Standard, hydrogen electrode potential

Equation (3.3) gives the potential dependence of the reaction free energy of Reaction (3.2). Since this reaction equilibrium defines the standard hydrogen electrode potential, we now have a direct fink between quite simple DFT calculations and the electrode potential. In a similar way, we can now calculate potential-dependent reaction free energies for other reactions, such as O - - H" " + e OH or OH - -+ e HzO. 

Table 3.2 Standard reduction potential ( ) for some redox pairs relative to the standard hydrogen electrode potential 0...

Thus, a more appropriate question to ask is Is it possible to measure the absolute potential of the hydrogen reaction, /H+(abs) Actually it is possible. Remembering the definition of a standard hydrogen electrode potential (see Section 6.3.4), this was defined as the potential obtained when a metal comes in contact with a solution containing H+ under thermodynamically reversible conditions at unit activity, and H2 at 1 atm, at 298 K. As to the identity of the metal base, it can in principle be any metal at which it is possible to observe the reaction H2 H+ + e taking place at equilibrium. In practice, the metals used as substrates can only be noble metals because most other metals enter into equilibria with their own species in solution. Usually platinum is the metal chosen. 

In the last section it was shown that instead of representing an electrode potential on a relative scale (arbitrarily setting the standard hydrogen electrode potential equal to zero), it is possible to numerically calculate the actual value of the latter, with a reference state of zero energy for the stationary electron at infinity in a vacuum. 

The standard hydrogen electrode potential °(H+/H2) is taken as zero at all temperatures by convention. 

Table 9.2. Standard equilibrium potential of ionic electrode reactions PZn is referred to the standard hydrogen electrode potential and e is the equilibrium electron in the ion transfer reactions.

Fig. 16.9 CB and VB energy levels of several semiconductors. (The semiconductors are in contact with aqueous electrolyte at pH 1. The energy scale is indicated in electron volts using either the normal hydrogen electrode (NHE) or vacuum level as reference. On the right the standard potentials of several redox couples are presented against the standard hydrogen electrode potential.) [Reprinted by permission from Macmillan Publishers Ltd [Nature] (Gratzel 2001), copyright (2001)]...

In electrochemistry, it is usual to measure potentials with respect to a stable and reproducible system, known as - reference electrode. For the vast majority of practical electrochemical problems there is no need to determine - absolute potentials. However, this is necessary in cases where one wants to connect the relative electrode potential with the absolute physical quantities of the system, like electronic energies, as is the case of the work function. It is possible to convert all relative values of electrode potential to absolute-scale values and to electronic energies. For aqueous systems the - standard hydrogen electrode potential corresponds to -4.44 V in the physical scale taking electrons at rest in vacuum as reference and the absolute potential is given by the relation T(abs) = T(SHE) + 4.44 [vii]. 

For comparison purposes it is necessary to determine each electrode potential in relation to an arbitrary standard. It is usual to assume the standard hydrogen electrode potential to be zero, i.e. the combination of hydrogen ions and electrons as given by Equation 10.2 simplified to... 

The standard electrode potential at the above mentioned standard state conditions is denoted by °. For the MCAT, the values of the standard electrode (reduction) potentials will be given to you if you are required to solve such a question. Do not try to memorize those values. The standard electrode potentials are based on an arbitration with reference to standard hydrogen electrode. The standard hydrogen electrode potential is considered to be 0 volt. 

In the laboratory a saturated calomel electrode (SCE) consisting of Hg, Hg2 CI2 /KCl (saturated) is commonly used. At 20°C this electrode has a potential 245 mV more positive than the standard hydrogen electrode potential. 

Fig. 11.5 Potential energy surface profile for the oxygen reduction reaction at the standard hydrogen electrode potential scale the proton was modeled by two shells of water molecules, H 0H2(H20)3(H20),5, and the data in parentheses are Gibbs free energies [50]...

Nonetheless, the S HE is a gas electrode that consists of a platinum foil suspended in sulfuric acid solution having unit activity = 1 mol/1) at 1 atm and 25 °C. In order to maintain a//+ = 1 mol/1 purified hydrogen (H2) gas is injected into the anodic half-cell in order to remove any dissolved oxygen. The platinum foil is an inert material in this solution and it ows the hydrogen molecules to oxidize, providing the electrons needed by the metal Mions to be reduced on the cathode surface. The concentration of M+ ions are also kept at unit activity. By convention, the standard hydrogen electrode potential is zero = 0. 

For a proton-electron transfer reaction, a computational standard hydrogen electrode potential can be used directly. To do this, we take advantage of the equivalence of the chemical potential of hydrogen gas and the proton-electron... 

At the standard condition (298 K, activity(H )= 1, H2 pressure = 1 bar), the reaction free energy is zero at the standard hydrogen electrode potential (0 Vshe)- The corresponding equilibrium relationship is written as... 

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