueous solutions to bereduced at cathode. For example silver ionsare reduced more readily than cupric ions,because silver is more electropositive.


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Metal ions above hydrogen are morereadily reduced than hydrogen ions with

100%efficiency. The metals in the series with highpositive potentials are recognized asmetals with good corrosion resistance.They show a little tendency to pass froma metallic state to an ionic state.Conversely, metals with high negativepotentials show a tendency to corrode,but whether they corrode or not

instance, iron has a potential of -0.440Vand indicates tendency to corrode, but ifit develops a film of oxide it would notcorrode. This effect is used informulating by an invisible oxide films.


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The metal with more negative potential isgenerally the anode, and the metal with a lessnegative potential, the cathode. if zinc and

aluminums are coupled, aluminum wouldbecome the anode and zinc the cathode. itmay be expected that in the presence ofmetals which are more negative than hydrogenin emf series, hydrogen reduction would be

the preferred process. That is, however, notalways the case. Metals as negative as zinccan be plated from an acid solution withoutliberation of hydrogen. In the case ofaluminum, however, hydrogen evolution would

be the preferred process, and, therefore,aluminum is deposited by electrolysis of a non-aqueous melt in order not to give any chancefor the liberation of hydrogen which mayenter metal and cause its embrittlement. Theemf series is also useful for the electrolyticrefining of the metals.


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1. The emf series lists only pure metals which haveonly a limited use in engineering application s.alloys are of major interest to engineers ratherthan pure metals.

2. The electrode potential has little relevance topotentials of metals in solution, in which thepotential of interest is the corrosion potentialand not electrode potential of the metal.

3. The position of metals in emf series is instance,

iron has a potential of -0.440V and indicatestendency to corrode, but if it develops a film ofoxide it would not remain active and hance,itwould not corrode. this effect is used informulating stainless steel, which are coveredby an invisible oxide film.


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4.The emf series predicts the tendency to

corrode but it cannot predict whethercorrosion would actually take place. Forinstance, on the basis of some negativepotential, iron shows tendency tocorrode, however, if it develops a passive

film in some environment it would notcorrode.5.The emf series cannot always predict the

effect of environment. For instance, in

food cans, tin can become active to iron.Such a change cannot be predicted byemf series.


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6.The effect of film formation on thetendency of the metal to passivity

solution cannot be predicted by emfseries. For insrance, titanium andaluminum are more negative than iron.However, in certain environments theyform a film which makes theirpotential less active than iron. Theeffect of film formation on tendencyof the metals to corrode is kinetic andcannot be predicted by thethermodynamic emf series.


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EMF Series Galvanic Series

Absolute relative

Quantitative qualitative

pure metals only metals & alloys

half-cell potential corrosion potential

standard conditions any specified conditions

based on thermodynamic analysis

based on thermodynamic

analysis

used for theoretical calculations used for practical applications


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Hardik Darji Ppt