The electrochemical series

Similar series can be drawn up for the affinity of the elements towards chlorine, fluorine, sulfur, etc. As with the electromotive series, these are all similar to each other, but not identical. Differences of order occur. Again, they generally arise where the values are close together sometimes, however they are more serious. 

Note that the electromotive series is an affinity series for the formation of compounds in solution. Likewise the displacement series. 

The contact series, the various electromotive series, the displacement series, and the various affinity series turn out to be remaikably similar to each other. Though by no means all identical, their similarities sufficiently outweigh their differences to encourage the formation of a single series as an approximate expression of all of them. This series is called the electrochemical series. 

Inevitably, there has been much discussion and argument about the exact order in which the elements should be put in the conflated series. Inorganic chemistry is not an exact science, as I have commented before. Nevertheless, all chemists are agreed about the general position that each element should have in the series - far left, mid right, etc. - and carry this assignment in their heads as a rough measure of the character of the element concerned. 

Classification of elements according to the electnx hemical series 

Some organic co-factors and metal centers in proteins act as electron transfer agents in a number of biological processes we need to be able to predict which species is reduced or oxidized in a redox reaction. 

We have seen that a cell reaction has 1C 1 if 0 and that 0 corresponds to reduction at the right-hand electrode. We have also seen that may be written as the difference of the standard potentials of the redox couples in the right and left electrodes (eqn 5.17, = r - El). A reaction corresponding to 

A couple with a low standard potential has a thermodynamic tendency to reduce a couple with a high standard potential. 

More briefly low reduces high and, equivalently, high oxidizes low. The same arguments apply to the biological standard values of the potentials. 

Consider the iron-containing protein ferredoxin, which participates in plant photosynthesis (Section 5.11), and cytochrome c, which participates in the last steps of respiration (Section 5.10). It follows from Table 5.2 that 

---

Metallic gold, which is found free in nature, has always been valued for its nobility, i.e. its resistance to chemical attack. This property is to be expected from its position in the electrochemical series. It... 

Almost all common metals and structural steels are liable to corrode in seawater. Regulations have to be followed in the proper choice of materials [16], In addition, there is a greater risk of corrosion in mixed constructions consisting of different metals on account of the good conductivity of seawater. The electrochemical series in seawater (see Table 2-4), the surface area rule [Eq. (2-44)] and the geometrical arrangement of the structural components serve to assess the possibility of bimetallic corrosion (see Section 2.2.4.2 and Ref. 17). Moreover the polarization resistances have considerable influence [see Eq. (2-43)]. The standards on bimetallic corrosion provide a survey [16,17]. 

Alumiojuffl resists corrosion not because of its position in the electrochemical series but because of the ra Hd formation of a coherent, inert, oxide layer. Contact with grafihite, Fe. Ni. Cu, Ag or Pb is disastrous for corrosion resistance, the effect of contact with steel, Zn and Cd depends on pH and exposure conditions. Protection is enhanced by anodizing the metal this involves immersing it in 15-20% H2SO4 and connecting it to the positive terminal so that it becomes coated with alumina ... 

In the electrochemical series of elements, copper is near the noble end and will not normally displace hydrogen, even from acid solutions. Indeed, if hydrogen is bubbled through a solution of copper salts, copper is slowly deposited (more rapidly if the process is carried out under pressure). (See Section 1.2 for thermodynamic considerations.)... 

Nickel occupies an intermediate position in the electrochemical series Ni2 /Ni = -0-227 V, so that it is more noble than Zn and Fe but less noble than Sn, Pb and Cu. Figure 4.21 shows a revised potential-pH equilibrium (Pourbaix) diagram for the Ni-H O system at 25°C. The existence of the higher anhydrous oxides Nij04, NijO, and NiOj shown in an earlier diagram appears doubtful in aqueous systems in the absence of positive identification of such species. It is seen that ... 

Zinc is relatively low in the electrochemical series and is widely regarded as an active metal. However, when high-purity zinc is placed in hydrochloric acid it will dissolve extremely slowly, if at all. It may be encouraged to... 

Concentrated hydrochloric acid will dissolve many metals (generally those situated above hydrogen in the electrochemical series), as well as many metallic oxides. Hot concentrated nitric acid dissolves most metals, but antimony, tin and tungsten are converted to slightly soluble acids thus providing a separation of these elements from other components of alloys. Hot concentrated sulphuric acid dissolves many substances and many organic materials are charred and then oxidised by this treatment. 

The question arises as to which metal is dissolved, and which one is deposited, when combined in an electrochemical cell. The electrochemical series indicates how easily a metal is oxidized or its ions are reduced, i.e., converted into positively charged ions or metal atoms respectively. The standard potential serves for the comparison of different metals. 

For transmetallations with a metal (metallo-de-metallations, Scheme 10-95) arylmercury compounds are particularly suitable due to the position of mercury as a noble metal in the electrochemical series of standard potentials (for examples see Makarova, 1970). 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

Corrosion is the unwanted oxidation of a metal. It cuts short the lifetimes of steel products such as bridges and automobiles, and replacing corroded metal parts costs billions of dollars a year. Corrosion is an electrochemical process, and the electrochemical series is a source of insight into why corrosion occurs and how to prevent it. 

Predict the spontaneous direction of a redox reaction by using the electrochemical series (Example 12.7). 

Because mercury lies above hydrogen in the electrochemical series, it is not oxidized by hydrogen ions. However, it does react with nitric acid ... 

The range of chemical reactivity of metals is wide, from the inertness of the platinum group to the extreme reactivity of some alkali metals. The order of metal reactivity follows essentially the order of the electrochemical series which is shown in Table 17.4 for the metals commonly deposited by CVD. 

The ease with which an atom gains or loses electrons is termed the electronegativity of the element. Tabulation of the elements in order of ease by which they lose electrons is called the electrochemical series and is shown in Table 6.10. Chapter 4 explains the importance of this to the formation and control of corrosion, and Chapter 6 discusses the relevance to predicting reactivity of metals towards water and their potential to become pyrophoric. 

Table 6.3 Variation in the nature of hydroxides of metals according to their positions in the electrochemical series.

Before dealing with various important applications of the electrochemical series with some practical examples, a break is given here to present a more detailed elaboration on the hydrogen electrode, reference electrodes, and some of the theoretical and general aspects pertaining to electrode potentials and free energy changes involved with cell reactions. 

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

The positive value of the standard voltage obtained in the example indicates that the cell reaction shown is spontaneous. Thus, the standard potentials in Table 6.11 can be used to predict whether a particular reaction will occur, or not. The advantage of Table 6.11 is that it provides quantitative as well as qualitative information. It not only conveys that nickel is a stronger oxidizing agent than silver (because nickel is positioned below silver in the electrochemical series), but it also conveys how much stronger, in terms of the cell emf of+1.05 V. 

Thus, in the case of iron coated with zinc (galvanized sheet), zinc would protect iron by sacrificing itself, i.e., by anodically dissolving in the corroding media. However, in the case of iron coated with tin (tinned sheet), tin would protect iron against corrosion by virtue of its own corrosion-resistance properties however, any flaw in the coating would enhance the corrosion of iron since it is anodically disposed to tin according to their placements in the electrochemical series. 

The electrochemical series table provides important clues as regards the chemical reaction. The evolution of hydrogen by the reaction of zinc and dilute sulfuric acid is facilitated by the impurity of copper or graphite in zinc. Copper and zinc form a local electrochemical... 

The principle of the displacement of one metal by another, or in other words of the displacement of nobler by base or not so noble metals, as described earlier, must be applied with due caution, without neglecting other effects that may not be immediately obvious from consideration of the electrochemical series. Some of these effects are illustrated in the following. Although the position of lithium is above that of sodium in the series, lithium cannot displace sodium from common salt solutions since both of these metals occupy positions higher up than hydrogen and will displace this element from the solution. It must be borne in mind, therefore, that the series applies to aqueous solutions, and the hydrogen ion, which is present in these solutions, can also take part in the displacement reactions. 

The electrochemical series corresponds only to the standard condition, i.e., for unit activity of the ions, since a change to another ionic concentration can alter the order of the electrode potentials of the elements very markedly. The case of nickel plating mentioned earlier may be taken as typically illustrative of the many practical examples of the effects and the consequences of nonstandard conditions. It must also be mentioned in the context of the examples of displacement reactions provided earlier that the concentrations and the electrode potentials frequently vary during a displacement reaction. 

The foregoing two examples have been taken to convey that the data of Table 6.11 can very well be used to determine the equilibrium constant for any reaction which is the overall reaction for a cell assembled with electrodes contained in the electrochemical series table. 

The various possible electrode reactions at the cathode and at the anode in electrolytic cells have been shown in Table 6.2. It has been pointed before that the outcome of an electrolytic process can be made on the basis of knowledge of electrode potentials and of overvoltages. The selection of the ion discharged depends on the following factors (i) the position of the metal or group in the electrochemical series (ii) the concentration and (iii) the nature of the electrode. Examples provided hereunder deliberate on these aspects. 

---