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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Kinetics > Rate Laws > Solution Phase Kinetics > Kinetic Salt Effect

Kinetic Salt Effect

Table of Contents

The kinetic Salt Effect is an effect on the rate of a reaction by salts present in the solution.

Introduction

In biological systems salts play an important role in how well a protein or even DNA is able to function. Salts are formed by ionic bonds, where there is a metal and an electromagnetic atom(s). Some examples of salts include NaCl, KCl, and NA2SO4. Salt molecules are able to disassociate, and form cations and anions. An increase in the charge(- or +) of a transition state or in an activated complex results in an increase in solvation (more order), and causes a decrease in the change of entropy (\(\Delta{S}\)). Whereas, a decrease in the charge of the transition state causes an increase in \( \Delta{S} \).

\(I^- + C^+ \rightleftharpoons E^o \rightarrow Product\)

Kinetic Salt Effect affects the reaction through stabilization. For example in the above reaction, both reactants have a charge. The negatively charged reactant is stabilized by the positive charges from the salt, while the positively charged reactant is stabilized by the negative charges from the salt.  As a result the rate at which the reactants come together decreases, thus decreasing the rate at which E forms. Since a charged intermediate will be stabilized in the solution as well, the half life of the intermediate at equilibrium will increase causing the reaction towards product formation. Since the rate of the product formation becomes higher because of increased amounts of the intermediate present on the solution first order kinetics is used to derive the rate constant equation:

\(logK_{TS} = logK_{TS^o} + 2Z_AZ_BI^{1/2}\) 

Where,

  • \(Z\) is the charge on the cation and ion from the salt. \(Z_AZ_B\) is a product value.
  • \(I\) is the ionic strength. \(I\) is also dependent on the solubility of the salt in the reaction mixture. Ionic strength is directly proportional to the solubility of the salt. Changing the ionic strength manipulates the solvation of the reactants and intermediates, thus changing the \(\Delta{S}\), and affecting the reaction rate.
  • \(logK_{TS^o}\) is the rate constant without the salt in the reaction mixture.
  • \(A\) is also a constant for the solvent the solution is in. The \(A\) value for water is 0.509 at 298 K.

A hard concept to understand is the relationship between \(Z_AZ_B\), \(I\), and the rate of the reaction is presented in tabular form to avoid confusion.

\(logK_{TS} = logK_{TS^o} + 2Z_AZ_BI^{1/2}\) 

ZAZB Rate of Reaction
+ Increases (salt present in the reaction mixture)
- Decreases (the ionic strength increases

                Note: I=0 at very dilute salt concentrations or if salt is inert. Reaction rate in this case is independent of salt

References

  1. Atkins, Peter and Julio de Paula. Physical Chemistry for the Life Sciences. New York: W.H. Freeman and Company, 2006.
  2. Chang, Raymond. Physical Chemistry for the Biosciences. USA: University Science Books, 2005.

Contributors

  • Artika Singh (UCD)

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Last Modified
09:38, 20 Dec 2013

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