Module-1: Electrode Potential And Cells 2015

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Howard Shaw
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1 Lecture-2 Standard Electrode potential Standard electrode potential is the electrode potential when the metal is in contact with a solution of its own ions of unit concentration (1M) at 298K. If the electrode involves gas, then the gas is measured at one atmosphere pressure. Standard electrode potential is denoted by E 0. Electromotive force [EMF] of the cell When a two electrode are coupled there is a flow of electrons from anode to cathode, indicates that the two electrodes have different potential. The potential difference between the two electrodes of a galvanic cell, which causes the flow of current from one electrode of higher potential to the other lower electrode, is called the electromotive force [EMF] of the cell or cell potential. Denoted by Ecell. EMF depends on the nature of the electrode, temperature and concentration of the electrolyte solution. Standard emf of a cell is defined as the emf of a galvanic cell, when the reactants and products of the cell reaction are at unit concentration [or unit activity] at 298 K and one atmosphere pressure. Relationship between EMF and free energy The cell reactions are the spontaneous reaction. In a spontaneous process free energy of the system decrease. Decrease in free energy is equal to maximum useful work done by the system. Representation of a cell [Cell notation] If two electrodes are combined to form a cell then the cell is represented by writing the electrode side by side with oxidation electrode [or anode] on the left and reduction electrode [or cathode] on the right. Separated by a salt bridge indicated by a double line. Hence the cell can be represented as, Ex: Daniel cell Copper- Silver nitrate cell: M 1 /M 1 n+ (C1) // M 2 n+ (C2) /M 2 Zn ZnSO 4 (1M)// CuSO 4 (1M)/Cu Cu/Cu 2+ (0.1M)//Ag + (0.1M)/Ag Sign Convention for cell 1) The overall electrochemical cell reaction is split into two half reaction i.e. oxidation half reaction and reduction half reaction. 2) Equal the number of electrons in the two half reactions by multiplying with a suitable number, if necessary.

3) The reaction, taking place at the left electrode is written as an oxidation reaction and that taking place at the right side is written as the reduction reaction.

2 3) The reaction, taking place at the left electrode is written as an oxidation reaction and that taking place at the right side is written as the reduction reaction. Then the overall cell reaction is the sum of these two reactions. Ex: Daniel cell Zn Cu e - Zn e - [Anode] Cu [Cathode] Zn + Cu 2+ Zn 2+ + Cu cell reaction The cell reaction is spontaneous and is positive. 4. The emf of the cell will be positive when the oxidation takes place at the left half cell and reduction takes place in right half cell as a result of spontaneous reaction. 4) EMF of the cell is the difference of electrode metals (Anode rod & Cathode rod) between the right half cell and left half cell. E cell = E righ t E left E cell =E cathode - E anode Standard emf is E 0 cell=e 0 cathode-e 0 anode The term electrode potential always refers to reduction potential. Construction of Galvanic cell Galvanic cell generally consists of two electrodes dipped in two electrolyte containing its own ions, which are separated by porous diaphragm or connected through a salt bridge. Ex : Daniel cell is an example for a galvanic cell. Daniel cell consist of two electrodes, one of which is of zinc rod dipped in 1M Zinc sulfate solution and the other is with a copper rod dipped in IM Copper sulphate solution. These two solutions are connected by a salt bridge. [salt bridge is a U tube containing a Paste of KCl or NH 4 NO 3 and Agar gel]. These two electrodes are connected externally through an ammeter by using a wire. The reaction taking place in a galvanic cell is the oxidation-reduction reaction. Zinc electrode undergoes oxidation i.e., Zinc goes in the solution as Zn 2+ ions with the liberation of electron in metal, hence it is called anode which is negatively charges electrode. The ZnSO 4 solution becomes richer with Zn 2+.

3 At anode Zn Zn e- [Oxidation] At the copper electrode reduction takes place. i.e., Cu 2+ ions takes two electrons. and get deposited as a metallic copper on copper rod. Reduction leads to positively charged electrode with copper solution of more dilute with respect to Cu 2 ions. At cathode Cu e- Cu [Reduction] Each electrode reaction is called half cell reaction. The net reaction is the sum of the two electrode reactions of which one is oxidation and the other is reduction called electrochemical reaction. The flow of electrons is from Zinc electrode to copper electrode. Hence the direction of flow of electrons from left hand side to right hand side (Anode to cathode). Whereas current flows from right hand side to left hand side i.e., Reduction compartment to oxidation compartment (Cathode to anode). Classification of Galvanic cells Galvanic cells are classified into 1) Primary Galvanic cells: The cell in which the cell reaction is not completely reversible are referred to as primary cells. They cannot be easily recharged electrically and hence are discarded after discharge of electricity Ex : Dry Cell. 2) Secondary galvanic cells: The cells in which the cell reaction is are called secondary cell. They are rechargeable by passing current through them in the direction opposite to that of discharge current. Ex : Lead acid accumulator Ni-Cadmium cell 3) Concentration cells: An electrochemical cell form by the combination of same electrode dipped in the same electrolyle solution of different concentration is called a concentration cell. Ag/AgNO 3 (0.01M)// AgNO 3 (0.1M)/Ag Derivation of Nernst equation for single electrode potential It shows the quantitative relationship between the electrode potential and concentration of reactants and products of a redox reaction. Let us consider a redox reaction

4 M < > M n+ + ne - (Oxidation & Reduction reaction) By convension, the reduction electrode reaction taken in the forward direction as electrode potential is the measure of reduction tendency. M n+ + ne -..>M For any reversible reactionn the Gibbs free energy change & equilibrium constant can be written as ΔG=RTlnK = RTln[products]/[Reactants].(1) R=Gas constant(8.314j/k/mol) K=[products]/[Reactants]=[M]/[M n+] According to Vant hoff s isothermic equation eqn (1) can be written as ΔG= ΔG 0 +RTln[M]/[M n+].(2) Also we know that, If the electrode potential is E volt and the electrode reaction involves transfer of n electrons with F is the Faradays of current passed, then the electrical work available from the electrode is nfe volt coulomb or Joules. Hence the decrease in free energy of the system is equal to electrical work done as the system. -ΔG= nfe or -ΔG 0 = nfe 0 Or ΔG= -nfe or ΔG 0 = -nfe 0 Substituting values of ΔG= -nfe or ΔG 0 = -nfe 0 in eqn (2) -nfe = -nfe o + RTln[M]/M n+ ].(3) Divide eqn(3) by nf E=E 0 - RTln[M]/M n+ ] E=E RT/nF log [M]/M n+ ] E=E RT/nF log [1]/[M n+ ] Since [M]=1 for a pure metal E=E RT/nF log [M n+ ] At 298K, E=E /n log[m n+ ] This expression is known as Nernst s equation for electrode potential at 298K. From this equation, it is clear that,

5 if concentration of solution (M n+ ) is increased, the electrode potential increases and vice versa. if temperature is increased, the electrode potential increases and vice versa. The Nernst equation can also be applied for the calculation of emf of a cell. Consider the cell reaction aa+ bb <=======> cc +dd The Nernst equation for the emf of the cell is E cell = E o cell RT/nF log [C] c [D] d /[A] a [B] b i.e. E cell = E o cell / n log [C] c [D] d /[A] a [B] b at 298 K. Where n is the number of electrons transferred during the cell reaction and E o cell is the standard emf of the cell. Additional information Importance of electrode potential: - When elements / electrodes are arranged in the increasing order of their standard electrode potentials is known as the electrochemical series. Electro Chemical series provides valuable information s that in constructing electrochemical cells. In electrochemical cells lower electrode potential metal acts as anode and higher electrode potential acts cathode. Metal ion Li + + e - Li (Base) Potential in Volts (Anode) K + + e - K Ca e - Ca Na + + e - Mg e - Al e - Zn e - Cr e - Na Mg Al Zn Cr Fe e - Fe Ni e - Ni

6 Sn e - Sn Pb e - Pb Fe e - Fe H + + e - ½ H (Reference) Cu e - Ag + + e - Pt e - Au + + e - Cu Ag Pt Au ½ F 2 + e - F - (Noble) (Cathode)

Review: Balancing Redox Reactions. Review: Balancing Redox Reactions

Review: Balancing Redox Reactions Determine which species is oxidized and which species is reduced Oxidation corresponds to an increase in the oxidation number of an element Reduction corresponds to a