CELL POTENTIALS UNDER STANDARD CONDITIONS

The cell potential of any voltaic cell is positive. The magnitude of the cell potential depends on the reactions that occur at the cathode and anode, the concentrations of reactants and products, and the temperature, which we will assume to be 25 C unless otherwise noted. In this section we focus on cells that are operated at 25 C under standard conditions. Recall from Table 19.2 that standard conditions include 1 M concentrations for reactants and products in solution and 1 atm pressure for gaseous reactants and products. The cell potential under standard conditions is called either the standard cell potential or standard emf and is denoted For the Zn-Cu voltaic cell 

Recall that the superscript indicates standard-state conditions. (Section 5.7) 

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Standard reduction potentials are used to calculate the cell potential under standard conditions. All half-reactions are shown in the reduction form. 

A concentration cell is any voltaic cell in which two half-cells consist of identical electrodes with different solution concentrations. For such a cell, its cell potential under standard conditions, 8°g j, is zero. 

Because any two oxidation-reduction reactions can be combined to make a cell, the tabulation of standard electrode potentials becomes a very efficient way of calculating cell potentials under standard conditions. As indicated by Eq. (54), if the electrode reactions involve the metals of the cell terminals, the metal-metal potential due to the cell terminals is automatically included in the result. A short table of standard electrode potentials is given in Table 2. 

CELL POTENTIALS UNDER STANDARD CONDITIONS We see that an important characteristic of a voltaic cell is its cell potential, which is the difference in the electrical potentials at the two electrodes and is measured in units of volts. Half-cell potentials are tabulated for reduction half-reactions under standard conditions standard reduction potentials). 

SECTION 20.4 Cell Potentials under Standard Conditions... 

SECTION 20.4 A voltaic cell generates an electromotive force (emf) that moves the electrons from the anode to the cathode through the external circuit. The origin of emf is a difference in the electrical potential energy of the two electrodes in the cell. The emf of a cell is called its cell potential, cell>3nd is measured in volts (1 V = IJ/C). The cell potential under standard conditions is called the standard emf, or the standard cell potential, and is denoted u. 

CELL POTENTIALS UNDER STANDARD CONDITIONS (section 20.4)... 

The zinc-copper galvanic cell is under standard conditions when the concentration of each ion is 1.00 M, as shown in Figure 19-13. The cell potential under these conditions can be determined by connecting the electrodes to a voltmeter. The measured potential is 1.10 V, with the Zn electrode at the higher (more negative) potential, so Zn gives up electrons and E eii = 1.10 V ... 

Provided the reaction is, in some sense, reversible, so that equilibrium can be attained, and provided the reactants and products arc all gas-phase, solution or solid-state species with well-defined free energies, it is possible to define the free energies for all such reactions under any defined reaction conditions with respect to a standard process this is conventionally chosen to be the hydrogen evolution/oxidation process shown in (1.11). The relationship between the relative free energy of a process and the emf of a hypothetical cell with the reaction (1.11) as the cathode process is given by the expression AC = — nFE, or, for the free energy and potential under standard conditions, AG° = — nFEl where n is the number of electrons involved in the process, F is Faraday s constant and E is the emf. 

To determine the E° of different cell arrangements, chemists use what are called standard reduction potentials for halfcells. A standard reduction potential is the electrical potential under standard conditions of a cell compared to the standard hydrogen electrode. The standard hydrogen electrode is a special half-cell that has been chosen as a reference to measure electrical potential. Just as sea level is a logical elevation for measuring gravitational potential,... 

This equation has a standard electrode potential of -0.447 V. Thus, the solution containing mercuric and chloride ions in contact with iron forms a battery. The reduction of the complex ions to metallic mercury is the cathodic reaction. The dissolution of iron is the anodic reaction. The overall reaction in the battery is given by the addition of Equation (13.42) and Equation (13.43). Due to the high value of its reversible cell voltage under standard conditions (0.85 V), it is expected that a very low equilibrium concentration of the complex ion can be achieved. 

The cell voltage under standard conditions is the potential difference between the right and left half cell U° = o2/h2o - - h+/h2 = 1-229 V. Since U° is positive, the Gibbs free energy of the reaction is negative according to Eq. (16)... 

So far, we ve considered cells with all components in their standard states. But most cells don t start at those conditions, and even if they did, the concentrations change after a few moments of operation. Moreover, in all practical voltaic cells, such as batteries, reactant concentrations are far from standard-state values. Clearly, we must be able to determine Ecc h the cell potential under nonstandard conditions. 

The Nernst equation says that a cell potential under any conditions depends on the potential at standard-state concentrations and a term for the potential at nonstandard-state concentrations. How do changes in Q affect cell potential From Equation 21.9, we see that... 

Electrochemical Series Some redox potentials under standard conditions (T = 298 K, p = 100 kPa, c = 1 kmolm in aqueous solutions) have been compiled in the following table (Table 23.1). This kind of sequence of standard redox potentials is also called the electrochemical series. The half-cells in question have been characterized by the corresponding Stockholm convention of 1953. In this convention, a phase boundary is denoted by a single vertical line. 

We calculate cell potentials under nonstandard conditions by using standard cell potentials and the Nernst equation. 

For each half-cell in a voltaic cell, the standard reduction potential provides a measure of the tendency for reduction to occur The more positive the value ofE° y the greater the tendency for reduction under standard conditions. In any voltaic cell operating under standard conditions, the value for the reaction at the cathode is more positive than the E°eelectrons flow spontaneously through the external circuit from the electrode with the more negative value of to the electrode with the more positive value of E°ej. 

One thought you may have about corrosion is that we cannot expect standard conditions in real world applications. What must we do to account for the differences that arise when standard conditions are not present This is a very important question with clear ties to thermodynamics. The equation that describes cell potentials under nonstandard conditions is called the Nemst equation ... 

Table 9.8 summarizes some experimental results from a number of voltaic cells operating under standard conditions. These cell potentials can also be calculated from standard electrode potentials (Chapter 19). 

The standard potential of the cell is determined by the type of active materials contained in the ceU. It can be calculated from free-energy data or obtained experimentally. A listing of electrode potentials (reduction potentials) under standard conditions is given in Table 1.1. A more complete list is presented in Appendix B. 

However, what if [Cu ] > IM and [Zn ] < IM For example, how would the cell potential for the following conditions be different from the potential under standard conditions ... 

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