Standard Electrode Potential E

Current flows from one half-cell to the other based on the difference in the electrical potential energy between the two electrodes. The difference in electrical potential energy per unit charge, as measured with a potentiometer, is called electromotive force (EMF), or cell potential. Because it is measured in volts, it is also referred to as the cell voltage. Note that potential difference is always measured, not the individual potentials of any half-cell. By convention, the term electrode potential always means the reduction potential, corresponding to a reduction reaction, such as. 

The cell potential is noted with AE, with the A symbol included because the measured voltage is a difference between two electrode potentials  

If all substances involved in the reaction are in their standard states (all concentrations are specified to be 1 M and the H2 gas is 1-atm pressure, temperature is 25°C), the cell is referred to as a standard cell, its voltage being denoted as superscript zero, or AE°. 

In order to determine the standard electrode potential for a metal, the galvanic cell is designed so that a half-cell is formed by a piece of metal immersed in a solution that contains 1.00 M of ions of that metal, and one half-cell with potential convention defined to be exactly zero volts. This electrode is called the standard hydrogen electrode (SHE) and it consists of a platinum electrode over which Hj gas at 1-atm of pressure is bubbled, immersed in a solution that contains 1.00 M of hydronium ion at 25°C. 

The notation of the cell formed by the half-ceUs described above can be  

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Standard Electrode Potential (E ) the equilibrium potential of an electrode reaction when the components are ail in their standard states. 

The potentials of the metals in their 1 mol U salt solution are all related to the standard or normal hydrogen electrode (NHE). For the measurement, the hydrogen half-cell is combined with another half-cell to form a galvanic cell. The measured voltage is called the normal potential or standard electrode potential, E° of the metal. If the metals are ranked according to their normal potentials, the resulting order is called the electrochemi-... 

In the introductory chapter we stated that the formation of chemical compounds with the metal ion in a variety of formal oxidation states is a characteristic of transition metals. We also saw in Chapter 8 how we may quantify the thermodynamic stability of a coordination compound in terms of the stability constant K. It is convenient to be able to assess the relative ease by which a metal is transformed from one oxidation state to another, and you will recall that the standard electrode potential, E , is a convenient measure of this. Remember that the standard free energy change for a reaction, AG , is related both to the equilibrium constant (Eq. 9.1)... 

The standard electrode potential e r is the value of E0,r obtained at standard conditions. 

Worked Example 7.19 We immerse a piece of silver metal into a solution of silver ions at unit activity and at s.t.p. The potential across the cell is 0.799 V when the SHE is the negative pole. What is the standard electrode potential E of the Ag+, Ag couple ... 

Variations in ionic strength are such an important concern that it is recommended for solutes to be analysed by a potentiometric procedure only if the ionic strength is known and controlled. Furthermore, calibration steps, i.e. to determine the standard electrode potential E should also be performed in a solution of the same, known, ionic strength, e.g. in a solution of perchloric acid of — 1.0 mol dm K Provided that 1 is always much higher than the concentration of the analyte, the latter does not contribute more than a tiny fraction of the overall ionic strength and so fluctuations in the activity coefficient y can be safely ignored. 

Consider a cell made up of two half cells, where one contains the Fe , Fe couple and the other the Cu, Cu couple. By looking up the respective values of the standard electrode potentials E given in Appendix 3, deduce the spontaneous cell reaction that would occur if the leads connecting the two half cells were allowed to short by touching. 

The formal electrode potential (also called the formal potential or the formal redox potential) is conceptually similar to the standard electrode potential, E . 

Next, we need to decide on what we think is occurring in terms of the system actually before us. Let s suppose that we have a CV which looks as though it describes a simple single reversible electron-transfer reaction. From the experimental trace of current against potential, it should be easy to obtain the standard electrode potential E . In addition, before we start, we measure the area of the electrode. A, and the thermodynamic temperature, T. Next, knowing A, T and E , we estimate a value for the exchange current lo, run a simulation, and note how similar (or not) are... 

The standard electrode potential, E, in which the suffix H indicates that the potential is on the H2-H+ scale, is derived as follows. We have... 

The standard electrode potentials, E°(V) for some chelates of the Fe /Fe redox couple areas follows o-phenanthroline, 1.20 2,2 -bipyridyl 1.096 water, 0.77 cyanide, 0.10 oxalate, -0.01 and 8-hydroquinone, -0.15 (Latimer, 1952). In the case of bipyridyl... 

Symbol Cd atomic number 48 atomic weight 112.41 a Group IIB (Group 12) metallic element ionization potential 8.994eV electron configuration [Kr]4di°5s2 valence state +2 standard electrode potential, E° -0.40V. The isotopes and their natural relative abundance are ... 

Symbol Ca atomic number 20 atomic weight 40.078 a Group IIA (Group 2) alkaline-earth metaUic element ionic radius 1.06 A (Ca2+) electron configuration [Ar]4s2 valence state +2 standard electrode potential, E° = -2.87V stable isotopes and their abundance Ca-40 (97.00%), Ca-44 (2.06%) Ca-42 (0.64%), Ca-48 (0.18%), Ca-43 (0.145%), and Ca-46 (0.003%) also the element has six unstable isotopes of which Ca-41 has the longest half-life, l.lxlO yr (decay mode electron capture), and Ca-38 has shortest half life 0.66 sec (P-decay). 

Symbol Na atomic number 11 atomic weight 22.9898 a Group lA (Group 1) alkali metal element electron configuration [NejSs valence +1 atomic radius 1.85A ionic radius, Na" in crystals 1.02A (for a coordination number 6) ionization potential 5.139 eV standard electrode potential, E°(Na+ + e Na) -2.71 V one naturally-occurring stable isotope, Na-23 (100%) sixteen artificial radioactive isotopes in the mass range 19-22, 24—35 longest-lived radioisotope, Na-22, ti/2 2.605 year shortest-lived isotope Na-35, ti/2 1.5 ms. 

A and 1.04A corresponding to CN 6 and 8, respectively standard electrode potential, E° for Tbs+ -i- 3e <-> Tb is -2.28V one naturally-occurring stable isotope, Tb-159 (100%) twenty-five artificial radioactive isotopes in the mass range 140-158, 160-165 the longest-lived radioisotope, Tb-158, ti/2 180 years shortest-lived isotope, Tb-142, tm 0.60 sec. 

Symbol Th atomic number 90 atomic weight 232.04 an actinide series radioactive element electron configuration XRn]6d27s2 valence state +4 atomic radius 1.80 A ionic radius, Th4+ 1.05 A for coordination number 8 standard electrode potential, E° for Th4+ -1- 4e Th is -1.899V all isotopes are radioactive the only naturally-occurring isotope, Th-232, ti/2 1.4xl0i° year twenty-six isotopes are known in the mass range 212-237. 

Symbol Sn atomic number 50 atomic weight 118.69 a Group IV A (Group 14) metaUic element of carbon family electron configuration Kr] 4di°5s25p2 valence states +2, +4 atomic radius 1.41 A electronegativity 1.7 standard electrode potential, E° for Sn2+ -i- 2e Sn is -0.1375 V ten naturally-occurring sta-... 

The chemistry of fluorine is dominated by its electronegativity, which is the highest of all elements. The colorless gas F2 has an estimated standard electrode potential E° (Chapter 15) of +2.85 V for reduction to F (cf. + 1.36 V for Cl2 to Cl-), and thus F2 immediately oxidizes water to oxygen (E° = +1.23 V), and 2% aqueous NaOH to the gas F20. Obviously, F2 cannot be made by electrolysis of aqueous NaF. The usual preparation involves electrolysis of HF-KF melts in a Monel (Cu-Ni alloy) or copper apparatus. 

If we choose a set of standard conditions (cf. Section 2.3) and one convenient half-cell to serve as a reference for all others, then a set of standard half-cell EMFs or standard electrode potentials E° (Appendix D)1-9 can be measured while drawing a negligible electrical current, that is, with the cell working reversibly so that the equations of reversible thermodynamics... 

Table 21, standard electrode potentials E in aqueous solutions AT 298 KiRELATIVE TO THE STANDARD HYDROGEN ELECTRODE... 

The first term on the right-hand side in eqn. (75) is the conditional or formal standard electrode potential E ° which contains the activity coefficients of O and R since a ratio of concentrations appears in the second term. 

Figure 5.9. Relative standard electrode potential E° of Cu/Cu2+ electrode.

The standard cell voltage e° can in turn be calculated from the standard electrode potentials E° for the partial reactions using the expression... 

The standard electrode potential (E on or sometimes referred to as E0) is defined as the potential that exists when the electrode is immersed in a solution of the ions... 

The SOFC consists of cathode, electrolyte and anode collectively referred to as the PEN - positive electrode, electrolyte, negative electrode. A single cell operated with hydrogen and oxygen provides at equilibrium a theoretical reversible (Nernst) or open circuit voltage (OCV) of 1.229 V at standard conditions (STP, T = 273.15 K. i> = 1 atm). With the standard electrode potential E°, universal gas constant R. temperature T. Faraday s constant F, molar concentration x and pressure p, the OCV is given by... 

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