Voltaic cells standard hydrogen electrode

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. 

It is impossible to measure directly the electrode potentials. Only the electromotive force (emf) of a voltaic cell arising from a combination of two electrodes can be directly measured, which is given as the arithmetical sum or difference of the two electrode potential depending upon their signs. If one of the electrode potential be accurately measured, that of the other may be calculated. The reference electrode arbitrarily chosen for this purpose is the standard hydrogen electrode. Hydrogen gas at 1 atm. pressure and at a temperature of 25°C is slowly bubbled over a platinised platinum electrode which is immersed in a solution of hydrogen ions of unit activity. By convention potential of the half cell reaction... 

A voltaic cell consists of a standard hydrogen electrode in one half-cell and a Cu/Cu" half-cell. Calculate [Cu" ] when ceii is 0.25 V. 

FIGURE 20.9 A voltaic cell using a standard hydrogen electrode (SHE). The... 

A voltaic cell is constructed with two hydrogen electrodes. Electrode 1 has = 1.00 atm and an unknown concentration of H" (ng). Electrode 2 is a standard hydrogen electrode = 1.00 atm, [H ] = 1.00Al).At 298 K the measured cell potential is 0.211 V, and the electrical current is observed to flow from electrode 1 through the external circuit to electrode 2. Calculate [ H ] for the solution at electrode 1. What is the pH of the solution ... 

Figure 20.9 A voltaic cell using a standard hydrogen electrode (SHE). The anode half-cell is Zn metal in a Zn(N03)2(aq) solution, and the cathode half-cell is the SHE in a HN03(aq) solution. 

We can develop a series of representative electrode potentials by measuring the potentials of other standard electrodes versus the SHE in the way we have described for the standard Zn-SHE and standard Cu-SHE voltaic cells. The standard electrode potential for any half-cell is its potential with respect to the standard hydrogen electrode, measured at 2 5"C when the concentration of each ion in the solution is 1 M and the pressure of any gas involved is 1 atm. 

Thus, if the standard half-cell is connected to a standard hydrogen electrode to form a voltaic or electrochemical cell, the measured voltage, called the electromotive force (EMF), is the standard electrode potential of that half-cell. 

The potential difference measured across the complete voltaic cell is easily measured and equals the sum of the electrode potentials for the two half-reactions. Individual electrode potential cannot be measured directly, because there can be no transfer of electrons unless both the anode and the cathode are connected to form a complete circuit A relative value for the potential of a half-reaction can be determined by connecting it to a standard half-cell as a reference. This standard halfcell, shown in Figure 2.8, is called a standard hydrogen electrode, or SHE. 

A voltaic cell designed to measure [Cu " ] is constructed of a standard hydrogen electrode and a copper metal electrode in the Cu solution of interest. If you want to construct a calibration curve for how the ceU potential varies with the concentration of copper(II), what do you plot in order to obtain a straight line What is the slope of the line ... 

To determine the value of E° for a sfandard electrode such as that to which half-cell reaction (19.6) applies, we compare it with a standard hydrogen electrode (SHE). In this comparison, the SHE is always taken as the electrode on the left of the cell diagram—the anode—and the compared electrode is the electrode on the right—the cathode. In the following voltaic cell, the measured potential difference is 0.340 V, wifh electrons flowing from the H2 to the Cu electrode. 

The voltaic cell in Figure 19-11 consists of two hydrogen electrodes. One is a standard hydrogen electrode (SHE), and the other is a hydrogen electrode immersed in a solution of unknown [H+], less than 1 M. The cell diagram is... 

Briefly describe each of the following ideas, methods, or devices (a) salt bridge (b) standard hydrogen electrode (SHE) (c) cathodic protection (d) fuel cell. Explain the important distinctions between each pair of terms (a) half-cell reaction and overall cell reaction (b) voltaic cell and electrolytic cell (c) primary battery and secondary battery (d) Eceii and E°en. 

Figure 21.8 Determining an unknown EhaH-uii with the standard reference (hydrogen) electrode. A voltaic cell has the Zn halfreaction in one half-cell and the hydrogen reference half-reaction in the other. The magnified view of the hydrogen half-reaction shows two HaO" ions being reduced to two H2O molecules and an H2 molecule,...

An electrode is prepared by dipping a silver strip into a solution saturated with silver thiocyanate, AgSCN, and containing 0.10 M SCN. The emf of the voltaic cell constructed by connecting this electrode as the cathode to the standard hydrogen half-cell as the anode is 0.45 V. What is the solubility product of silver thiocyanate ... 

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