Standard emf series

The standard emf series based on hydrogen is obviously not applicable to molten salt electrolysis systems. No emf series similar to that for aqueous systems has been established for molten electrolytes this is due to the nonavailability of accepted standard electrodes and the use of numerous molten electrolytes involving widely differing tamperers, consequent to the widely varying melting temperatures of the salts used. In spite of these, many emf series have been compiled, using a variety of molten salts with different stand-... 

As is evident from Eq. (11.4), copper and zinc are very far apart in the standard EMF series, so alloy codeposition seems next to impossible. Fortunately, the difference can be eliminated (even reversed) by changing the values of the activities. This can be achieved by inducing a considerable change in ionic concentrations via complex ion formation, as discussed in detail below. 

Table 2.1 Standard aqueous half-cell potentials at 25 °C (also known as standard electrode, redox, or oxidation potentials, and as the standard emf series)(a)...

From the reversible electrode potential in the standard emf series of metals, it is possible to predict whether a particular metal will spontaneously dissolve. 

The potential between metals exposed to solutions containing approximately one atom gram weight of their respective ions (unit activity) are precisely measured at constant temperature. Table 2.1 presents the standard emf series of metals. 

For the standard emf series, ranking is based on the magnitude of the voltage generated when the standard cell of a metal is coupled to the standard hydrogen electrode at 25°C (77°F). 

The half-cell potentials in the standard emf series are thermodynamic parameters that are valid only at equilibrium corroding systems are not in equilibrium. Furthermore, the magnitudes of these potentials provide no indication as to the rates at which corrosion reactions occur. 

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. 

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. 

Each electrochemical reaction has its own reversible potential, just as each element has its own melting temperature. A list of these reversible potentials under standard conditions is called an electromotive (emf) series. 

The standard reversible potential is that listed in the EMF series of Table 1 and represents a special case of the Nernst equation in which the second term is zero. The influence of the solution composition manifests itself through the logarithmic term. The ratio of activities of the products and reactants influences the potential above which the reaction is thermodynamically favorted toward oxidation (and conversely, below which reduction is favored). By convention, all solids are considered to be at unit activity. Activities of gases are equal to their fugacity (or less strictly, their partial pressure). 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

The values obtained for the standard emf of Cell A at the nine temperatures (t in °C) were fitted to power series equations in t — 25, with the following results ... 

By combining many pairs of half-cells into voltaic cells, we can create a list of reduction half-reactions and arrange them in decreasing order of standard electrode potential (from most positive to most negative). Such a list, called an emf series or a table of standard electrode potentials, appears in Appendix D, with a few examples in Table 21.2 on the next page. 

Describe how standard electrode potentials (Ehair-ceii values) are combined to give Ecen and how the standard reference electrode is used to find an unknown haif-ceiii explain how the reactivity of a metal is related to its Ehaif-ceih write spontaneous redox reactions using an emf series like that in Appendix D ( 21.3) (SPs 21.3,21.4) (EPs 21.24-21.40)... 

The emf series of the standard half-ceU electrode potential on the hydrogen scale are given in Table 2.2. The reactions in this table are written as reduction reactions from left to right at T=25 °C. They have the same polarity as the reduction potential, which is measured experimentally. 

The Emf Series is an orderly arrangement of the standard potentials for all metals. The more negative values correspond to the more reactive metals (Table 3.2). Position in the Emf Series is determined by the equilibrium potential of a metal in contact with its ions at a concentration equal to unit activity. Of two metals composing a cell, the anode is the more active metal in the Emf Series, provided that the ion activities in equilibrium are both unity. Since unit activity corresponds in some cases to impossible concentrations of metal ions because of restricted solubility of metal salts, the Emf Series has only limited use for predicting which metal is anodic to another. 

The standard reference point in the EMF series is the hydrogen electrode, which consists of gaseous hydrogen at 1 atm bubbling over a platinum electrode in an acidic solution with activity 1 for the hydrogen ion (Figure 11-6). Similar electrodes can be made for some other non-metallic elements, and a few of these elements are included in the table, as well as some other oxidation-reduction pairs. 

Electromotive force series (EMF Series) A list of elements arranged according to their standard electrode potentials, the sign being positive for elements whose potentials are cathodic to hydrogen and negative for those anodic to hydrogen. 

In addition, galvanic corrosion can be predicted by using the electromotive force (emf) or standard potential series for metal reduction listed in Table 2.1. These reactions are reversible. The standard metal potential is measured against the standard hydrogen electrode (SHE), which is a reference electrode having an arbitrary standard potential equals to zero. Details on types of reference electrodes are included in chapter 2. 

For carbon steel stmctures, the NACE recommended protective potential, Ec = -0.85 Vcufouso t and the standard potential or emf series (Table 2.2) of an element, which is anodic to iron, can be used to determine the ceU potential and the free energy change. Thus, E — —0.85 Vcu/ouso + Ea where Ea from Table 2.2 is for a reduction process and therefore, its sign must be changed and the its magnitude be changed from Vshe to Vou/ouso -... 

The standard emf and galvanic series are rankings of metallic materials on the basis of their tendency to corrode when coupled to other metals. 

Each metal or metal area will develop an electrode with a measurable electrical potential. This potential can be referenced to that of a standard hydrogen electrode, which by convention is set at zero. Thus, all metals have either a higher or lower potential compared to hydrogen, and a comparative list of metals can be produced indicating their relative nobility. This list is the galvanic or electrochemical series and measured as an electromotive force (EMF). 

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. 

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