Voltaic cells standard reduction

A voltaic cell converts chemical energy into electrical energy. It consists of two parts called half-cells. When two different metals, one in each half-cell, are used in the voltaic cell, a potential difference is produced. In this experiment, you will measure the potential difference of various combinations of metals used in voltaic cells and compare these values to the values found in the standard reduction potentials table. 

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. 

Understanding voltaic cells, anodes, and cathodes Figuring standard reduction potentials and electromotive force Zapping current into electrolytic cells... 

You have probably worked with tables of standard reduction potentials before. These tables provide the reduction potentials of various substances. It describes an oxidized species s ability to gain electrons in a reduction half-reaction (like copper in the voltaic cell example). According to this definition, we can use a value from the table to represent the E°red in the expression above, but how do you find the E°ox ... 

In voltaic cells, it is possible to carry out the oxidation and reduction halfreactions in different places when suitable provision is made for transporting the electrons over a wire from one half-reaction to the other and to transport ions from each half-reaction to the other in order to preserve electrical neutrality. The chemical reaction produces an electric current in the process. Voltaic cells, also called galvanic cells, are introduced in Section 17.1. The tendency for oxidizing agents and reducing agents to react with each other is measured by their standard cell potentials, presented in Section 17.2. In Section 17.3, the Nernst equation is introduced to allow calculation of potentials of cells that are not in their standard states. 

Over the years, chemists have measured and recorded the standard reduction potentials, abbreviated of many different half-cells. Table 21-1 lists some common half-cell reactions in order of increasing reduction potential. The values in the table are based on using the half-cell reaction that is being measured as the cathode and the standard hydrogen electrode as the anode. All of the half-reactions in Table 21-1 are written as reductions. However, in any voltaic cell, which always contains two halfreactions, the half-reaction with the lower reduction potential will proceed in the opposite direction and will be an oxidation reaction. In other words, the half-reaction that is more positive will proceed as a reduction and the half-reaction that is more negative will proceed as an oxidation. 

You are given the half-cell descriptions for a voltaic cell and standard reduction potentials in Table 21-1. In any voltaic cell, the half-reaction with the lower reduction potential will proceed as an oxidation. With this information, you can write the overall cell reaction and calculate the standard cell potential. 

The standard potential for this voltaic cell seems reasonable given the reduction potentials of the half-cells that comprise It. The mathematical operations with negative numbers are correct and the answer is correct to the thousandths place. 

The standard potential of a voltaic cell is the difference between the standard reduction potentials of the half-cell reactions. 

Use standard reduction potentials, to calculate the potential of a standard voltaic cell, E%... 

A voltaic cell containing a standard Fe /Fe electrode and a standard Ga /Ga electrode is constructed, and the circuit is closed. Without consulting the table of standard reduction potentials, diagram and completely describe the cell from the following experimental observations, (i) The mass of the gallium electrode decreases, and the gallium ion concentration increases around that elec-... 

A voltaic cell is designed with a copper electrode immersed in 1.0 M copper (II) sulfate solution, CuSO j(aq), and a lead electrode immersed in 1.0 M lead (II) nitrate solution, Pb(NO3)2(aq) at 25 °C. Given the standard reduction potentials shown below, determine the potential of the cell in volts. 

We can generalize this result for any voltaic cell the standard cell potential is the difference between the standard electrode potential of the cathode (reduction) half-cell and the standard electrode potential of the anode (oxidation) half-cell ... 

By combining many pairs of half-cells into voltaic cells, we can create a list of reduction half-reactions and arrange them in decreasing order of standard electrode potential (from most positive to most negative). Such a list, called an emf series or a table of standard electrode potentials, appears in Appendix D, with a few examples in Table 21.2 on the next page. 

Plan The standard half-cell reactions are identical, so °eii is zero, and we calculate Eceii from the Nernst equation. Because half-cell A has a higher [Ag ], Ag ions will be reduced and plate out on electrode A. In half-cell B, Ag will be oxidized and Ag ions will enter the solution. As in all voltaic cells, reduction occurs at the cathode, which is positive. 

The Example Problems showed you how to use the data from Table 20.1 to calculate the standard potential (voltage) of voltaic cells. Another important use of standard reduction potentials is to determine if a proposed reaction under standard conditions will be spontaneous. How can standard reduction potentials indicate spontaneity Electrons in a voltaic cell always flow from the half-cell with the lower standard reduction potential to the half-cell with the higher reduction potential, giving a positive cell voltage. To predict whether any proposed redox reaction will occur spontaneously, simply write the process in the form of half-reactions and look up the reduction potential of each. Use the values to calculate the potential of a voltaic cell operating with these two half-cell reactions. If the calculated potential is positive, the reaction is spontaneous. If the value is negative, the reaction is not spontaneous. However, the reverse of a nonspontaneous reaction will occur because it will have a positive cell voltage, which means that the reverse reaction is spontaneous. 

On the basis of the standard reduction potentials shown above, which standard cell notation correctly represents its voltaic cell ... 

Based on the standard reduction potentials given above, if a silver electrode and a chromium electrode are connected in a voltaic cell, which electrode will undergo oxidation and which will undergo reduction Explain how you can tell. 

It is possible to design a redox reaction such that the oxidation occurs at one location and the reduction occurs at another location. The device is called a galvanic or voltaic cell. The cathode (usually a metal bar or carbon rod) is the electrode where reduction takes place the anode (usually a metal bar or carbon rod) is the electrode where oxidation takes place. The salt bridge allows ions to slowly migrate from one beaker to the other to maintain electrical neutrality in each half-cell. The voltmeter measures the voltage (or potential), V, between the two electrodes. If the temperature is 298 K, and the solutions are 1 M, then the beakers with the electrodes are each considered to be a standard half-cell. 

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). 

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 Etea- 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 Ered- The standard cell potential, ceU> is the standard reduction potential of the cathode reaction, (cathode), minus the standard reduction potential of the anode reaction, (anode) ... 

The standard reduction potentials for other half-reactions can be determined in a fashion analogous to that used for the Zx " jZvr half-reaction. TABLE 20.1 lists some standard reduction potentials a more complete list is found in Appendix E. These standard reduction potentials, often called half-cell potentials, can be combined to calculate values for a large variety of voltaic cells. 

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. 

The fact that the standard cell potential is the difference between the standard reduction potentials of cathode and anode is illustrated graphically in M FIGURE 20.10. The more positive E° value identifies the cathode, and the difference between the two standard reduction potentials is the standard cell potential. -4 FIGURE 20.11 shows E values for the two half-reactions in the Zn-Cu voltaic cell of Figure 20.5. 

In writing the equation this way, we have dropped the subscript cell to indicate that the calculated emf does not necessarily refer to a voltaic cell. Also, we have generalized the standard reduction potentials by using the general terms reduction and oxidation rather than the terms specific to voltaic cells, cathode and anode. We can now make a general statement about the spontaneity of a reaction and its associated emf, E A positive value of E indicates a spontaneous process a negative value of E indicates a nonspontaneous process. We use E to represent the emf under nonstandard conditions and E° to indicate the standard emf. 

Rusting of iron requires both oxygen and water, and the process can be accelerated by other factors such as pH, presence of salts, contact with metals more difficult to oxidize than iron, and stress on the iron. The corrosion process involves oxidation and reduction, and the metal conducts electricity. Thus, electrons can move through the metal from a region where oxidation occurs to a region where reduction occurs, as in voltaic cells. Because the standard reduction potential for reduction of e (aq) is less positive than that for reduction of O2, Fe(s) can be oxidized by 02(g) ... 

The standard cell potential of a voltaic cell is the difference between the standard reduction potentials of the half-reactions that occur at the cathode and the anode u = (cathode) — (anode). The value of u is positive for a voltaic ceU. 

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