Complexation Reactions

A complexation chemical reaction consists of a number of electron transfer steps with some chemical steps that precedes or succeeds the electron transfer. Almost all organic electrochemical reactions are complex reactions.  Let’s go through an example. Add Cl^- to a solution containing Ag⁺. A precipitate will be formed as AgCl is insoluble in water. Continue adding Cl^- to the precipitate of AgCl. Due to common ion effect it is expected that the solubility of AgCl decrease continuously as Cl^- concentration increases. The solubility of AgCl increases as AgCl₂^- and AgCl₃^-. This type of compound is called a complex and it can intensify the solubility of an insoluble salt as complex forms.

Metals having oxidation states of +1, +2, +3 or +4 can exist as complexes in water. Metals with high oxidation states do not exist as complexes in solution. They form oxyanions.

Complexation Reactions in Nuclear separations

Complexation reactions are very important in nuclear separations that are mainstay in nuclear industry. Nuclear separations have applications in all parts of nuclear fuel cycle from mining and milling of ores. It is also applied in the purification of nuclear materials, uranium enrichment, reactor fuel fabrication, reactor spent fuel reprocessing and radioactive waste management. Separations in these areas rely to a very large extent on complexation reactions. Complexation reactions of each element depend largely on the valance states of elements. If the valance of an element is changes its characteristics change a lot.

Complex Reaction Mechanisms

The simple gas-phase dissociation of a molecule takes into account that collisions are necessary to provide the activation energy for the reaction. Perhaps this may be the most famous and important statement about complexation reactions.

Consider the complexation reaction:

H₂ + Br₂ → 2HBr

The rate law for the reaction is:

d (HBr) /    dt         =   k(H₂)(Br₂)^1/2    /      1   +  k’(HBr)  / (Br₂)

where k & k’ are rate constants.

Br₂ →  2Br

Br + H₂  →  HBr + Br

H  + HBr  → H₂ + Br

Br  + Br  → Br₂

Complexation Reactions in Hydrological systems

Complexation reactions that are happening at the mineral –water interface affect the transport and transformation of metals and organic proportions and other things like nutrient availability in soils, formation of ore deposits, ore deposits formation, acidification of water sheds etc...  It is essential to explore the applicability of microscopic models of surface complexation and field scale system to sophisticate surface chemical and hydrological modeling approaches.

Questions:

• Define complexation Reaction?
• Explain the role of complexation reactions in nuclear separations?