Standard cell potentials definition

The standard cell potential for the reduction of hydrogen ions to hydrogen gas is, by definition, 0.00 V. This potential is for the standard hydrogen electrode, SHE, which is the reference to which we compare all other cell potentials. All metals above hydrogen on the Activity Series will displace hydrogen gas from acids. (See Chapter 4) Metals below hydrogen will not displace hydrogen gas. 

LT is the standard cell potential difference, which is determined only by the reactants in definited standard states. This quantity results as the difference of standard electrode potentials. The power term Ila contains the corrected composition quantities a, (fugacities and activities) with the stoichiometric coefficients v, of the gases and condensed substances taking part in the cell reaction [10,12]. If a sensor at equilibrium delivers signals in agreement with Equation (25-7) then we have a reaction celt. In this case at solid electrolytes with oxide ion vacancies Vo> two reactions can be found besides... 

What is the definition of the standard ceU potential (fceii) What does a large positive standard cell potential imply about the spontaneity of the redox reaction occurring in the cell What does a negative standard cell potential imply about the reaction ... 

Ideally a standard cell is constmcted simply and is characterized by a high constancy of emf, a low temperature coefficient of emf, and an emf close to one volt. The Weston cell, which uses a standard cadmium sulfate electrolyte and electrodes of cadmium amalgam and a paste of mercury and mercurous sulfate, essentially meets these conditions. The voltage of the cell is 1.0183 V at 20°C. The a-c Josephson effect, which relates the frequency of a superconducting oscillator to the potential difference between two superconducting components, is used by NIST to maintain the unit of emf. The definition of the volt, however, remains as the Q/A derivation described. 

Electrochemical cells can be constructed using an almost limitless combination of electrodes and solutions, and each combination generates a specific potential. Keeping track of the electrical potentials of all cells under all possible situations would be extremely tedious without a set of standard reference conditions. By definition, the standard electrical potential is the potential developed by a cell In which all chemical species are present under standard thermodynamic conditions. Recall that standard conditions for thermodynamic properties include concentrations of 1 M for solutes in solution and pressures of 1 bar for gases. Chemists use the same standard conditions for electrochemical properties. As in thermodynamics, standard conditions are designated with a superscript °. A standard electrical potential is designated E °. 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

You have probably worked with tables of standard reduction potentials before. These tables provide the reduction potentials of various substances. It describes an oxidized species s ability to gain electrons in a reduction half-reaction (like copper in the voltaic cell example). According to this definition, we can use a value from the table to represent the E°red in the expression above, but how do you find the E°ox ... 

According to its definition, the standard (reduction) potential of the A/A couple is the standard electromotive force of a cell in which an A/A electrode (where the activities of A and A are made unity) is opposed to an NHE (normal hydrogen electrode) whose potential is assigned to zero by convention. 

Because of thermodynamic and electrochemical conventions, standard potentials are defined in the direction of reduction, independently of the respective chemical stabilities of the molecules involved. Thus for the oxidation of toluene to its cation radical, E° refers to the reduction of the highly unstable cation radical into the highly stable toluene. To overcome such a priori chemical nonsence, E is frequently designated as the standard oxidation potential of toluene for example. However, such a term should not be accepted according to canonical rules because it formally implies that the cell now operates in a driven mode, that is, is connected to an external power supply [19]. Thus in this chapter we prefer to use the denomination standard reduction potentials, rather than the usual temi standard potential, as a reminder of the E° definition, although such as expression is basically a pleonasm. 

As already discussed, the standard hydrogen electrode (SHE) is the chosen reference half-cell upon which tables of standard electrode potentials are based. The potential of this system is zero by definition at all temperatures. Although this reference electrode was often used in early work in electrochemistry, it is almost never seen in chemical laboratories at the present time. It is simply too awkward to use because of the requirement for H2 gas at 1 bar pressure and safety considerations. 

Because the standard-state half-cell potential, , is measured relative to the zero potential of the hydrogen half-cell, = El, and the definition of " given by equation 7.27 is substituted into equation 7.22 to give... 

To determine the standard electrode potential of an element M we set up a cell as illustrated in Fig. 7,6. The element is placed in a solution of its ions at unit activity (standard state, based on the unit-molality definition) and coupled to a standard hydrogen electrode.f The potential of element M with respect to the platinum of the hydrogen electrode is called the standard electrode potential of M. (If the element M is positive with respect to the hydrogen electrode then the standard electrode potential of M is positive and vice versa.) If the metal in the cell is zinc we find... 

Erhe andELHE are equilibrium half-cell potentials, or electrode potentials, which depend in sign on the definitions of positive and negative electricity and assignment of Eu + = 0 at standard condi-... 

Table 19.1 lists standard reduction potentials for a number of half-cell reactions. By definition, the SHE has an E° value of 0.00 V. Above the SHE the negative standard reduction potentials increase, and below it the positive standard reduction potentials increase. It is important to know the following points about the table ... 

The overall reaction in a cell is made up of the reactions occurring at the electrodes. It is reasonable to assume that the overall emf of the cell is also made up of contributions from each electrode. To put this on a quantitative and comparative basis it is necessary to couple each electrode to one standard electrode. The Pt(s)IH2(g) 1 atmlHCl(aq) (unit activity) electrode is such a standard and by definition is taken to have zero electrode potential. Every other electrode potential is relative to this standard hydrogen electrode which Is always written on the left of the cell. 

The cathode consists of platinum which is an inert conductor in contact with the 1 M ions surrounded by hydrogen gas at 1 atm. Such an electrode is called a standard hydrogen electrode which per definition has a half cell potential (symbolised at 298 K by the symbol of s°) of 0.00 volt. The figure below shows the principle in the build up of the standard hydrogen electrode. 

The standard hydrogen electrode has per definition a half cell potential of 0.0 Volt at a Tf concentration of 1,0 M. 

The total voltage developed under standard conditions is -1-0.76 V. But the voltage of the SHE is 0 by definition therefore the standard reduction potential of the Zn half-cell is ... 

Equation (5) or (11) can be applied directly to half-cell reactions such as (6) and (7) and the resulting potentials obtained will be identical to those obtained from the overall reactions (9) and (10) because of the definition of the SHE as the universal standard. A selection of standard potentials of half-cell reactions is shown in Table 1 [5]. By international convention, electrode reactions in thermodynamic tables are always written as reduction reactions, so the more noble metals have a positive standard potential. Lists such as that in Table 1 are also called electromotive force series or tables of standard reduction potentials. 

As we shall see in Section 21-14, this arbitrary definition will provide a basis on which to describe the relative potential (and hence the relative spontaneity) of any half-ceU process. We can construct a standard cell consisting of a standard hydrogen electrode and some other standard electrode (half-cell). Because the defined electrode potential of the SHE contributes exactly 0 volts to the sum, the voltage of the overall cell is then attributed entirely to the other half-cell. 

Various standard electrode potentials can be calculated for a half-cell after the definition of standard electrode potential. Currently, most of standard electrode potentials can be obtained from the text. 

Definitions. Define briefly (a) difference of potential, (b) electromotive force, (c) salt bridge, (d) anode, (e) positive electrode, (f) reference half-cell, (g) standard electrode potential, (h) decomposition potential, (i) overvoltage, (j) sacrificial anode. 

Since the measured cell potential difference is actually the potential difference between two electrodes, it immediately comes to mind to assimilate each of the bracketed terms into the potential of each of the electrodes. They are called electrode potentials. E° and °2, which are in the two subgroups, exhibit characteristic values of both couples Oxi/Redi and Oxa/Reda. These constants are called standard potentials of both couples and are symbolized (Oxi/Redi) and °(Ox2/Red2). Assigning numerical values to and E°2 has been a problem since the experimental determination of absolute electrode potentials hence, assigning those to standard electrode potentials is impossible (see the electrochemistry part). It was solved by assigning relative values to them. The strategy was based on the fact that if absolute electrode potentials are not measurable, the difference between them can be. Thus, an electrode standard potential has been chosen conventionally for the couple H+w/H2(g) (hydrogen electrode). Its standard electrode has been set definitively to the value 0.0000 V at every temperature ... 

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