Nernst half-cell equations

The Nernst equation can be used for complete cell reactions also. The number of moles of electrons (not shown in the equation for the cell reaction because the electrons cancel when the half-cell equations are combined) is the number in each half-cell just before those equations are combined. 

The Nernst equation can also be used to determine the effect of changes in concentration on the voltage of an individual half-cell, E or Consider, for example, the half-reaction... 

As a consequence, the equilibrium potential of the single half-cell also depends on the concentrations of the compounds involved. The Nernst equation [Eq. (24)], which is one of the most important electrochemical relations, explains this context... 

Referring to the discussion of the fundamental concepts regarding half cells and the Nernst equation in Chapter 5 (Section 5.3.1) it is possible to briefly summarize the similarities and differences of these two sets of systems. It is important to recognize the ways in which they are different when considering the behavior of complex multivariate systems such as the oceans and clouds, or a lake-river system. 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

For the standard half-cell, [Zn2+] = 1 M. Substituting these data into the Nernst equation, we have 0.0592 1 0.0592 1... 

Therefore, the Nernst equation predicts that the voltage of a standard half-cell equals E°. 

Electrode and therefore cell potentials are very important analytically as their magnitudes are determined by the activities of the reactants and products involved in the electrode reactions. The relation between such activities and the electrode potential is given by the Nernst equation. For a general half-cell reaction written as a reduction, i.e. aA + bB +. .. ne = xX + yY +. . ., the equation is of the form... 

Though it is relatively easy to formulate relations like the Nernst equation here for a cell, Equation (7.41) properly relates to a half-cell. 

The purpose of the silver-silver chloride combination is to prevent the potential that develops from changing due to possible changes in the interior of the electrode. The potential that develops is a membrane potential. Since the glass membrane at the tip is thin, a potential develops due to the fact that the chemical composition inside is different from the chemical composition outside. Specifically, it is the difference in the concentration of the hydrogen ions on opposite sides of the membrane that causes the potential—the membrane potential—to develop. There is no half-cell reaction involved. The Nernst equation is... 

From the Nernst equation, calculate the pressure of hydrogen involved if alHsO ) = 0.011, and Eh+,h2 = 0.008 V. Take p = 10 Pa. (Remember from the balanced half-cell reaction that n = 2.)... 

The Nernst equation tells us the electrical potential of a half-cell when the reactants are not at unit activity. 

Step 2 Write a Nernst equation for the right half-cell, which is attached to the positive terminal of the potentiometer. This is E+. 

What if you had written the Nernst equation for the right half-cell with just one electron instead of two ... 

For non-standard conditions, cell (or half-cell) potentials, E, can be calculated with the Nernst equation... 

The standard states to which this E° value refers are 1 atm for oxygen gas and 1 mol/L for H+. We can calculate E for the above half-cell for neutral solutions, in which [H+] = 10-7, by using the Nernst equation. Assuming the oxygen remains at its standard state, P(O2) = 1 atm,... 

Ions of opposite charge tend to associate into loosely-bound ion pairs in more concentrated solutions, thus reducing the number of ions that are free to donate or accept electrons at an electrode. For this reason, the Nernst equation cannot accurately predict half-cell potentials for solutions in which the total ionic concentration exceeds about 10-3 M. 

The Nernst Equation accurately predicts half-cell potentials only when the equilibrium quotient term Q is expressed in activities. Ionic activities depart increasingly from concentrations when the latter exceed 10-10-3 M, depending on the size and charge of the ion. 

The Nernst equation (Equation 18.4) can be used to relate the cell potential to the concentration of the solutions in each half-cell. 

This equation, the Nernst redox equation, provides a way of relating E and E0 for any redox reaction or half-cell reaction. 

Half-cell reaction — The redox reaction (- electrode reaction) proceeding in a half-cell. The half-cell reaction changes the ratio of the activities of the reduced and oxidized forms. When the half-cell reaction is electrochemically reversible (see reversibility), the -> Nernst equation will describe the dependence of the -> electrode potential on the ratio of the activities of the reduced and oxidized forms. 

Nernst equation (17.3) An equation relating the potential of a cell or half-cell to the concentrations or pressures of its reagents. 

The Nernst equation can be applied to half-reactions. Calculate the reduction potential at 25°C of each of the following half-cells. 

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