Example 4.5

Example . How would you account for the increasing oxidising power in the series VO + < Cr O – < MnO – ? This is due to the increasing stability of the lower species to which they are reduced. For the first row transition metals the E o values are: E o Co Ni Cu (M + /M) – . – . – . – . – . – . + . Explain the irregularity in the above values. The E o (M + /M) values are not regular which can be explained from the irregular variation of ionisation enthalpies ( i i   H H ) and also the sublimation enthalpies which are relatively much less for manganese and vanadium. Why is the E o value for the Mn + /Mn + couple much more positive than that for Cr + /Cr + or Fe + /Fe + ? Explain. Much larger third ionisation energy of Mn (where the required change is d to d ) is mainly responsible for this. This also explains why the + state of Mn is of little importance. . . Magnetic Properties Transition metals vary widely in their chemical reactivity. Many of them are sufficiently electropositive to dissolve in mineral acids, although a few are ‘noble’—that is, they are unaffected by single acids. The metals of the first series with the exception of copper are relatively more reactive and are oxidised by 1M H + , though the actual rate at which these metals react with oxidising agents like hydrogen ion (H + ) is sometimes slow. For example, titanium and vanadium, in practice, are passive to dilute non oxidising acids at room temperature. The E o values for M + /M (Table . ) indicate a decreasing tendency to form divalent cations across the series. This general trend towards less negative E o values is related to the increase in the sum of the first and second ionisation enthalpies. It is interesting to note that the E o values for Mn, Ni and Zn are more negative than expected from the general trend. Whereas the stabilities of half-filled d subshell ( d ) in Mn + and completely filled d subshell ( d ) in zinc are related to their E e values; for nickel, E o value is related to the highest negative enthalpy of hydration. An examination of the E o values for the redox couple M + /M + (Table . ) shows that Mn + and Co + ions are the strongest oxidising agents in aqueous solutions. The ions Ti + , V + and Cr + are strong reducing agents and will liberate hydrogen from a dilute acid, e.g., Cr + (aq) + H + (aq) ® Cr + (aq) + H (g) . . Chemical Reactivity and E o Values