REDOX REACTIONS IN TERMS OF ELECTRON TRANSFER REACTIONS PART 02

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Reducing agent :Donor of electron s .
Competitive Electron TransferReactions. Place a strip of metallic zinc in an aqueoussolution of copper nitrate for about one hour. You may notice that thestrip becomes coated with reddish metalliccopper and the blue colour of the solutiondisappears. Formation of Zn2+ ions among theproducts can easily be judged when the bluecolour of the solution due to Cu2+ hasdisappeared. If hydrogen sulphide gas ispassed through the colourless solutioncontaining Zn2+ ions, appearance of white zinc. sulphide, ZnS can be seen on making thesolution alkaline with ammonia.The stateof equilibrium for the reaction represented byequation 11. For this purpose, let us placea strip of metallic copper in a zinc sulphatesolution. No visible reaction is noticed and. attempt to detect the presence of Cu2+ ions bypassing H2S gas through the solution toproduce the black colour of cupric sulphide,CuS, does not succeed. Cupric sulphide has such a low solubility that this is an extremelysensitive test; yet the amount of Cu2+ formedcannot be detected. We thus conclude that thestate of equilibrium for the reaction 11 greatly favours the products over the reactants.Example 2: Electron transfer reaction nowto copper metal and silver nitrate solution inwater and arrange a set-up as shown inThe solution develops blue colour dueto the formation of Cu2+ ions on account of thereaction:Redox reaction between zinc and aqueous solution of copper nitrate occurring in a beaker. Here, Cu s is oxidised to Cu2+aq andAg+aq is reduced to Ag s. Equilibrium greatlyfavours the products Cu2+aq and Ag s .By way of contrast, let us also compare thereaction of metallic cobalt placed in nickelsulphate solution. The reaction that occurshere is :Redox reaction between copper and aqueous solution of silver nitrate occurring in a beaker.At equilibrium, chemical tests reveal that bothNi2+aq and Co2+aq are present at moderate. concentrations. In this case, neither thereactants Co s and Ni2+aq nor the productsCo2+aq and Ni s are greatly favoured.This competition for release ofelectronsincidently reminds us of the competition forrelease of protons among acids. this table inwhich metals and their ions are listed on thebasis of their tendency to release electrons justas we do in the case of acids to indicate thestrength of the acids. Theelectron releasing tendency of the metals is inthe order: Zn Cu Ag is a simple example of metal activityseries or electrochemical series. Thecompetition for electrons between variousmetals helps us to design a class of cells,named as Galvanic cells in which the chemicalreactions become the source of electricalenergy.