Standard cell potential Nemst equation

Electrochemistry electrolytic and galvanic cells Faraday s laws standard half-cell potentials Nemst equation prediction of the direction of redox reactions... 

E = E °-------In Q The Nemst equation is used to convert between standard cell potentials and potentials of... 

Thus far, we have based all of our calculations on the standard cell potential or standard half-cell potentials—that is, standard state conditions. However, many times the cell is not at standard conditions—commonly the concentrations are not 1 M. We may calculate the actual cell potential, Ecell, by using the Nemst equation ... 

It is possible to obtain values for "ceU experimentally, although it is usual in the laboratory to work with solutions of concentrations measure values of Eoett (rather than standard cell potentials). Such values are dependent on solution concentration (strictly, activity), and ceii and "ceU related by the Nemst equation (see equation 8.21). ... 

To do this problem, you need the Nemst equation. This means we must determine the number of electrons transferred and we must be able to calculate the standard cell potential from tabulated data. We can plug this information into the Nemst equation and adjust the concentration to obtain the desired voltage. 

Consider a Zn /Zn half-cell (at 298 K) in which [Zn ] = O.lOmoldm , i.e. non-standard conditions. The Nemst equation (eq. 8.21) shows how the reduction potential varies with the concentrations of the species present. 

Nemst equation The equation relating the cell potential of an electrochemical cell to the standard cell potential and the reaction 0.0592 V... 

QB For this cell because the electrodes are identical, the standard electrode potentials are numerically equal and subtracting one from the other leads to the value c°dl = 0.000 V. However, because the ion concentrations differ, there is a potential difference between the two half cells (non-zero nonstandard voltage for the cell). [Pb2+] = 0.100 M in the cathode compartment. The anode compartment contains a saturated solution of Pbl2. We use the Nemst equation (with n = 2) to determine [Pb2+] in the saturated solution. 

The reduction potential of a half-cell depends not only on the chemical species present but also on their activities, approximated by their concentrations. About a century ago, Walther Nemst derived an equation that relates standard reduction potential ( ") to the reduction potential (E) at any concentration of oxidized and reduced species in the cell ... 

The standard reduction potential would be observed if the half-cell of interest (with unit activities) were connected to a standard hydrogen electrode, as it is in Figure 14-7. It is nearly impossible to construct such a cell, because we have no way to adjust concentrations and ionic strength to give unit activities. In reality, activities less than unity are used in each half-cell, and the Nemst equation is employed to extract the value of E° from the cell voltage.12 In the hydrogen electrode, standard buffers with known pH (Table 15-3) are used to obtain known activities of H+. 

The standard potential applies to a half-cell when all the reactants are present at unit activity that is, when the solution species are near a concentration of 1 molar. The actual half-cell potential is a function of the solution concentrations and is related to these and to the standard potential E° by the Nemst equation. The Nemst equation for the generalized half-reaction... 

By use of an electromotive series (E° values) for standard half-reactions written as reductions (see Chapter 2) the potential of each half-cell can be calculated by means of the Nemst equation ... 

By convention, the potentials of all half-reactions, E°, are found tabulated for the reduction process under standard conditions of temperature (298.15 K), pressure (1 atm), and solute concentrations (1 molar). For nonstandard conditions, the reduction potentials, and hence the cell voltage, will differ. The concentration dependence on the cell voltage is given by the Nemst equation ... 

While the redox titration method is potentiometric, the spectroelectrochemistry method is potentiostatic [99]. In this method, the protein solution is introduced into an optically transparent thin layer electrochemical cell. The potential of the transparent electrode is held constant until the ratio of the oxidized to reduced forms of the protein attains equilibrium, according to the Nemst equation. The oxidation-reduction state of the protein is determined by directly measuring the spectra through the tranparent electrode. In this method, as in the redox titration method, the spectral characterization of redox species is required. A series of potentials are sequentially potentiostated so that different oxidized/reduced ratios are obtained. The data is then adjusted to the Nemst equation in order to calculate the standard redox potential of the proteic species. Errors in redox potentials estimated with this method may be in the order of 3 mV. 

In Equation (18b), the activity quotient is separated into the terms relating to the silver electrode and the hydrogen electrode. We assume that both electrodes (Ag+/Ag and H+/H2) operate under the standard condition (i.e. the H+/H2 electrode of our cell happens to constitute the SHE). This means that the equilibrium voltage of the cell of Figure 3.1.6 is identical with the half-cell equilibrium potential E°(Ag+l Ag) = 0.80 V. Furthermore, we note that the activity of the element silver is per definition unity. As the stoichiometric number of electrons transferred is one, the Nemst equation for the Ag+/Ag electrode can be formulated in the following convenient and standard way ... 

In a voltaic cell where all ions have a concentration of 1M, the cell potential is equal to the standard potential. For cells in which ion concentrations are greater or less than 1M, as shown below, an adjustment must be made to calculate cell potential. That adjustment is expressed by the Nemst equation ... 

The graphing calculator can run a program that calculates the equilibrium constant for an electrochemical cell using an equation called the Nemst equation, given the standard potential and the number of electrons transferred. Given that the standard potential is 2.041 V and that two electrons are transferred, you will calculate the equilibrium constant. The program will be used to make the calculations. 

Tlie concentration pel is just a type of galvanic cell. If la never at standard conditions, so the Nemst equation is required to solve for the cell potential. 

S5.4 The potential difference of the cell from Example 5.45 was calculated to be +1.23 V. To calculate the potential difference under non-standard conditions we start with the Nemst equation given by the formula ... 

To determine the potential of a half-cell reaction when the reactants and products are not at unit activity, the Nemst equation is used. Consider again the general half-cell reaction given by Equation (26.13). When this reaction is combined with the standard hydrogen electrode reaction (Equation (26.20)), the overall cell reaction is... 

Equations 2.72 and 2.73 are Nemst half-cell equations. For example, with Eq 2.73, when aMm+= 1, E m Mn,+ = E m+ Hence, E Mm+ is the half-cell potential at unit activity of the ions (i.e., the standard electrode half-cell potential). Values of the standard potentials of many electrode reactions are available in the literature, some of which are given in Table 2.1 (Ref 2, 7, 8). All values are given in sign and magnitude relative to the standard hydrogen electrode as previously discussed. 

The potentials, relative to the standard hydrogen electrode, of several half cells used as reference electrodes are given in Table 6.1 (the section Examples of Half-Cell Reactions and Nemst-Equation Calculations in Chapter 2 provides discussions of half-cell-potential calculations). There are a number of factors that contribute to the selection of components and to the design of a satisfactory reference half cell ... 

Further we looked at galvanic cells where it was possible to extract electrical energy from chemical reactions. We looked into cell potentials and standard reduction potentials which are both central and necessary for the electrochemical calculations. We also looked at concentration dependence of cell potentials and introduced the Nemst-equation stating the combination of the reaction fraction and cell potentials. The use of the Nemst equation was presented through examples where er also saw how the equation may be used to determine equilibrium constants. 

The Nemst equation is used to calculate electrode potentials and cell potentials for concentrations and partial pressures other than standard-state values. 

---