INTRODUCTION :
Electro chemistry is the study of production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy
to bring about non-spontaneous chemical transformations. The subject is of importance both for theoretical and practical considerations. A large
number of metals, sodium hydroxide, chlorine, fluorine and many other chemicals are produced by electrochemical methods. Batteries and fuel cells
convert chemical energy into electrical energy and are used on a large scale in various instruments and devices. The reactions carried out electrochemically
can be energy efficient and less polluting. Therefore, study of electro chemistry is important for creating new technologies that are ecofriendly. The transmission ofsensory signals through cells to brain and vice versa and communication between the cells are known to have electrochemical origin. Electro chemistry, is
therefore, a very vast and interdisciplinary subject. In this Unit, we will cover only some of its important elementary aspects.
electrochemical cells : we had studied the construction and functioning
of Daniell cell. This cell converts the chemical energy liberated
during the redox reaction. zns+cu2+aq→zn2+aq+custo electrical energy and has an electrical potential equal to 1.1 V when concentration of Zn2+ and Cu2+ ions is unity (1 mol dm–3). Such a device is called a galvanic or a voltaic cell.
If an external opposite potential is applied in the galvanic cell and increased
slowly, we find that the reaction continues to take place till the opposing voltage reaches the value 1.1 when, the reaction stops altogether and no current flows through the cell. Any further increase in the external potential again starts the reaction but in the opposite direction. It now functions as an electrolytic cell, a device for using electrical energy to carry non-spontaneous chemical reactions. Both types of cells are quite important and we shall study some of their salient features in the following.
galvanic cells : As mentioned earlier a galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy. In this device the Gibbs energy of the spontaneous redox reaction is converted into electrical work which may be used for running a motor or other electrical gadgets like heater, fan, geyser, etc.
Daniell cell discussed earlier is one such cell in which the following redox reaction occurs.
zn(s)+cu2+(aq)->zn2+(aq)+cu(s)
These reactions occur in two different portions of the Daniell cell. The reduction half reaction occurs on the copper electrode while the oxidation half reaction occurs on the zinc electrode. These two portions of the cell are also called half-cells or redox couples. The copper electrode may be called the reduction half cell and the zinc electrode, the oxidation half-cell.
We can construct innumerable number of galvanic cells on the pattern
of Daniell cell by taking combinations of different half-cells. Each halfcell consists of a metallic electrode dipped into an electrolyte. The two half-cells are connected by a metallic wire through a voltmeter and a switch externally. The electrolytes of the two half-cells are connected internally through a salt bridge. Sometimes, both the electrodes dip in the same electrolyte solution and in such cases we do not require a salt bridge.
At each electrode-electrolyte interface there is a tendency of metal ions from the solution to deposit on the metal electrode trying to make it positively charged. At the same time, metal atoms of the electrode have a tendency to go into the solution as ions and leave behind the electrons at the electrode trying to make it negatively charged. At equilibrium, there is a separation of charges and depending on the tendencies of the two opposing reactions, the electrode may be positively or negatively charged with respect to the solution. A potential difference develops between the electrode and the electrolyte which is called
electrode potential. When the concentrations of all the species involved
in a half-cell is unity then the electrode potential is known as standard
electrode potential. According to IUPAC convention, standard reduction potentials are now called standard electrode potentials. In a galvanic cell, the half-cell in which oxidation takes place is called anode and it has a negative potential with respect to the solution. The other half-cell in which reduction takes place is called cathode and it has a positive potential with respect to the solution. Thus, there exists a potential difference between the two electrodes and as soon as the switch is in the on position the electrons flow from negative electrode to positive electrode. The direction of current flow is opposite to that of
electron flow.
