Oxidation Reduction Chemistry of the Elements
Periodic Trends in Standard Reduction Potentials of Oxo Anions and Acids
p-Block ElementsConsider oxo anions and acids of the p-block in their highest oxidation state (charge equal to group number) being reduced by two electrons.
As oxidation number increases across a Period from left to right, Eo becomes more positive. Therefore, stability decreases across the Period and the ability to act as an oxidizing agent increases.
d-Block ElementsThe trends across a Period are the same as in the p-Block Elements while the trends down a family are different. In the d-block, the elements in Period 4 show the greatest reluctance to adopt the group oxidation number (the most easily reduced.)
Redox Reactions and the Real WorldMost redox reactions occur under nonstandard conditions. As a reaction proceeds, the concentrations of reactants and products change altering the driving force for the reaction.
The effect of nonstandard concentrations on potential at room temperature is given by the Nernst equation.
Consider the reduction of a metal ion to a metal.
The Synthesis of Oxo AnionsThe dependence of potential on concentration is more complicated for oxo anions, oxo acids, oxides and hydroxides than it is for simple metal cations. Consider the reduction of ferrate to ferric ion.
Decreasing the concentration of the hydrogen ion shifts the equilibrium to the left favoring ferrate stability in strongly basic solution.
The synthesis of a salt of an oxo anion with a very highly oxidized central atom is normally carried out in basic solution.
To carry out oxidations in basic solution, an oxidizing agent such as chlorine that involves few or no hydrogen ions in the reduction is preferred.
A very unstable species such as the ferrate ion is normally prepared in strongly alkaline solution, using chlorine as the oxidizing agent.
The equilibrium is driven to the product side by precipitating the ferrate ion with an appropriate cation (Ba2+ is a good choice since the ferrate ion is feebly basic).
Hydrogen peroxide is another oxidizing agent that can be used in alkaline solution.
Many oxo anions can be prepared by the electrolysis of aqueous solution containing the elements in low oxidation states. In these cases, water is reduced at the cathode generating hydroxide ion lowering the pH while the oxo anion is produced at the anode.
If the oxo anion has a standard reduction potential in the range +0.3 to +1.2 V, basic conditions are not usually necessary and strongly oxidizing acids such as nitric acid may be used as the oxidizing acid.
The element may also be oxidized in air generating its acidic oxide. This acidic oxide may be converted into its oxo acid by reaction with water or directly to the oxo salt by reaction of the acidic oxide with a basic oxide.
If the desired oxo acid or anion has a reduction potential below about +0.2V, acid-base reactions may be sufficient since the element may well occur naturally in the desired oxidation state.