Electromotive series table

The tendency for a species to become oxidized or reduced determines the sign and potential of the half-cell. The tendency is strongly related to the chemical reactivity of the species concerned in aqueous systems. Based on the potential developed in a half-cell under controlled conditions, the elements may be arranged in an order known as the activity series or electromotive series (Table 15.1). In general, the metals at the top of the activity series are most chemically reactive and tend to give up electrons easily, following the reaction of M° M" - - e. The metals at the bottom of the series are more noble and therefore less active. They do not give up electrons easily in fact, their cations will accept electrons from metals above them in the activity series. In the process, the cations become neutral metal atoms and plate out of solution, while the more active metals become ionic and dissolve. This is illustrated as follows ... 

Nowadays, tables of standard electrode potentials are used instead of the electromotive series. They include electrode reactions not only of metals but also of other substances [Table 3.1 for detailed tables, see the books of Lewis and Rendall (1923) and Bard et al. (1985)]. 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

ACTIVITY SERIES- Also referred to as the electromotive series or the displacement series, this is an arrangement of the metals (other elements can be included) in the order of their tendency to react with water and acids, so that each metal displaces from solution those below itiu the series and is displaced by those above it. See Table 1. Since the electrode potential of a metal in equilibrium with a solution of its ions cannot be measured directly, the values in the activity series are, in each case, the difference between the electrode potential of the given metal tor element) in equilibrium with a solution of its ions, and that of hydrogen in equilibrium with a solution of its ions. Thus in the table, it will be noted that hydrogen lias a value of 0.000. In experimental procedure, the hydrogen electrode is used as the standard with which the electrode potentials of other substances are compared. The theory of displacement plays a major role in electrochemistry and corrosion engineering. See also Corrosion and Electrochemistry. 

Standard [reduction] potentials for hundreds of electrodes have been determined (mostly in the period 1925-45, during which time they were referred to as oxidation potentials ) and are usually tabulated in order of increasing tendency to accept electrons. This ordering is also known as the electromotive series of the elements. As can be seen in the abbreviated version in Table 1, sodium is the most active of the metallic elements in the sense that its oxidation product Na+ shows the smallest tendency (as indicated by the highly negative voltage) to undergo reduction. 

It can be seen from this equation that the greater the difference between the standard potentials of the two metals Mi and M2, the larger will be the equilibrium ratio of activities (or concentrations) of the respective ions. The greater the difference between the standard potentials, therefore, the more completely will one metal displace another from a solution of its ions. The metal with the more positive (oxidation) potential, as recorded in Table XLIX, will, in general, pass into solution and displace the metal with the less positive potential. The series of standard potentials, or electromotive series, as it is sometimes called, thus gives the order in which metals are able to displace each other from solution the further apart the metals are in the series the more completely will the higher one displace the lower one. It is not true, however, to say that a metal lower in the series will not displace one higher in the series some displacement must always occur until the required equilibrium is established, and the equilibrium amounts of both ions are present in the solution. 

TABLE 3.4. Position of Thngsten in the Electromotive Series of Metals in Different Molten Salts... 

D) Acids do not spontaneously spit out a proton Despite our way of writing ionization equilibria as shown on the next page, acids do not give up a proton unless a base comes by to take the proton away. The reactions as drawn in the table should be considered half-reactions, just as the reactions in the electromotive series were half-reactions for balancing oxidation-reduction reactions in general chemistry. 

A Table of the Standard Potentials of the Elements, at 25°. The Electromotive Series. In Table II the standard potentials of the... 

Whereas there is no concensus of opinions as to the cause of overvoltage, its existence helps to explain quite a number of electrochemical phenomena. Thus if the (usually comparatively small) effect of ion activities is ignored, any metal above hydrogen in the electromotive series given in Table II of Chapter 14 should react, in an acid solution, with the evolution of hydrogen gas. Actually, as is well known, the metals, from zinc downward in the series, if moderately pure, react comparatively slowly in acid solutions. For instance, if a piece of pure zinc is placed in dilute sulfuric acid the reaction... 

In chemistry, metals are distinguished by their chemical properties, and there are two main groups. Reactive metals, such as the alkali metals and alkaline-earth metals, are electropositive elements. They are high in the electromotive series and tend to form compounds by losing electrons to give positive ions. They have basic oxides and hydroxides. This typical metallic behavior decreases across the periodic table and increases down a group in the table. 

When the standard electrode potentials are listed in decreasing value as shown in Table 14.6, an electromotive series is created which has the hydrogen half-cell reaction listed at a potential of zero. These values are reduction potentials at 25°C referred to the standard hydrogen electrode (SHE). Metals listed at the top of the series are noble or less reactive. Metals listed below the hydrogen reaction are reactive, that is, they corrode more readily. A metal listed below another metal will displace it from a solution containing the higher metal s ions. Iron, for example, wUl have copper metal plated on it when placed in a copper sulfate solution. This is an iron corrosion reaction. Metals listed below hydrogen will displace the H" " ions from acid solutions. 

The electromotive series is a list of the elements in accordance with their electrode potentials. The measurement of what is commonly known as the "single electrode potential", the "half-reaction potential" or the "half-cell electromotive force" by means of a potentiometer requires a second electrode, a reference electrode, to complete the circuit. If the potential of the reference electrode is taken as zero, the measured E.M.P. will be equal to the potential of the unknown electrode on this scale. W. Ostwald prepared the first table of electrode potentials in 1887 with the dropping mercury electrode as a reference electrode. W. Nernst selected in 1889 the Normal Hydrogen Electrode as a reference electrode. G.N. Lewis and M. Randall published in 1923 their table of single electrode potentials with the Standard Hydrogen Electrode (SHE) as the reference electrode. The Commission of Electrochemistry of the I.U.P.A.C. meeting at Stockholm in 1953 defined the "electrode potential" of a half-cell with the SHE as the reference electrode. 

If we do a last step and postulate that the considered couple should also given standard conditions, then the potential E changes to the standard redox potential This potential is useful for classifying redox couples with respect to their oxidizing or reducing capability. Now we can arrange redox couples in the order of their individual standard potentials. The resulting tables are called electrochemical series or electromotive series. 

For maximum utility the encyclopedia has been arranged into five sections. Part I is an electromotive series. Part II lists tables of EMF values according to elements. Part III lists EMF values of common Inorganic anions. Part IV lists electrode potentials of various organic functional groups. Finally, Part V lists EMF data for the inorganic nitrogen bases, NH3 and NjH. ... 

While some electromotive series differentiate between reactions in the acid and basic ranges, our tables are all-encompassing, as the... 

For each environment, a galvanic series can be con-stmcted in which metals are arranged in order of their corrosion potential, with the most active metals at the top, and the most inactive metals at the bottom. These series usually are similar to, but are not exactly the same, as the well-known electromotive series. The best defined (and the most commonly used) galvanic series is that based on corrosion potentials in salt solution, which is reproduced in Fig. 4. Lists of corrosion potentials of specific aluminum alloys can be found in Chapter 2 (see Tables 2 to 5). It is important to remember that various aluminum alloys have sufficiently (Bf-ferent corrosion potentials (by as much as 0.4 V in some cases) to cause strong galvanic cells when in contact with each other. Care must be taken therefore that all alloys and tempers are compatible, even in an all-aluminum stmcture. 

The above condition is commonly referred to as the Dirichlet boundary condition. The known potential value is usually selected from the electromotive force series table. 

Table 6.11 lists, to the right of the arrows, reducing agents or disposition to electron loss or disposition to oxidation in order of increasing strength. Such a list is more popularly called the electromotive force, or emf, series. The maximum potential difference which can be measured for a given cell is called the electromotive force (abbreviated emf) and represented by the symbol Ecell. It may be recounted that the emf values reported in Table 6.11 are for those cells under specified standard conditions in which all the concentrations are 1 M and pressures are 1 atm. The emf of such a cell is said to be its standard electromotive force, and is given by the symbol E ell. 

---