Zinc reduction potentials

To calculate the E°, the voltage, of an electrochemical cell, the voltage for the oxidation half reaction at the anode is added to the voltage for the reduction half reaction at the cathode E°a ll = E°oxid react + E°red react. For an electrochemical cell with zinc and copper electrodes, E° = 0.76 + 0.34 = 1.10 V, voltage equals 0.76 + 0.34, which equals 1.10 volts. The sign for the zinc reduction potential half reaction is changed because the half reaction is reversed to show that oxidation occurs at the zinc electrode. The two half reactions can also be added to show the overall reaction in the electrochemical cell ... 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

The silver reductor has a relatively low reduction potential (the Ag/AgCl electrode potential in 1M hydrochloric acid is 0.2245 volt), and consequently it is not able to effect many of the reductions which can be made with amalgamated zinc. The silver reductor is preferably used with hydrochloric acid solutions, and this is frequently an advantage. The various reductions which can be effected with the silver and the amalgamated zinc reductors are summarised in Table 10.11. ... 

Reductant equivalent weights of, 847 Reduction 409 by chromium(II) salts, 409 by hydrogen sulphide, 416 by Jones reductor (zinc amalgam), 410 by liquid amalgams, 412 by silver reductor, 414 by sulphurous acid, 416 by tin(II) chloride, 415 by titanium(II[), 410 by vanadium(II), 410 see also Iron(III), reduction of Reduction potentials 66 Reference electrodes potentials, (T) 554 Relative atomic masses (T) 819 Relative error 134 mean deviation, 134... 

The Zn /Zn reduction potential is more negative than the H3 O /H2 reduction potential (-0.76 V vs. 0 V), so zinc is the anode in this cell. Zinc is oxidized and hydronium ions are reduced, causing electrons to flow from the more negative zinc electrode to the less negative SHE. Again, we reverse the direction of the half-reaction with the more negative potential and find E by subtracting the half-cell potentials ... 

The overall voltage generated by a standard galvanic cell is always obtained by subtracting one standard reduction potential from the other in the way that gives a positive value for E (.gH Example applies this reasoning to zinc and iron. 

C19-0031. Tin cans are made of iron, coated with a thin film of tin. After a break occurs in the film, a tin can corrodes much more rapidly than zinc-coated iron. Using standard reduction potentials, explain this... 

The table of standard reduction potentials assists in the determination as to whether species can react with each other, or not. This can be substantiated by considering the reaction of hydrogen with two metals, copper and zinc. In order to determine whether or not a reaction takes place spontaneously under standard conditions, one calculates the standard potential using hydrogen ions and the metal as reactants. 

The zinc complex is more difficult than the free Zn2+ to reduce as deduced from the more negative standard reduction potential ... 

For a Daniell cell, you know that copper is the cathode and zinc is the anode. The relevant half-reactions and standard reduction potentials from Table 11.1 are as follows. 

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... 

Standard reduction potentials of iron and zinc are as follows ... 

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

Of the Group 12 elements, zinc, cadmium and mercury, only Hg has a water-stable -I-1 state, and all three elements have + 2 states that are water-stable. Their reduction potentials are summarized in the Latimer diagram ... 

Notice, however, that if the reaction at the Zn/Zn2+ interface is reversed and written as an electronation (reduction) rather than a deelectronation, then this electronation does not proceed spontaneously and its free-energy change is positive. This positive value of AG° = -nFE0 implies that E° must be negative. The standard reduction potentials for the zinc and copper systems are, therefore, -0.76 and +0.34 V, in contrast to the standard oxidation potentials, which are +0.76 and -0.34 V, respectively. 

Then, we tabulate the E° values for the half-reactions, remembering that E° for oxidation of zinc is the negative of the standard reduction potential (—0.76 V). We do not multiply the E° value for reduction of Ag + (0.80 V) by a factor of 2, however, because an electrical potential does not depend on how much reaction occurs. 

Zinc metal is produced by reducing ZnO with coke, and magnesium metal is produced by the electrolysis of molten MgCl2. Look at the standard reduction potentials in Appendix D, and explain why magnesium can t be prepared by the method used for zinc. 

Let us then compare zinc with other metallic elemental substances with respect to the formation of M2+(aq) in acid solution at a pH of zero. The relevant data are summarised in Table 5.7. Note that the ° values given refer (in accordance with the European Convention) to reduction potentials for the half-reactions ... 

Sodium amalgam is effective for promoting defluorination (Scheme 14), to produce an interesting series of dienes (38), (40) and (42), especially (38) [7, 8, 36], but tetrakis(dimethylamino)ethene (TDAE) (43) (Scheme 15) is a much safer system to use and, consequently, with the latter reagent, the process may be scaled up [36]. TDAE (43) is successful because its donor capacity is high, i.e. it has been variously described as being similar to that of zinc [37] or to alkali-metals [38] and will react, therefore, with most perfluorinated alkenes or -cycloalkenes. Furthermore, the fact that the dienes (38), (40) and (42) may be isolated derives from the fact that the dienes have more CF= sites than the starting alkenes and consequently their respective reduction potentials vary by... 

At the present time, tables of E° values use reduction half reactions called standard reduction potentials. Because the value of E° is affected by the concentration of the electrolyte solution, these values are given for 1 molar solutions. On a table of standard reduction potentials, we would find the E° value for zinc to be -0.76 V, the negative value resulting because the zinc oxidation half reaction must be reversed for a reduction potential table When the half reaction is reversed, the sign must also be reversed. Some standard reduction potential half reactions include the following ... 

Electrochemical reduction of the salts (4) provides radicals (18) which dimerize or undergo further reduction to anions (32) or dianions (80MI43100). The reduction potentials are not much affected by substituents. Reduction with zinc in aprotic conditions gives bi(l,2-dithiol-3-yls) (59), and 3-chloro-l,2-dithiolylium salts (35a X = Cl) are converted into bi(l,2-dithiol-3-ylidenes) (20) (75TL3473). Divalent chromium converts the 3,5-dimethyl-l,2-dithiolylium cation into a dithioacetylacetonate ligand (72AJC2547). The reaction of 3,5-diamino-l,2-dithiolylium salts (8) or alkyl derivatives with thiols provides dithiomalonamides (60) by electron transfer (63ACS163). 

Very conveniently, the oxidation potential is the reverse of the reduction potential. For example, consider the oxidation of zinc in our previous example ... 

Answer To solve the problem we need to know the values for n and E. Let s begin by looking at E. Zinc is the anode in this reaction, and silver is the cathode. You can tell this in two ways the first is that the reduction potential for silver is more positive than zinc, and second because silver is being reduced in the reaction. Knowing this, we can use Equation 18.2 to determine E ... 

---