specific reaction with thiosulphate ions (S O – ), which too is a redox reaction: I (aq) + S O – (aq) → 2I – (aq) + S O – (aq) ( . ) I , though insoluble in water, remains in solution containing KI as KI . On addition of starch after the liberation of iodine from the reaction of Cu + ions on iodide ions, an intense blue colour appears. This colour disappears as soon as the iodine is consumed by the thiosulphate ions.
Thus, the end-point can easily be tracked and the rest is the stoichiometric calculation only. . . Limitations of Concept of Oxidation Number As you have observed in the above discussion, the concept of redox processes has been evolving with time.
This process of evolution is continuing. In fact, in recent past the oxidation process is visualised as a decrease in electron density and reduction process as an increase in electron density around the atom(s) involved in the reaction. . Redox Reactions and Electrode Processes The experiment corresponding to reaction ( .
), can also be observed if zinc rod is dipped in copper sulphate solution. The redox reaction takes place and during the reaction, zinc is oxidised to zinc ions and copper ions are reduced to metallic copper due to direct transfer of electrons from zinc to copper ion. During this reaction heat is also evolved. Now we modify the experiment in such a manner that for the same redox reaction transfer of electrons takes place indirectly.
This necessitates the separation of zinc metal from copper sulphate solution. We take copper sulphate solution in a beaker and put a copper strip or rod in it. We also take zinc sulphate solution in another beaker and put a zinc rod or strip in it. Now reaction takes place in either of the beakers and at the interface of the metal and its salt solution in each beaker both the reduced and oxidized forms of the same species are present.
These represent the species in the reduction and oxidation