Reduction half-reaction potentials

B. The reduction half reaction potential is greater than die oxidation half reaction potential,... 

In general, only the reduction half-reaction potentials are listed in tables, as in Table 9.1. The potential of an oxidation half-reaction is the negative of the value of the reduction half-reaction. Moreover, it is convenient to standardise the concentrations of the components of the cells. If the ceU components are in their standard states, standard electrode potentials, E°, are recorded ... 

Standard-state potentials are generally not tabulated for chemical reactions, but are calculated using the standard-state potentials for the oxidation, E°o, and reduction half-reactions, fi°red- By convention, standard-state potentials are only listed for reduction half-reactions, and E° for a reaction is calculated as... 

Standard Reduction Potentials for Several Biological Reduction Half-Reactions ... 

Standard half-cell voltages are ordinarily obtained from a list of standard potentials such as those in Table 18.1 (page 487). The potentials listed are the standard voltages for reduction half-reactions, that is,... 

Standard potentials are also called standard electrode potentials. Because they are always written for reduction half-reactions, they are also sometimes called standard reduction potentials. 

The more positive the potential, the greater the electron-pulling power of the reduction half-reaction and, therefore, the more strongly oxidizing the redox couple (the stronger the tendency for the half-reaction to occur as a reduction). 

TABLE 12.1 Standard Potentials Species i at 25°C Reduction half-reaction E° (V)... 

Use the information in Appendix 2B to determine the standard potential for the redox couple Ce4"7Ce, for which the reduction half-reaction is... 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

STRATEGY Find the standard potentials of the two reduction half-reactions in Appendix 2B. The couple with the more positive potential will act as an oxidizing agent (and be the site of reduction). That couple will be the right-hand electrode in the cell diagram corresponding to the spontaneous cell reaction. To calculate the standard emf of the cell, subtract the standard potential of the oxidation half-reaction (the one with the less-positive standard potential) from that of the reduction half-reaction. To write the cell reaction, follow the procedure in Toolbox 12.2. 

One of the most useful applications of standard potentials is in the calculation of equilibrium constants from electrochemical data. The techniques that we develop here can be applied to any kind of reaction, including neutralization and precipitation reactions as well as redox reactions, provided that they can be expressed as the difference of two reduction half-reactions. 

The aluminum-air fuel cell is used as a reserve battery in remote locations. In this cell aluminum reacts with the oxygen in air in basic solution, (a) Write the oxidation and reduction half-reactions for this cell, (b) Calculate the standard cell potential. See Box 12.1. 

This is a quantitative problem, so we follow the standard strategy. The problem asks about an actual potential under nonstandard conditions. Before we determine the potential, we must visualize the electrochemical cell and determine the balanced chemical reaction. The half-reactions are given in the problem. To obtain the balanced equation, reverse the direction of the reduction half-reaction with the... 

They are the basis of many products and processes, from batteries to photosynthesis and respiration. You know redox reactions involve an oxidation half-reaction in which electrons are lost and a reduction half-reaction in which electrons are gained. In order to use the chemistry of redox reactions, we need to know about the tendency of the ions involved in the half-reactions to gain electrons. This tendency is called the reduction potential. Tables of standard reduction potentials exist that provide quantitative information on electron movement in redox half-reactions. In this lab, you will use reduction potentials combined with gravimetric analysis to determine oxidation numbers of the involved substances. 

It is not possible to prepare F2 by electrolysis of an aqueous NaF solution. In electrolysis, the most easily oxidized and reduced species are the ones involved. To prepare F2, the oxidation of F would have to occur. However, water is more easily oxidized than is F, as seen by its position in the standard reduction potential chart (Appendix J and below). By inspection, H20 is a stronger reducing agent than F because the reduction half-reaction has a less positive E°. So H20 s oxidation is preferable to F s oxidation. F2 can be prepared from molten NaF, but not aqueous NaF. 

Since reactions (i) and (iii) are both reduction half reactions, we cannot simply subtract the potential for (i) from the potential for (iii). Instead, we are forced to obtain the voltage for (ii) via the free energy changes for the three half reactions. Thus,... 

The standard reduction potential, symbolized by E°, is a number reflecting the relative tendency of a reduction half-reaction to occur. In a table of standard reduction potentials, such as Table 14.1, those half-reactions at the top have positive numerical values and a strong tendency to occur. Those near the bottom have negative values and a tendency to go in the reverse direction. 

This means that the Ni electrode is the anode and must be involved in oxidation, so its reduction half-reaction must be reversed, changing the sign of the standard half-cell potential, and added to the silver half-reaction. Note that the silver half-reaction must be multiplied by two to equalize electron loss and gain, but the half-cell potential remains the same ... 

Applying Concepts Write the half-reactions for the anode and cathode in each of the voltaic cells in the data table. Look up the half-reaction potentials from the standard reduction potentials table (Table 21-1) and record these in the data table. 

Each half-reaction is written as a reduction. The half-cell potential for a reduction half-reaction is called a reduction potential. Look at the molecules and ions on the left side of each half-reaction. The most easily reduced molecules and ions (best oxidizing agents), such as F2, Mn04, and O2, are near the top of the list. The least easily reduced molecules and ions (worst oxidizing agents), such as Na", Ca ", and H2O, are near the bottom of the list. 

The half-cell potential for an oxidation half-reaction is called an oxidation potential. If the reduction half-reaction is as follows,... 

Step 1 The oxidation and reduction half-reactions are as follows. Oxidation half-reaction (occurs at the anode) 2r(aq) l2(s) + 2e Reduction half-reaction (occurs at the cathode) Br2( ) -i- 2e 2Br (aq) Step 2 The relevant reduction potentials in the table of standard reduction potentials are ... 

Step 3 The standard electrode potential for the reduction half-reaction is Rred = 1.066 V. Changing the sign of the potential for the oxidation half-reaction gives... 

In this section, you learned that you can calculate cell potentials by using tables of half-cell potentials. The half-cell potential for a reduction half-reaction is called a reduction potential. The half-cell potential for an oxidation half-reaction is called an oxidation potential. Standard half-cell potentials are written as reduction potentials. The values of standard reduction potentials for half-reactions are relative to the reduction potential of the standard hydrogen electrode. You used standard reduction potentials to calculate standard cell potentials for galvanic cells. You learned two methods of calculating standard cell potentials. One method is to subtract the standard reduction potential of the anode from the standard reduction potential of the cathode. The other method is to add the standard reduction potential of the cathode and the standard oxidation potential of the anode. In the next section, you will learn about a different type of cell, called an electrolytic cell. 

Tabulated E values can be used to calculate the for any reaction, as illustrated in Table 7.2 for the Zn/Cu galvanic cell. The redox reaction is spontaneous when the half-reaction (Cu /Cu) with the larger reduction (+0.34V) acts as the oxidizing agent. In this case, the other half-reaction (Zn /Zn) proceeds as an oxidation. The halfcell potential for this reduction is +0.76 V as it represents the reverse of the half-cell reduction potential as listed in Table 7.2. The sum of the oxidation and reduction half reactions is +0.34V + 0.76 V = +1.10 V. Thus for the galvanic Zn/Cu cell is +1.10V. 

The E (vertical) axis is a reflection of the potential values in volts (v) of reduction half-reactions describing the conditions under which changes in the aqueous oxidation state of the element occur. These E values range from -1-3.00 V to —4.00 V. The pH (horizontal) axis gives pH values ranging from a pH of —1.0 (10 molar hydrogen ion) to a pH of 15.0 (10 molar hydrogen ion). The sloped dashed lines have to do with the behavior of the solvent water. This will be discussed in detail later. 

Table 19-1 lists some standcird reduction potentials along with the reduction half-reactions associated with them. The table is ordered from the most negative (most likely to oxidize) to the most positive E° (most likely to be reduced). The reactions with negative E° are therefore reactions that happen at the anode of a voltaic cell, while those with a positive... 

Table 19-1 Reduction Half-Reactions and Standard Reduction Potentials ...

The cell potential is the sum of potentials of oxidation and reduction half reactions. 

Since neither of the two half-reaction potentials is known absolutely, it is necessary to propose an arbitrary /cm, relative to which all half-reaction potentials may be quoted. The half-reaction chosen to represent the arbitrary zero is the hydrogen electrode1 in which the half-reaction is the reduction of the aqueous hydrogen ion to gaseous dihydrogen ... 

A large positive value of F implies that the oxidized form of the couple is a good oxidizing agent. For example, the reduction potential for the reduction of dichlorine to aqueous chloride ion is + 1.36 V, and the reduction half-reaction ... 

Calculations ol the reduction potential of a half-reaction from I two other half-reaction potentials were described. 

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