Oxidation States and EMFs of Groups

The emf values for oxidation of the oxygen in water are given in Eqs. 10.119 to 10.121. These determine the minimum reduction emf necessary for a species to effect oxidation of the oxygen 1 M acid. ° + 1.229 V neutral solution. E +0.81S V 1 M base, ° +0.401 V. There are several oxidation states of manganese that are reduced by water, but the protonated manganate ion is typical  

24 Predictions based on emf values are not always borne out in the laboratory. For example, pure Mn(s) would be predicted to react with neutral water but no reaction is observed. In some cases reactions arc extremely slow and are not observed for kinetic reasons. In others, products of the reaction, such as oxide coatings, protect the reactant surfaces. Furthermore, reactions are usually not run at standard conditions and then ° values do not reflect the true spontaneity of the reaction. 

Disproportionation occurs when a species is both a good reducing agent and a good oxidizing agent. In basic solution, for example, CU disproportionates to Cl and CIO- ions  

Other applications of emfs include the prediction of thermodynamically possible redox reactions [e.g.. will Sri +oxidize Fez+ to Fe3+7] and the stabilization of oxidation states through the formation of complexes. The former is a straightforward application of thermodynamics and will not be discussed further here. The second is of great importance. It was introduced in Chapter 11 and will be discussed further below. 

The Nemst equation was given before (Eq. 10.115), and in this chapter the effect of pH on the reduction potential of the hydrogen ion has been mentioned, but the effect in general should be emphasized. There are several types of reactions in which concentrations of the reactants and products affect the stability of various oxidation stales. This can be understood through application of the Nernst equation. The reduction potential of hydrogen will vary with the concentration of the hydrogen ion hence the commonly known fact that many reasonably active metals dissolve in acid but not in base. 

Having compared in genera) terms the properties of transition metak both on the bask of the d-electron configuration aixl the properties of the light versus heavier metals, we shall now look more specifically at the stabilities of the various oxidaticxi states of each element in aqueous solution. Every oxidation state will not be examined in detail, but the emf data to make such an evaluation will be presented in the form of a Latimer diagram. 

If you are not thoroughly famOiar with the principles of electrocbemistiy, you should review Chapter 10 and the Latimer diagram derived there (below) befbre considering the following discussion for determining the stability of oxidation states  

There are three sources of thermodynamic instability for a particular oxidation state ct an element in aqueous solution (l)The element may reduce the hydrogen in water or hydronium ions (2) it may oxidize the oxygen in water or hydroxide ions or (3) it may disproportionate. 

The emf values for reduction of hydrogen in water are given in Eqs. 10.116 to 10.118. These determine the minimum oxidation ends necessary fora species to effect 

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The Chemistry of the Heavier Transition Metals 587 Oxidation States and EMFs of Groups ]-12 588 The Lanthanide and Actinide Elements 599 Coordination Chemistry 608 The Transactinide Elements 613... 

The elements copper, silver, and gold show such anomalies that there sometimes appears to be little congruence as a family, with the member that is least reactive as a metal (Au) being the only one that has an appreciable chemistry in the +3 oxidation state and also the only one to reach the - I and +5 oxidation slates (CsAu and AuFj), although both copper and silver may be oxidized to +4. The members of the family more or less routinely (silver less frequently) violate the very useful rule of thumb you have seen earlier The maximum oxidation state of an element is equal to or less than its group number (IB, IVB, VIIB, etc.). Thus we have CUSO4, AgF, and [AuClJ . Each member of the family has a different preferential oxidation state (Cu, +2 Ag, + 1 Au, +3). The one property they do have in common is that none has a positive emf for M -> M therefore, the free metals arc not affected by simple acids, nor are they readily oxidized otherwise, leading to their use in materials intended to last.31... 

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