Electromotive series

When a metal is in contact with a solution containing its ions, a difference in electrical potential arises due to the resultant effect of two tendencies of the metal on the one hand to throw off positive ions and thus charge the solution positively, and of the metal ions on the other hand to deposit on the metal and impart then-charges to it. 

The elements for which no figures are given are placed in approximately their correct position in the series. 

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The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

Volta discovered that when he used different metals in his pile some combinations had a stronger effect than others. From that information he constructed an electromotive series. How would Volta have ordered the following metals, if he put the most strongly reducing metal first Fe, Ag, Au, Zn, Cu, Ni. Co, Al ... 

Other organometallic compounds that are hydrolyzed by water are those of sodium, potassium, lithium, zinc, and so on, the ones high in the electromotive series. Enantioselective protonation of lithium enolates and cyclopropyllithium compounds have been reported. When the metal is less active, stronger acids are required. For example, R2Zn compounds react explosively with water, R2Cd slowly, and R2Hg not at all, though the latter can be cleaved with concentrated HCl. How-... 

Many organometallic compounds are best prepared by this reaction, which involves replacement of a metal in an organometallic compound by another metal. The compound RM can be successfully prepared only when M is above M in the electromotive series, unless some other way is found to shift the equilibrium. That is, RM is usually an unreactive compound and M is a metal more active than M. Most often, RM is R2Hg, since mercury alkyls are easy to prepare and mercury is far down in the electromotive series." Alkyls of Li, Na, K, Be, Mg, Al, Ga, Zn, Cd, Te, Sn, and so on have been prepared this way. An important advantage of this method over 12-36 is that it ensures that the organometallic compound will be prepared free of any possible halide. This method can be used for the isolation of solid sodium and potassium alkyls." If the metals lie too close together in the series, it may not be... 

When a metal is immersed into the solution of salt of another metal farther to the right in the electromotive series, the first metal dissolves (is oxidized) while the second metal is deposited (its ions are reduced). Thus, the first metal displaces the second from its solution. 

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)]. 

The process of precipitation of a metal from an aqueous solution of its salt by another metal is the well-known cementation process, so named because the precipitated metal is usually cemented on the metal introduced into the system. The process prediction stems from consideration of electrode potentials of metals. The metal positioned with greater (oxidation) potential in the electromotive series will pass into solution and remove a metal positioned with a less positive potential. The larger the spread of the positions of the two metals in the series, the greater is the possibility or feasibility of cementing out one by the other. 

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... 

The Leclanche dry cell with zinc replaced by magnesium offers considerable advantages. In the electromotive series, magnesium occupies a more basic position as compared to zinc ( n = -0.76 V / g = -2.38 V), and this means that the voltage of a magnesium cell is greater than that of a similar zinc cell. 

The compilation of such data may further be used to tabulate electromotive series of redox potentials in any given donor solvent. 

Metal Corrosion.—When a metal above hydrogen in the electromotive series is placed in a solution of an electrolyte there is a tendency for the metal to ionize i.e., to split into ions and electrons for instance, with iron, the following reaction tends to take place... 

The Electrodeposition of Metals.—If hydrogen were evolved at a cathode at its reversible potential under all conditions no metal higher in the electromotive series than hydrogen could be deposited electro-lytically from an aqueous solution. Due to hydrogen overvoltage, however, it is possible to obtain deposits of these metals as high in the... 

Al possesses a great affinity for oxygen and when finely divided (powdered, flaked, etc) it bums in the air. It burns also when made in the form of a thin ribbon similar to that of Mg. When Al powder is mixed and heated with an oxide of a metal below it in the electromotive series, displacement takes place, for instance in Thermite (qv) ... 

Using the scale in which hydrogen is zero, the electrode potential of arsenic in contact with normal arsenious chloride is -0-27 volt.8 Using an electrode formed by plating arsenic on copper, Marquis 7 made a series of potential measurements in an alcoholic solution of arsenious chloride and from his results concluded that arsenic should be placed between hydrogen and copper in the electromotive series the nature of the electrolyte used, however, influences the relative positions of... 

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. 

In his original design Volta stacked couples of unlike metals one upon another in order to increase the intensity of the current. This arrangement became known as the "voltaic pile." He studied many metallic combinations and was able to arrange the metals in an "electromotive series" in which each nielal was positive when connected to the one below it in the series. Volta s pile was the precursor of modem batteries. 

Additional explanatory material is added throughout the book. Some of the material explains further and simplifies concepts under discussion, while other material adds new information. In Chapter 2 paint pigments and binders are discussed. New information on surprising origins of ancient paint pigments and binders is added. In Chapter 3 supports for paints are described, and information is added about supports used in 50 B.C. that are still used today. In Chapter 6, in which jewelry making and the study of metals and electrochemistry are combined, an electromotive series is added to enhance cell voltage calculations. 

By use of an electromotive series (E° values) for standard half-reactions written as reductions (see Chapter 2) the potential of each half-cell can be calculated by means of the Nemst equation ... 

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. 

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