Reducing agent : Donor of electron.

Competitive Electron Transfer Reactions. Place a strip of metallic zinc in an aqueous solution of copper nitrate for about one hour. You may notice that the strip becomes coated with reddish metallic copper and the blue colour of the solution disappears. Formation of Zn2+ ions among the products can easily be judged when the blue colour of the solution due to Cu2+ has disappeared. If hydrogen sulphide gas is passed through the colourless solution containing Zn2+ ions, appearance of white zinc. sulphide, ZnS can be seen on making the solution alkaline with ammonia. The state of equilibrium for the reaction represented by equation 11. For this purpose, let us place a strip of metallic copper in a zinc sulphate solution. No visible reaction is noticed and. attempt to detect the presence of Cu2+ ions bypassing H2S gas through the solution to produce the black colour of cup ric sulphide,CuS, does not succeed. Cup ric sulphide has such a low solubility that this is an extremely sensitive test; yet the amount of Cu2+ formed cannot be detected. We thus conclude that the state of equilibrium for the reaction 11 greatly favours the products over the reactants.

Example : Electron transfer reaction now to copper metal and silver nitrate solution in water and arrange a set-up as shown in the solution develops blue colour due to the formation of Cu2+ ions on account of the reaction : Redox reaction between zinc and aqueous solution of copper nitrate occurring in a beaker. Here, Cu s is oxidised to Cu2+aq and Ag+aq is reduced to Ag. Equilibrium greatly favours the products Cu2+aq and Ag. By way of contrast, let us also compare the reaction of metallic cobalt placed in nickel sulphate solution.

The reaction that occurs here is : Redox reaction between copper and aqueous solution of silver nitrate occurring in a beaker. At equilibrium, chemical tests reveal that both Ni2+aq and Co2+aq are present at moderate. concentrations. In this case, neither the reactants Co s and Ni2+aq nor the products Co2+aq and Ni are greatly favoured. This competition for release of electrons in cidently reminds us of the competition for release of protons among acids. this table in which metals and their ions are listed on the basis of their tendency to release electrons just as we do in the case of acids to indicate the strength of the acids.

The electron releasing tendency of the metals is in the order : Zn Cu Ag is a simple example of metal activity series or electro chemical series. The competition for electrons between various metals helps us to design a class of cells, named as Galvanic cells in which the chemical reactions become the source of electrical energy.

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