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a, , ELECTROCHEMISTRY, , , , 4.1, ELECTROCHEMICAL CELLS, , Broadly speaking, electrochemical cells are of two types viz., electrolytic cells, and galvanic (or voltaic) cells. In electrolytic cells, a chemical reaction or more, accurately electrolysis is carried out by the use of electric current, whereas in, galvanic cells an electric current is produced due to the result of some spontaneous, chemical reaction. In other words, an electrolytic cell is a device to convert electrical, energy into chemical energy, whereas in a galvanic cell the chemical energy is, converted into electrical energy. We will, however, discuss the working of these two, , types of cells along with their sign conventions., [l] Electrolytic Cell ae, , Suppose the electrolysis of molten sodium chloride is carried out in an, electrolytic cell using platinum électrodes as shown in figure (1). The left side, , , , , , , , Fig. 1. Electrolytic cell., , electrode is ¢ ive terminal of an external battery which, , onnected to the negative term ee en a, Supplies electrons to this electrode. Therefore, the Teft side electrode becomes, Nepati, , atively charged. The electrons are taken off from the right side electrode by a, Positive terminal of the battery, Huse of this, this electrode aoauues ©, Positive ¢ arge. Within the solution, the electric current 1s carried out ds the, Movement of ions. The positive ions (i.e., cations) are attracted towar, , Negati Soe i .’».. anions) are attracted towards the, positive electrode and the negative ions (i.e., anions) veetede, the, , Tate electrode. As cations are attracted toward ee ee called the anode., Th €r is called the cathode. Similarly, the positive electroc ade tion at the anode,, § ® electrode reactions involve reduction at the cathode and oxidatio, Shown below : = :, , , , Scanned with CamScanner

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INTEGRATED CHEMIST,, 126, he, At cathode : Na +e, = 52h, +¢°, At anode : cl 2 2, , The overall change or process is ;, Na‘ +Cl> ——>Na+ S Cl,, , [Il] Galvanic Cell, , Agalvanic cell is a device which converts chemical energy (or potential energ, into electrical energy due to the redox reaction occurring within the cell., , One of the simplest galvanic cell is a Daniell cell. In it, zinc sulphate is placedi, a porous pot, in which a zinc rod is dipped. Copper sulphate solution is taken in, glass vessel in which a copper rod is dipped. The porous pot is then placed in tt, glass vessel. In it, the two solutions seep through the porous pot and, therefor, come in contact with each other at once. The reactions occurring at each electrod, are :, , At zinc electrode :, , Zn (s) == Zn?* (aq) + 2e7 (Oxidation, At copper electrode :, , Cu? (s)+ 2e7 == Cu(s) (Reduction, The cell reaction is thus represented as :, Zn(s) + Cu** (aq) == Zn?*, , If the two partial reactions are carried out ii, electrons will have to flow from a, , (aq) + Cu(s), , In a Daniell cell, a zinc electrode is di, , and a copper electrode (or some unreacti' luti a %, , ions, as shown in figure (2). From zinc electrode, a small aaioucl nay e, , the solution as Zn** ions, leaving a negative charge on the Cleckaes ie can an, e (because 0, , ped into a solution co:, , oe 4:, : ntaining Zn~~ 100, ve metal) into a so ef 3, , , , , , e, , Ammeter, , Porous plug, , , , ZnSO, solution CuSO, solution, Fig. 2. Daniell cell. £4», , Scanned with CamScanner

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Y, ELECTROCHEMISTR oF, , electrons left behind) and a positive charge in the solution. These charges stop the, jonisation process of zine almost instantaneously, for every Zn* ions entering the, solution, it or another ion will be driven back to the zinc electrode by the attraction, of the negative electrode and the repulsion of the positive solution. Similarly, a few, Cu~* ions from the solution may be deposited as copper atoms on the copper, electrode. This change will make the electrode positive and the solution negative, and so the deposition process is stopped. However, these processes at the two, electrodes, i.e., oxidation of zinc electrode and reduction at copper electrode can be, resumed if we connect the two electrodes by a wire to allow the electrons to flow, from zinc electrode to the electron deficient copper electrode and bring the two, solutions into contact by means of a salt bridge or a porous plug. This will, neutralise the charges of the two solutions and prevent a direct flow of Cu?" ions, towards the zinc electrode., , In this cell, the zinc electrode becomes negatively charged as the electrons, released during the oxidation reaction (Zn — Zn7* +2e~) accummulate at this, electrode. Similarly, the copper electrode becomes positively charged as the, electrons are taken out of this electrode for the reduction process (Cu** +2e7 >, Cu) which occurs at this electrode. As in the electrolytic cell, the electrode at which, oxidation reaction occurs is called the anode and that where reduction reaction, occurs is called the cathode. So, the negative electrode of the galvanic cell is called, the anode and the positive electrode as the cathode., , [Ill] Salt Bridge, A salt bridge is an inverted U-tube that contains an electrolyte. It is used to, connect the oxidation and reduction half-cells of a galvanic (voltaic cell). It, maintains electrical neutrality while charge flow with in the internal circuit. Which, in turn prevent the cell from rapidly running its reaction to equilibrium., Application of salt bridge : Consider the galvanic cell shown below :, , , , , Cu plate, , , , , , , , , , CuSO,, , Cathode half cell, , Anode half cell, , Fig. 3, , bove cell, an exc ;, Silas a. On the other hand an ex!, , 1. So, the flow of electrons, , fZn2* ions (positive, ee 02" ions, cess of SO4, , If there is no salt bridge i :, hrough the wire, , charge) would be there in the anode ce, , (negative charge in the cathode half cell, Would soon come to halt., , Scanned with CamScanner