Standard electrode potential values

It must be emphasised that standard electrode potential values relate to an equilibrium condition between the metal electrode and the solution. Potentials determined under, or calculated for, such conditions are often referred to as reversible electrode potentials , and it must be remembered that the Nernst equation is only strictly applicable under such conditions. 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

When the follow-up reaction leads to an electroactive product, the effects of the redox properties of the product (Z, above) on the current-potential curves also must be considered. For more than one electroactive product, the nomenclature for the model system changes from and to A and B, below, each compound having its own E (standard electrode potential) value ... 

Standard electrode potentials ( values) may be used to decide whether redox reactions are allowed to occur. The key point to remember here is that the halfreaction having the more positive electrode potential occurs as a reduction, while the half-reaction having the more negative electrode potential occurs as an oxidation. To help you apply this easily ... 

Before we discuss standard electrode potential, we will talk about electromotive force (emf). The electromotive force of a cell is the potential difference between the two electrodes. This can be measured using a voltmeter. The maximum voltage of a cell can be calculated using experimentally determined values called standard electrode potentials. By convention, the standard electrode potentials are usually represented in terms of reduction half-reactions for 1 molar solute concentration. The standard electrode potential values are set under ideal and standard-state conditions (latm pressure and 25°C temperature). From the MCAT point of view, you can assume that the conditions are standard, unless stated otherwise. Table 12-1 shows a list of standard electrode potentials (in aqueous solution) at 25°C. 

From Table 12-1, we can take the standard electrode potential values. The cell containing the copper electrode has a standard potential value of 0.34 V. For the other half-cell, the reaction is oxidation. Since the value given in the table is in terms of reduction half-reactions, we have to reverse the sign of the standard... 

Thus g, the cell voltage at zero current, can be estimated from the standard free energy change or from the standard electrode potentials, values of which are available in the literature (Perry and Chilton, 1976). 

Since discharge at the cathode involves reduction, ions that accept electrons readily will be reduced first. Therefore, strong oxidizing agents with more positive standard electrode potential values will be preferentially discharged compared to those with less positive values. For example, it is easier to discharge copper(ii) ions than zinc ions at the cathode ... 

Step 2 Write the possible reduction reactions that could occur at the cathode. Refer to page 23 of the IB Chemistry data booklet and quote the necessary standard electrode potential values ( reduction)-... 

The above standard state (Table 2.2) conforms to the convention adopted by the International Union of Pure and Applied Chemistry (lUPAC), which requires that aU tabulated electromotive force (emf) or standard electrode potential values ( °) be so with respect to the standard hydrogen electrode (SHE) for reduction reactions of pure metals, such as A/+ + ze = M. In addition. Table 2.3 illustrates standard potentials for some compounds. 

Finally, let us point out that the absolute standard electrode potential value of the couple H+w/H2(g) is actually about 4.5 V. This value cannot be verified since we cannot measure an absolute potential. It was obtained by using thermodynamic cycles, taking into account some thermodynamic data such as the proton hydration enthalpy and entropy. These last ones have been approached by considering the quadrupole model of water (see Chap. 1). It is quite evident that the value of 4.5 V differs considerably from the conventional one (0.00 V). However, it does not change the redox phenomena provision since only the standard electrode potential differences are taken into account. 

Standard electrode potential values for all half-cells are measured relative to this electrode. When connected to another half-cell, the value read on the voltmeter gives the standard electrode potential for that half-cell. 

Standard electrode potential values, E, give us a measure of how easy or difficult it is to oxidise or reduce a species. We can compare the oxidising and reducing powers of... 

Source Values are compiled from the following sources Bard, A. J. Parsons, R. Jordon, J., eds. Standard Potentials in Aqueous Solutions. Dekker New York, 1985 Milazzo, G. Carol , S. Sharma, V. K. Tables of Standard Electrode Potentials. Wiley London, 1978 Swift, E. H. Butler, E. A. Quantitative Measurements and Chemical Equilibria. Freeman New York, 1972. 

To calculate the open circuit voltage of the lead—acid battery, an accurate value for the standard cell potential, which is consistent with the activity coefficients of sulfuric acid, must also be known. The standard cell potential for the double sulfate reaction is 2.048 V at 25 °C. This value is calculated from the standard electrode potentials for the (Pt)H2 H2S04(yw) PbS04 Pb02(Pt) electrode 1.690 V (14), for the Pb(Hg) PbS04 H2S04(yw) H2(Pt) electrode 0.3526 V (19), and for the Pb Pb2+ Pb(Hg) 0.0057 V (21). 

Direct subtraction of the standard electrode potential would have given the incorrect value u2+/cu = 0-346 - 0-522 = -0- 176 V.)... 

The equilibrium potentials and E, can be calculated from the standard electrode potentials of the H /Hj and M/M " " equilibria taking into account the pH and although the pH may be determined an arbitrary value must be used for the activity of metal ions, and 0 1 = 1 is not unreasonable when the metal is corroding actively, since it is the activity in the diffusion layer rather than that in the bulk solution that is significant. From these data it is possible to construct an Evans diagram for the corrosion of a single metal in an acid solution, and a similar approach may be adopted when dissolved O2 or another oxidant is the cathode reactant. 

The values in Table 2.16 show how the potentials obtained under service conditions differ from the standard electrode potentials which are frequently calculated from thermodynamic data. Thus aluminium, which is normally coated with an oxide film, has a more noble value than the equilibrium potential 3 + / = — 1-66V vs. S.H.E. and similar considerations apply to passive stainless steel (see Chapter 21). 

When metals are arranged in the order of their standard electrode potentials, the so-called electrochemical series of the metals is obtained. The greater the negative value of the potential, the greater is the tendency of the metal to pass into the ionic state. A metal will normally displace any other metal below it in the series from solutions of its salts. Thus magnesium, aluminium, zinc, or iron will displace copper from solutions of its salts lead will displace copper, mercury, or silver copper will displace silver. 

The values of Hn and E are zero for water, by virtue of the constants 1.74 and 2.60. In these definitions, pKa refers to the acid ionization constant of the conjugate acid of the nucleophile, and E° to the standard electrode potential for the two-electron half-reaction ... 

If no value is given unity activity has been used and thus a standard electrode potential is given. 

This is an example of a reversible reaction the standard electrode potential of the 2PS/PSSP + 2c couple is zero at pH 7. The oxidation kinetics are simple second-order and the presence of a radical intermediate (presumably PS-) was detected. Reaction occurs in the pH range 5 to 13 with a maximum rate at pH 6.2, and the activation energy above 22 °C is zero. The ionic strength dependence of 2 afforded a value for z Zg of 9 from the Bronsted relation... 

STANDARD ELECTRODE POTENTIALS FOR REDUCING IONS IN AQUEOUS SOLUTION All values taken from Ref. 19 unless otherwise indicated. 

Knowledge of the value of ij (abs) makes it possible to convert all relative values of electrode potential to an absolute scale. For instance, the standard electrode potentials of the oxygen electrode, the zero charge of mercury, and the hydrated electron, in the absolute scale are equal to -5.67,. 25, and 1.57 V, recpectively. ... 

Parameter E° in Eqs. (3.30) and (3.32) is called the standard electrode potential it corresponds to the value of electrode potential that is found when the activities of the components are unity. Values E° differ somewhat from values E°. For a more distinct differentiation between these parameters, E° is called the formal electrode potential. 

The standard electrode potential of reaction (15.20) calculated thermodynamically is 1.229 V (SHE) at 25°C. For reachons (15.21) and (15.22), these values are 0.682 and 1.776 V, respechvely. The equihbrium potenhals of all these reactions have the same pH dependence as the potential of the reversible hydrogen electrode therefore, on the scale of potentials (against the RHE), these equilibrium potenhals are... 

Consequent to substitution of the value of the standard electrode potentials as obtained from entries in Table 6.11 and conversion of natural logarithm to the base 10 logarithm, the following relationship is obtained ... 

---