Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s)
The experiment corresponding to the above 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 half reactions.
A redox couple is defined as having together the oxidised and reduced forms of a substance taking part in an oxidation or reduction half reaction.
This is represented by separating the oxidised form from the reduced form by a vertical line or a slash representing an interface (e.g. solid/solution). For example in this experiment the two redox couples are represented as Zn2+/Zn and Cu2+/Cu. In both cases, oxidised form is put before the reduced form.
Now we put the beaker containing copper sulphate solution and the beaker containing zinc sulphate solution side by side (Figure).
We connect solutions in two beakers by a salt bridge (a U-tube containing a solution of potassium chloride or ammonium nitrate usually solidified by boiling with agar agar and later cooling to a jelly like substance).
This provides an electric contact between the two solutions without allowing them to mix with each other. The zinc and copper rods are connected by a metallic wire with a provision for an ammeter and a switch. The set-up as shown in Figure is known as Daniell cell.
When the switch is in the off position, no reaction takes place in either of the beakers and no current flows through the metallic wire. As soon as the switch is in the on position, we make the following observations:
The transfer of electrons now does not take place directly from Zn to Cu2+ but through the metallic wire connecting the two rods as is apparent from the arrow which indicates the flow of current.
The electricity from solution in one beaker to solution in the other beaker flows by the migration of ions through the salt bridge. We know that the flow of current is possible only if there is a potential difference between the copper and zinc rods known as electrodes here.
The potential associated with each electrode is known as electrode potential. If the concentration of each species taking part in the electrode reaction is unity (if any gas appears in the electrode reaction, it is confined to 1 atmospheric pressure) and further the reaction is carried out at 298K, then the potential of each electrode is said to be the Standard Electrode Potential.
By convention, the standard electrode potential (E- of hydrogen electrode is 0.00 volts. The electrode potential value for each electrode process is a measure of the relative tendency of the active species in the process to remain in the oxidised/reduced form.
A negative E- means that the redox couple is a stronger reducing agent than the H+/H2 couple. A positive E- means that the redox couple is a weaker reducing agent than the H+/H2 couple.
The standard electrode potentials are very important and we can get a lot of other useful information from them. You will learn more about electrode reactions and cells in Class XII.