We have seen that different metal/metal ion combinations have different values of standard electrode potentials. The various elements can be arranged in order of increasing or decreasing values of their standard reduction potentials. The arrangement of various elements in the order of increasing values of standard reduction potentials is called electrochemical series. The electrochemical series, also called activity series consisting of some electrodes along with their respective reduction reactions has been given in Table 33.1.
Table 33.1. Standard Electrode Potentials
APPLICATIONS OF THE ELECTROCHEMICAL SERIES
Some of the important applications of the electrochemical series have been discussed as follows:
1. Calculation of the standard EMF of the cell. From the electrochemical series, the standard reduction potentials of electrodes are found out. The electrode with higher reduction potential is taken as cathode and other as anode. From this EMF0 of the cell is calculated as:
EMF o = E o cathode – E o anode
However, if the reaction taking place in the cell is also to be determined, the following steps are followed:
(a) Write reduction equations for both the electrodes along with their standard reduction potentials, separately.
(b) Balance each reaction with respect to the number of atoms of each kind and the electrical charges.
(c) Multiply the reactions by a suitable number so that the number of electrons involved in both the half reactions are equal.
(d) Subtract the equation for reaction having the lower standard reduction potential from the other reaction having higher standard reduction potential. The difference gives the standard EMF of cell.
The electrode having higher standard reduction potential acts as cathode and the other electrode acts as anode. Now let us solve some numerical problems to understand the applications of this method in calculating standard EMF.
2. Comparison of the reactivities of metals. The metals which occupy higher position in electrochemical series have lower reduction potentials. This indicates that ions of such metals will not be reduced to metals easily. On the contrary, such metals would be easily oxidized to their ions by losing electrons. Therefore, from the position of the metal in the electrochemical series, it is possible to predict the relative reactivities of metals. The metal having smaller reduction potential can displace metals having larger reduction potential from the solutions of their salts. In other words, the metal occupying higher position in the series can displace the metals lying below it from the solutions of their salts.
Thus, we may conclude that the metals occupying higher positions in the electrochemical series are more reactive in displacing the other metals from the solutions of their salts. For example, zinc lies above copper in the series and hence, it can displace copper from copper sulphate solution. Copper cannot displace zinc from zinc sulphate solution because it lies below zinc in the series and hence, it is less reactive.
3. Predicting the feasibility of a redox reaction. With the help of electromotive series we can predict whether a given redox reaction will take place or not. From the given equation, the substances undergoing oxidation and reduction are identified. The substance undergoing oxidation will act as anode and the substance undergoing reduction will act as a cathode. The EMF of this hypothetical cell is calculated as under :
EMF o = E o cathode – E o anode
If EMF o comes out to be positive the given redox reaction will take place and if EMF0 comes out to be negative the given redox reaction will not take place.
4. To predict the reaction of a metal with dilute acids to liberate hydrogen gas. Some metals like Fe, Zn react with dil. acids like HCl, H2SO4 to liberate H2 gas while some metals like Cu, Ag do not liberate H2 gas with dil. HCl, dil. H2SO4. A prediction about ability of a given metal to produce H2 gas by its reaction with dilute acids can be easily made from the knowledge of electromotive series. Chemical reaction between metal and acid can be represented, in general as
For the above reaction to occur, the E o red of metal (M n+ M) must be lower than that of hydrogen. Hence, it can be concluded that metals which lie above hydrogen in the electromotive series can reduce H+ ions to hydrogen and hence, liberate hydrogen gas on reaction with dil acids. In other words, metals having negative reduction potentials can displace hydrogen from acids. For example, zinc (E o zn+2 / z n = -0.76 volt) lies above hydrogen in the series and hence, it can displace hydrogen from dilute acids, whereas copper (E0 eu•2teu = +0.34 volts) which is lying below hydrogen in the series cannot displace hydrogen from acids.
5. Relative Oxidising and Reducing Powers of Various Substances. Substance with higher value of standard reduction potential have greater tendency to undergo reduction. For example F 2 has highest reduction potential which means it is most easily reduced to p– ions. In other words, F2 is best oxidising agent. Li+ ion, on the other hand, had lowest reduction potential. Hence Li+ is weakest reducing agent or conversely Li metal is best reducing agent. Thus, it can be concluded that substances with higher reduction potentials are strong oxidising agents while substances with lower reduction potentials are strong reducing agents.