Standard reduction potential 846

Standard reduction potentials for several of the half-reactions involved in the cells discussed in the text. A more extensive table of potentials appears in Appendix I. 

Such a sequence of reactivities based on standard reduction potentials is sometimes called 

With this information, we can determine the standard cell potential for any pair of half-reactions by using the equation  

We must interpret the nature of an electrochemical system based on the information available in a table of standard reduction potentials. With two half-reactions there are only two possible outcomes—and one outcome yields a negative value for the cell potential. Because we know that a galvanic cell cannot have a negative E° value, we must determine the combination of half-reactions that provides a positive value for °. 

Iron must be oxidized for a combination of these two half-reactions to yield a positive cell potential  

One of the few periodic trends of the metals not to show a strong diagonal effect is the standard reduction potential. In fact, this trend follows more of a horizontal rule. The standard reduction potential, E°, is defined in Equation (5.21). The standard reduction potential for the normal hydrogen electrode (N.H.E.), or the half-reaction shown in Equation (5.22), is given a value of zero. Metal atoms with E s more n ative than the N.H.E. are easier to oxidize and harder to reduce. Metal atoms with s more positive than the N.H.E. are easier to reduce and harder to oxidize  

The standard reduction potential can be calculated using the thermodynamic cycle shown in Equations (5.23)-(5.26)  

TABLE 5.11 Standard reduction potentials for selected elements at 25 °C. 

Many of the transition metals can take more than one oxidation state. Whenever this occurs, it is useful to show the relationship between the standard reduction potentials graphically using a Latimer diagram. Consider the stepwise reduction potentials for Cu and Cu + shown by Equations (5.27) and (5.28). In order to determine the skip potential for the two-electron reduction of Cu shown by Equation (5.29), we cannot simply add the two stepwise potentials together to get the final result. 

The reason for this becomes apparent when the equation relating the standard reduction potential to the Gibbs free energy is considered, as shown by Equation (5.30), where F is Faraday s constant (96,485 C/mol). While the value of n in Equation (5.30) is I for Equations (5.27) and (5.28), it is 2 for Equation (5.29)  

Source Values are compiled from the following sources Bard, A. J. Parsons, R. Jordon, J., eds. Standard Potentials in Aqueous Solutions. Dekker New York, 1985 Milazzo, G. Carol , S. Sharma, V. K. Tables of Standard Electrode Potentials. Wiley London, 1978 Swift, E. H. Butler, E. A. Quantitative Measurements and Chemical Equilibria. Freeman New York, 1972. 

H3ASO4 + 2H3O++ 2e- HASO2 + 4H2O Conditions for formal potentials E° ) are listed next to the potential. 

In addition, the Pt serves as the electrical conductor to the external circuit. Under standard state conditions, that is, when the H2 pressure equals 1 atm and the ideal concentration of the HCl is 1 M, and the system is at 25°C, the reduction potential for the reaction given in Eq. (15.8) is exactly 0 V. (The potential actually depends on the chemical activity of the HCl, not on its concentration. The relationship between activity and concentration is discussed subsequently. For an ideal solution, concentration and activity are equal.) The potential is symbolized by where the superscript zero means standard state conditions. The term standard reduction potential means that the ideal concentrations of all solutes are 1 M and all gases are at 1 atm other solids or liquids present are pure (e.g., pure Pt solid). By connecting the SHE half-cell with any other standard half-cell and measuring the voltage difference developed, we can determine the standard reduction potential developed by the second half-cell. 

Consider, for example, a cell at 25°C made up of the two half-cells  

This cell has a Zn half-cell as the anode and the SHE as the cathode. All solutes are present at ideal 1 M concentrations, gases at 1 atm and the other species are pure solids, and so both half-cells are at standard conditions. The measured cell emf is -1-0.76 V and this is the standard cell potential, because both half-cells are in their standard states. From Eq. (15.7), we can write  

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  

We have determined the Zn standard reduction potential even though the galvanic cell we set up has Zn being oxidized. By substituting other half-cells, we can determine their electrode potentials (actually, their relative potentials) and build a table of standard reduction potentials. If we set up a galvanic cell with the SHE and Cu, we have to make the SHE the anode in order for a spontaneous reaction to occur. This cell. 

The reaction in a galvanic cell is always an oxidation-reduction reaction that can be broken down into half-reactions. It would be convenient to assign a potential to each half-reaction so that when we construct a cell from a given pair of half-reactions, we can obtain the cell potential by summing the halfcell potentials. For example, the observed potential for the cell shown in Fig. 11.5(a) is 0.76 volt, and the cell reaction is 

For this cell the anode compartment contains a zinc metal electrode with Zn and S04 ions in an aqueous solution that bathes the electrode. The anode reaction is the oxidation half-reaction  

Each zinc atom loses two electrons to produce a Zn ion that enters the solution. The electrons flow through the wire. For now we will assume that all cell components are in their standard states, so in this case the solution in the anode compartment will contain 1 M Zn. The cathode reaction of this cell is 

The cathode consists of a platinum electrode (used because it is a chemically inert conductor) in contact with 1 M ions and bathed by hydrogen gas at 

Such an electrode, called the standard hydrogen electrode, is shown in Fig. 11.5(b). 

Although we can measure the total potential of this cell (0.76 V), there is no way to measure the potentials of the individual electrode processes. Thus, if we want potentials for the half-reactions (half-cells), we must arbitrarily divide the total cell potential. For example, if we assign the reaction 

The standard cell potential of a voltaic cell, depends on the particular cathode and anode half-cells. We could, in principle, tabulate the standard cell potentials for all possible cathode-anode combinations. However, it is not necessary to undertake this arduous task. Rather, we can assign a standard potential to each half-cell and then use these half-cell potentials to determine The cell potential is the difference between two half-cell potentials. By convention, the potential associated with each electrode is chosen to be the potential for reduction at that electrode. Thus, standard half-cell potentials are tabulated for reduction reactions, which means they are standard reduction potentials, denoted The standard cell potential, E n, is the standard reduction potential of the cathode reaction, (cathode), minus the standard reduction potential of the anode reaction, B ed (anode)  

It is not possible to measure the standard reduction potential of a half-reaction directly. If we assign a standard reduction potential to a certain reference half-reaction, however, we can then determine the standard reduction potentials of other halfreactions relative to that reference value. The reference half-reaction is the reduction of H aq) to H2(,g ) under standard conditions, which is assigned a standard reduction potential of exactly 0 V  

Why do Na ions migrate into the cathode half-cell as the cell reaction proceeds  

Cathode half-cell (standard hydrogen electrode, SHE) 

When the cell is operated under standard conditions, the cell potential is +0.76 V. By using the standard cell potential ( °eii = 0.76 V), the defined standard reduction potential of H ( °ed = 0 V) and Equation 20.8, we can determine the standard reduction potential for the Zrf /Zn half-reaction  

An electrode designed to produce this half-reaction is called a standard hydrogen electrode (SHE). An SHE consists of a platinum wire connected to a piece of platinum foil covered with finely divided platinum that serves as an inert surface for the reaction (V FIGU RE 20.8). The SHE allows the platinum to be in contact with both 1 M H (aq) and a stream of hydrogen gas at 1 atm. The SHE can operate as either the anode or cathode of a cell, depending on the nature of the other electrode. 

M FIGURE 20.8 The standard hydrogen electrode (SHE) is used as a reference electrode. 

Why does Na migrate into the cathode haif-ceii as the ceii reaction proceeds  

The single vertical line represents a phase boundary. For example, the zinc electrode is a soUd and the Zn ions (from ZnS04 are in solution. Thus, we draw a line between Zn and Zn to show the phase boundary. The double vertical lines denote the salt bridge. By convention, the anode is written first, to the left of the double lines and the other components appear in the order in which we would encounter them in moving from the anode to the cathode. 

The choice of an arbitrary reference for measuring electrode potential is analogous to choosing the surface of the ocean as the reference for altitude, calling it zero meters, and then referring to any terrestrial altitude as being a certain number of meters above or below sea level. 

Second, it serves as an electrical conductor to the external circuit. 

Under standard-state conditions (when the pressure of H2 is 1 atm and the concentration of the HCl solution is 1 M see Table 18.2), the potential for the reduction of at 25°C is taken to be exactly zero  

The superscript denotes standard-state conditions, and E° is the standard reduction potential, or the voltage associated with a reduction reaction at an electrode when all solutes are 1 M and all gases are at 1 atm. Thus, the standard reduction potential of the hydrogen electrode is defined as zero. The hydrogen electrode is called the standard hydrogen electrode (SHE). 

The reaction in a galvanic cell is always a redox reaction that ay be divided into two half-cell reaction is a similar fashion as we have seen it earlier. As well as there is a cell potential for the whole cell there is also a half-cell potential associated with the half-cell reaction. The ell potential for the entire cell is thereby the sum of the two half cell-reactions. This we will look further into in the following example. 

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The name ga/van/cce//honors Luigi Gaivani (1737-1798), an itaiian scientist generaiiy credited with the discovery of eiectricity. These ceiis are sometimes caiied voltaic cells after Aiessandro Voita (1745-1827), another itaiian, who first constructed ceiis of this type around 1800. 

The measured potential for this cell is 0.76 V at 25 C. We can write the half-cell reactions as follows  

One indkation of the diEction ot electron flow is the fact that as the reaction proceeds, the mass of the zirK electrode decreases as a result of ihc oxidation half-reaction  

By convention, the standard cell potential, E ceii, which is composed of a contribution from the anode and a contribution fi om the cathode, is given by 

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The UV-visible absorption spectrum of Ru(2,2 -bipyridine)3 maximum at about 450 nm, from which the energy in volts for process XI-39 may be estimated. The standard reduction potential for the R" /R couple is about 1.26 V at 25°C. Estimate from this information (and standard reduction potentials) the potential in volts for processes XI-40 and XI-41. Repeat the calculation for alkaline solutions. 

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