Redox Reactions

Oxidation Reduction Chemistry of the Elements

So far we have considered that all elements in positive oxidation states begin their existence in water as hydrated cations and then undergo reactions between the cations and the water to "neutralize" their positive charge. These reactions ultimately alter the pH of the solution.

Review of Redox Concepts

Redox reactions normally require the presence of two reactants

in one reactant has the oxidation number of an element is decreased (reduction)
in one reactant has the oxidation number of an element is increased (oxidation)

A redox reaction will occur if DG for the reaction is negative
Experimentally cell voltage is measured

In electrochemical cells, the oxidation and reduction occur in separate compartments (half-cells), and the overall voltage is the sum of the half-cell voltages
Often one half-cell uses the standard hydrogen electrode (SHE)

The voltage generated depends on the conditions of the reactants and products

gases

standard state is a pressure of 1.000 atm

solutes

concentration that gives an activity of 1.00
(often 1M or 1 m)

Standard reduction potentials

the amount of voltage generated in a half-cell when coupled with the SHE

Periodic Trends in Standard Reduction Potentials of Oxo Anions and Acids

The less positive the value of the standard reduction potential (E^o), the more stable the species is and the less easily it is reduced.

p-Block Elements

Consider 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, E^o becomes more positive. Therefore, stability decreases across the Period and the ability to act as an oxidizing agent increases.

In a given family, the group oxidation number is least stable at the top of the family (second Period) and at the bottom of the family (sixth Period). The group oxidation number is most stable in the third Period. This is possibly due to the fact than an oxidation number above 5 in an oxo anion requires a coordination number of at least 4. In Groups VIA and VIIA, the stability of the highest oxidation state follows the sequence:

Period 3 element >>Period 4 element > Period 5 element

d-Block Elements

The 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 World

Most 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.

Increasing the concentration of the metal ion will tend to drive the reaction to the right increasing the potential

The Synthesis of Oxo Anions

The 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.

Practical Consequence:
The synthesis of a salt of an oxo anion with a very highly oxidized central atom is normally carried out in basic solution.

Oxo anions salts with very highly oxidized central atoms tend to be useful oxidizing agents which work most effectively in acidic solutions.

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.

Typically, for species with standard reduction potentials greater than +1.2 V in acidic solution, the use of strongly basic solutions and strong oxidizing agents is required.

A very unstable species such as the ferrate ion is normally prepared in strongly alkaline solution, using chlorine as the oxidizing agent.

2 Fe(OH)₃ + 3 OCl^- + OH^-

2 FeO₄^2- + 5 H₂O + 3 Cl^-

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.

2 Cr(OH)₃ + 3 H₂O₂ + 4 OH^-

2 CrO₄^2- + 8 H₂O

Examples of Oxo Anion Syntheses in Alkaline Solution
Oxo Anion	Starting Material	Oxidizing Agent
periodate	iodate	chlorine
tellurate	tellurite	chlorine
perxenate	xenon trioxide	ozone
perbromate	bromate	fluorine

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.

Examples of Electrolytic Oxo Anion Syntheses
Oxo Anion	Starting Material
hypochlorite	aqueous sodium chloride
permanganate	manganate ion

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.

Examples Using Nitric Acid
Oxo Anion	Starting Material
iodic acid	iodine
selenous acid	selenium
arsenic acid	As₂O₃

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.

PCl₃ + 3 H₂O

H₃PO₃ + 3 HCl

Syntheses for oxo acids and anions can be found in:

Brauer, G. ed. Handbook of Preparative Inorganic Chemistry, 2 vols, 2nd ed., Academic Press, New York, 1963.