Electro Chemical Cells
The electrochemical cells can be divided into two categories. These are (1) electrolytic cells, and (2) galvanic cells. In an electrolytic cell, a desired chemical reaction is carried out with the help of current, whereas in a galvanic cell, current is generated as a result of some spontaneous chemical reaction that occurs in the cell. In this section, we describe the main characteristics of galvanic cells.
For simplicity, we take the example of a Daniel cell in which the spontaneous reaction is
Zn + Cu2+(aq) Zn2+(aq) + Cu
Zn Zn2+(aq) + 2e-
Cu2+(aq) + 2e- Cu
in two different places. The most obvious way to carry out these two partial reactions is to insert the Zn electrode into a solution containing Zn2+ and the Cu electrode (or some unreactive metal) into a solution containing Cu2+ . Oxidation reaction takes place at Zn electrode and reduction at Cu electrode. These two electrodes can be connected by an electrical wire to allow the electrons to flow from the Zn electrode to the electron-deficient Cu electrode. The two solutions can be brought into contact through a porous plug or a salt bridge to prevent a direct flow of Cu2+ towards the Zn electrode.
In the cell shown the Zn electrode becomes negatively charged as the electrons released in the oxidation reaction (Zn Zn2+ + 2e-) reside at this electrode. Similarly, the Cu electrode becomes positively charged as the electrons are consumed for the reduction reaction (Cu2+ + 2e- Cu) that takes place at this electrode.
In both types of electrochemical cells (i.e. electrolytic and galvanic), the electrode at which oxidation reaction occurs is known as the anode and the where reduction reaction occurs is knows as cathode. Thus, the negative electrode of a galvanic cell is known as anode and the positive one as the cathode.
Following the recommendations given above, we can write the expressions for cell reaction and the cell potential. Taking an example of the Daniel cell written as
Zn| Zn2+(aq)|| Cu2+(aq)| Cu
In the Daniell cell, electrons flow from Zn electrode to Cu electrode. This is due to the fact that Zn atom can be more easily oxidized to Zn2+ than Cu atom to Cu2+. On the other side. Cu2+ can be more easily reduced than the Zn2+. Consequently, Zn is oxidized to Zn2+ and the electrons see free at the Zn electrode move towards Cu electrode where Cu2+ is reduced to Cu. Basically, we can say that the flow of electrons is due to the difference in oxidation tendencies of the two atoms or it is due to the difference in reduction tendencies of the two ions. The relative oxidation tendencies of atoms can be represented by the oxidation potentials. A more easily oxidizable atom will have a large value of oxidation potential and a less easily oxidizable atom will have a low value of oxidation potential. Thus, Zn atom has a large oxidation potential than the Cu atom. Electrons in the external circuit flow from the electrode of higher oxidation potential to the electrode of lower oxidation potential. The difference in potential which causes a current flow from the electrode of higher potential to the one of lower potential is known as the electromotive force, abbreviated as emf, of the cell and is expressed in volts. We will represent emf of a cell by the symbol Ecell. Thus
Ecell = Higher oxidation potential - Lower oxidation potential
As per recommendations of IUPAC (International Union of Pure and Applied Chemistry), Ecell is expressed in terms of reduction potentials (known as standard potentials) of the two electrodes. The reduction potentials measure the relative reduction tendencies of ions and their values are simply the negative values of oxidation potentials of the corresponding atoms. This follows from the fact that an atom with the maximum tendency of oxidation (i.e. highest value of oxidation potential) will yield an ion with the least tendency of reduction (i.e. the minimum reduction potential) and vice versa.
Ecell = - (- Higher oxidation potential) + (- Lower oxidation potential)
= - Lower reduction potential + Higher reduction potential
Ecell = Higher reduction potential - Lower reduction potential
The emf of a galvanic cell can be determined experimentally by using a calibrated potentiometer or an electronic voltmeter, in which negligible current is withdrawn from the cell. In the potentiometric method, the emf of the cell is matched with the varying potential difference of an external standard cell. During this matching, it is also possible to determine the negative (or anode) and positive (or cathode) terminals of the cell under study. In fact, the terminal attached to the negative (or positive) terminal of the external cell is negative (or positive) end of the cell.