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XII 03 Electrochemistry - VII
EMF of a Galvanic cell Every Galvanic or voltaic cell is made up of two half cells, the oxidation half cell and reduction half cell. The potentials of these cells are always different. Due to this difference the current moves from higher potential to the elctrode with lower potential i.e from cathode to anode. Anode Electrons Cathode Current The difference in potentials of two half cells is known as emf ofthe cell or cell potential
(i) When oxidation potential of anode and reduction potential of cathode are taken: E"cell = oxidation potential + reduction potential of anode of cathode. = Eoanode-E cathode (ii) When reduction potentials of both the electrodes are taken into account: E cell E cathode - E anode E Right- E , (ii) When oxidation potentials of both electrodes are taken E"cell = oxidation potential of anode-oxidation potential ofcathode.
Difference between emf and potential difference: Emf Potential difference 1. It is the potential difference It is the difference in the electrode between two electrodes when potential when current is flowing no current is flowing through through the cell the circuit 2. It is maximum voltage that a It is always less than the maximum value cell can deliver of voltage which the cell can deliver 3. It is responisible for the steady flow of current in the of current in the cell cell It is not responsible for the steady flow
Reversible and irreversible cells: Daniel cell has emf value 1.1 volt. Ifan opposing emf exactly equal to 1.1 volt is applied to the cell, the cell reaction Stops butifit is increased beyond 1.1 volt the reaction is reversed Cu Zn2+ZnCu2+ Such a cell is called reversible cell. For reversibility the main conditions are: (D) The chemical reaction of the cell stops when an exactly equal opposing e.m.fis applied The chemical reaction of the cell is reversed and the current flows in opposite direction when the opposing emfis slighty greater than that ofthe cell. (ii) The cells which do not obey the above two condition are called irreversible cells. For e.g a cell consisting of zinc and copper electrodes dipped in H2S04 solution is ireversible in nature.
Prediction for occurance of a redor reaction: > Any redox reaction would occur spontaneous by if the free energy AG is negative. The free energy is related to the emf of the cell in the following manner: G can be negative only if EoCu 1s +ve
Electrode and cell potentials- Neinst equation: The electrode potential and the emf of the cell depend upon the nature of the electrode. temperature and the activities (concentration ofions in solution) The Gibbs free energy change in given by: AG AG 2.303RTlog a 72 an +ay m1z a-. activities ofreactants and products Go-free energy change for a reaction at standard conditions cel,--nFpo cell-2.303 RT [Anode] (Cathode 2.303 RT Anode) CellEO cell At 25 C putting the values of R' and , we get 2.303X8.314x298 (Anode Eelln 96500 O (Cathode) au = Ecell-0.591 log anode) (Cathode)
Potential of a single elctrode Applying Nernst equation: M Mn+ + ne- Applving Nernst equation Ecell = Eocell-0.059 log(Mn +) cell m log(Mn+) Equillibrium constant from E ll ow that 0,059 1os (cathed 0.059, [Anodel Ecell cell Cathode]
Equillibrium constant from E We know that Ecell E cell 0.059 [Anodel Cathodel For the Galvanic cell: Zn2 + Cu2+-Zn2+Cu log fcuat [Cu2+] At equilibrium cell conc. of products conc. of reactants nlog K or Eoll = 0.059 log K cell