Redox Reactions

(i) In aqueous solution, AgNO₃ ionises to give Ag⁺(aq) and NO₃^– (aq) ions.
AgN0₃(aq) ——–> Ag⁺(aq) + NO₃^– (aq)
Thus, when electricity is passed, Ag⁺(aq) ions move towards the cathode while NO₃^– ions move towards the anode. In other words, at the cathode, either Ag⁺(aq) ions or  H₂O molecules may be reduced. Which of these will actually get discharged would depend upon their electrode potentials which are given below:
Ag⁺(aq) +e^–———-> Ag(s); E ° = +0.80 V …(i)
2H₂O(Z) + 2e^–  ————> H₂(g) + 2OH^–(aq); E ° = -0.83 V …(ii)
Since the electrode potential (i.e., reduction potential of Ag⁺(aq) ions is higher than that of H₂O molecules, therefore, at the cathode, it is the Ag⁺(aq) ions (rather than H₂O molecules) which are reduced.
Similarly, at the anode, either Ag metal of the anode or H₂O molecules may be oxidised. Their electrode potentials are:
Ag(s) ———–> Ag⁺(aq) + e^–; E ° = -0.80 V …(iii)
2H₂O(l) ————–> 02(g) +4H+(aq)+4e^– ; E ° = -1.23 V …(iv)
Since the oxidation potential of Ag is much higher than that of H₂O, therefore,
at the anode, it is the Ag of the silver anode which gets oxidised and not the H₂O molecules. It may, however, be mentioned here that the oxidation potential of  N0₃^–ions is even lower than that of H₂O since more bonds are to broken during reduction of N0₃ ions than those in H₂O.
Thus, when an aqueous solution 0f AgN0₃  is electrolysed, Ag from Ag anode dissolves while Ag⁺(aq) ions present in the solution get reduced and get deposited on the cathode.
(ii) If, however, electrolysis of AgN0₃  solution is carried out using platinum electrodes, instead of silver electrodes, oxidation of water occurs at the anode since Pt being a noble metal does not undergo oxidation easily. As a result, O₂ is liberated at the anode according to equation (iv).
Thus, when an aqueous solution of AgNO₃ is electrolysed using platinum electrodes, Ag⁺ ions from the solution get deposited on the cathode while 02 is liberated at the anode.
(iii) In aqueous solution, H₂S0₄ionises to give H⁺(aq) and SO₄^2-(aq) ions.
H₂S0₄(aq) ——> 2H⁺(aq) +S0₄^–(aq)
Thus, when electricity is passed, H⁺  (aq) ions move towards cathode while SO₄^2-(aq) ions move towards anode. In other wode either H⁺(aq) ions or H₂O molecules are reduced. Their electrode potentials are:2H⁺(aq)2e^– ——->  H₂(g); E ° = 0.0 V
H₂O(aq) + 2e^– ——-> H₂(g) + 2OH^–((aq); E ° = -0.83 V
Since the electron potential (i.e., reduction potential) of H⁺(aq) ions is higher than that of H₂O, therefore, at the cathode, it is H⁺(aq) ions (rather than H₂O molecules) which are reduced to evolve H₂  gas.
Similarly at the anode, either SO₄^2-(aq) ions or H₂O molecules are oxidised. Since the oxidation potential of SO₄ is expected to be much lower (since it  involved cleavage of many bonds as compared to those in H20) than that of HjO molecules, therefore, at the anode, it is H₂O molecules (rather than SO₄^2- ions) which are oxidised to evolve O₂ gas.
From the above discussion, it follows that during electrolysis of an aqueous solution of H₂S0₄ only the electrolysis of H₂O occurs liberating H₂ at the cathode and O₂ at the anode.
(iv) In aqueous solution, CuCl₂ ionises as follows:
CuCl₂(aq) ——-> CU²⁺(aq) + 2Cl^–(aq)
On passing electricity, CU²⁺(aq) ions move towards cathode and CU²⁺(aq) ions move towards anode.
Thus, at cathode, either  CU²⁺(aq) or H₂O molecules are reduced. Their electrode potentials are:
CU²⁺+ 2e^–  ———> Cu(s); E ° = +0.34 V
H₂O(l) + 2e^–  ——–> H₂(g) + 2OH^–; E ° = -0.83 V
Since the electrode potential of CU²⁺(aq) ions is much higher than that of H₂O, therefore, at the cathode, it is CU²⁺(aq) ions which are reduced and not H₂Omolecules.
Similarly, at the anode, either Cl^–(aq) ions or H₂O molecules are oxidised. Their oxidation potentials
2Cl^–(aq) ——> Cl₂(g) + 2e^–; AE ° = -1.36 V
2H₂O(l) ——>O₂(g) + 4H⁺(aq) + 4e^–; ∆E ° = -1.23 V
Although oxidation potential of H₂O molecules is higher than that of Cl^– ions, nevertheless, oxidation of Cl^–(aq) ions occurs in preference to H₂O since due to overvoltage much lower potential than -1.36 V is needed for the oxidation of H₂O molecules.
Thus, when an aqueous solution of  CuCl₂ is electrolysed, Cu metal is liberated at the cathode while Cl₂ gas is evolved at the anode.