Standard potential of zinc

The first attempt to determine zero charge potential for single-crystal sp metal was described for zinc [3, 6] from C-E capacitance curves in dilute solutions. Epzc for the face of Zn(OOOl) was about 80 mV more positive than that for Zn(lOlO). Later, it was pointed out [6, 10] that the determination of Epzc directly from C-E dependencies was not possible for zinc because the potential is close to the reversible standard potential of zinc in aqueous solution. 

SOLUTION The zinc electrode is the anode, so zinc is oxidized and copper is reduced. The standard potential of zinc is —0.76 V. The copper electrode must contribute a potential such that overall the cell potential is 1.10 V ... 

The Standard Potential of Zinc. The determination of the standard potential of zinc will also be considered in detail since it can be obtained from measurements on several different types of galvanic cells. In addition, a discussion of the computations from the different data brings up most of the questions involved in this field. The types of cell on which the measurements have been made are ... 

Together with 1.0292 volts for the o value for cell (24) we have another value for the standard potential of zinc... 

This standard potential has been used for computing the activity coefficients of zinc sulphate given in Table V, Chapter 8. From this value the standard potential of zinc can obviously be obtained if an Eq value of the cell... 

In accord with the foregoing discussion, the standard potential of zinc, which is not separately measurable, refers to the emf of a cell with two electrodes— zinc and the standard hydrogen electrode ... 

The reduction is thermodynamically favored, because the standard potential of the couple Cu2+/Cu is positive (E° = +0.34 V). Metals with negative standard potentials, such as zinc (E° = —0.76 V) and nickel (E° = —0.23 V), cannot be extracted hydrometallurgically. 

A galvanic cell can be constmcted from a zinc electrode immersed in a solution of zinc sulfate and an iron electrode immersed in a solution of iron(II) sulfate. What is the standard potential of this cell, and what is its spontaneous direction under standard conditions ... 

Although zinc is formally a 4-block element, some of its chemical properties are similar to those of the alkaline earth metals, especially those of magnesium. This is mainly due to zinc s exclusive exhibition of the +2 oxidation state in all its compounds and its appreciable electropositive character. With a standard potential of —0.763 V, zinc is considerably more electropositive than copper and cadmium. 

One more example demonstrates how to use standard reduction potentials to determine the standard potential of a cell. Let s say you wanted to construct a cell using silver and zinc. This cell resembles the Daniell cell of the previous example except that a silver electrode is substituted for the copper electrode and a silver nitrate solution is used in place of copper sulfate. From Table 14.2, it is determined that when silver and copper interact silver is reduced and copper oxidized. The two relevant reactions are... 

If the standard reduction potential of zinc is -0.76 V, what will be the standard oxidation potentials of the other metals ... 

The electrode potential is defined as the potential difference between the terminals of a cell constructed of the half-cell in question and a standard hydrogen electrode (or its equivalent) and assuming that the terminal of the latter is at zero volts. Note therefore that the electrode potential is an observable physical quantity and is unaffected by the conventions used for writing cells. The statement. . . the electrode potential of zinc is —0.76 volts. . . implies only that a voltmeter placed across the terminals of a cell consisting of standard hydrogen electrode and the zinc electrode would show this value of potential difference, with the zinc terminal negative with respect to that of the hydrogen electrode. An electrode potential is never a metal/solution potential difference , not even on some arbitrary scale. 

Note that aZn(s) = 1 since zinc is a pure substance. A more complex relationship is required if one component is a non-stoichiometric solid which can exist over a range of compositions. E % is known as the standard electrode potential of zinc. 

The hydrogen electrode is then used to define the standard potential of any other electrode. For example, to find the standard potential of a zinc electrode, we measure the standard potential of the cell in which the hydrogen electrode is one electrode and a zinc electrode is the other ... 

The value quoted here, like all the values in this text, is for 25°C. Because, according to convention, the hydrogen electrode contributes 0 to the standard cell potential, the standard potential of the cell, 0.76 V, can be attributed entirely to the zinc electrode. Moreover, because the zinc electrode is known by experiment to be the anode, its standard potential is subtracted from that of hydrogen ... 

The standard potential of a zinc electrode is —0.76 V and the standard potential of the cell... 

The standard potentials for (Zn2+ Zn) and (Ag+ Ag) are —0.763 V and 0.799 V from Table 19-1. The standard potential of the cell is the difference between these two numbers, 0.779 V —(—0.763) = 1.542 V. The silver potential is higher, and this means that the silver ion is the oxidizing agent. The zinc couple provides the reducing agent and is the negative electrode. The equation for the cell is... 

The standard oxidation potential of zinc = 0.76 V. The overall reaction is the sum of oxidation and reduction potentials (see Figure 1.3). 

The zinc reductor, commonly known as the Jones ° reductor, generally is prepared from amalgamated zinc. The addition of mercury does not affect the standard potential of the Zn -Zn couple (—0.7628 V) as long as solid zinc is present. The rate of reduction, however, depends on the concentration of zinc at the surface of the amalgam. With relatively strong oxidants, such as Fe(III) and Ce(IV), which are reduced by mercury, a mercury content of 1 or even 5% may be used at high acid concentrations to control the rate of hydrogen evolution. With weaker oxidants the mercury content should be minimized so that the reduction reaction is not retarded. 

In order to determine the standard potential of a metal forming a soluble, highly dissociated chloride, e.g., zinc, the measurements are made on cells of the type... 

The Standard potential at the anode plus the standard potential at the cathode gives the standard cell potential. The potential of the SHE is 0.000 volt, and the standard cell potential is found to be 0.763 volt. So the standard potential of the zinc anode must be 0.763 volt. The Zn Zn2+(1.0 7W) H+(1.0 M), H2(l atm) Pt cell is depicted in Eigure 21-9. 

We can evaluate the standard reduction potential of zinc, zn-" /zn. Oy reversing the oxidation half-reaction ... 

Besides the stable -i- II oxidation state, Cd may also form very unstable oxidation state -t-l compounds. Cd has a relatively low standard potential of -0.402 V (for -t-2 e = M), which resembles more that of zinc (-0.762 V) than that of mercury (-1-0.854 V). This low standard potential reflects the reducing power of Cd. 

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