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Electrochemistry - VIII XIl - o3
Electrochemical seri es:- By measuring the potentials of various electrodes versus standard hydrogen electrode, a series of standard electrode potential has been established. When the electrodes (metals and non-metals) in contact with their ions are arranged on the basis of the values of their standard reduction potentials or standard oxidation potentials the resulting series is called the elctrochemical, electromotive or activity series of the elements
Eleetrode Reaction (Reduetion) Standard Electrode Reduetion potential E.vol Element 3.05 2.925 - 2.87 Na 2.714 2.37 Na+e-Na 1.66 0.7628 0.74 0.44 | 0.25 0.14 0.00 +0.337 2H + + 2e-= H2 0.535 Ag +0.799 +0.88s Br Cl2 1.36 +2.87 5780358 09873 suonoola 201 01 kauapua lusapul ir 2 luate Suis!pixo se upiuans Suseasul Mg Zn Cr Fe ca Ni Sn H, Cu //// Au F,
Characteristics of electrochemical series The negative sign of standard electrode potential indicates that an electrode when ioned with SHE acts as anode and oxidation occurs at this electrode. The positive sign of standard electrode potential indicates that when joined with SHE it acts as cathode and reduction occurs at theis electrode All those elements which have a negative value of reduction potential are strong reducing agents and are placed above hydrogen in the series. All those which have positive value of reduction potential are strong oxidizing agents and are placed below hydrogen >The metals placed above hydrogen are active metals because of their larger reactivities. The activities of metals decreases from top to bottom.
Application of Electrochemical series: 1. Reactivity of metals: >The activity of metals depend on its tendency to loose electrons (M"+) >The metal which have high negative value (or smaller positive value) of standard reduction potential readily loses the electron and is converted into cation Alkali metals and alkaline earth metals have lower value of standard reduction potentials are chemically active. These react with cold water and evolve hydrogen >Metals like Fe, Pb, Sn, Ni, Co etc will not react with cold water but react with hot water > Metals like Cu, Ag, Au which lie below hydrogen are less reactive.
2. Electropositive character of metals: More the value of-ve reduction potential more is the electropositive character (a) Metals having E values near about 2.0 V or more negative are strongly electropositive in character. Metals having E value between 0.0 and -2.0 V are moderately electropositive Al, Zn, Fe, Ni, Co etc. (b) (c)Metals which are below hydrogen and posess positive value of reduction potential are weakly electrolpositive for e.g Cu, Hg, Ag etoc.
3. Displacement reactions:- To predict whether a given metal will displace another from its salt solution: The metal having low standardreductionpotential wll displace the metal from its salt solution which has higher value of standardreduction potential. Fore.g zinc and displace copper. Displacement ofhydrogen from dilute acids by metals: Metals occupying top positions in the elctrochemical series readily liberate hydrogen from dilute acids and on descending in the series tendency to liberate hydrogen gas from dilute acids decreases. For example metals like Cu, Hg, Au, Pt. etc. do not evolve hydrogen from dilute acids Displacement ofone no-metal from its salt solutionby anothernon-metal A non-metal having higher value ofreduction potential will displace another non-metal with lower reduction potential. For e.g. Cl2 +2KI 2KCI+12 Chlorine can displace iodine as the value ofchlorineis +1.36V and that ofiodine is 0.54 V
4. Reducing Power of metals More the negative reduction potential more is the tendency to loose electrons and more is the reducing power Element Na Zn Fe Reduction Potential -271-0.736-0.44 Reducing nature decreases 5. Oxidising natue of non-metals More the value of reduction potential higher in the tendency to accept electron increases and hence the oxidizing power also increases. Element 12 Br2 C2 F2 Reduction Potential +0.53 +1.06+1.36+2.85
6. Thermal stabilitv of metallic oxides:- The thermal stability ofmetal oxide depends on its electropositive nature. As the electropositive character decreases the thermal stability ofthe oxide also decreases. The stability of the oxide also decreases. The oxides ofmetals having high positive reductionpotentials are not stable towards heat Ag20 2Ag02 2Hg0-2 Hg + 02 Bao Na20No decompositions AL203
7. Product of electrolvsis:- If two cations and two anions are present in solution then during electrolysis the ions which has high value of standard reduction potential is discharged first at the cathode Ca2+,Na+,Mg2 +,A13+Zn2+,Fe2+,H+,Cu2+Ag+,Au3+ inc.order Similarly the anion having low value of standard reduction potential is liberated first at the anode. inc.order of discharge
Acidic medium: FeFe -Fe2+/Fe2+ 0.77V re Fe2+ , Alkaline medium: Eo Fe Indicates that iron will dissolve in acid medium to form Fe2+ ion. Since hence +2 state ofiron will be more stable than +3 state Fe2+ Fe3+ is negative