PPT on Reaction Kinetics in Solution | Kinetic Salt Effects

Reaction Kinetics in Solution:

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Comparison of gas and liquid phase reactions{T=298 K & p= 1 atm}

• Molecules in the gas phase occupy ≈0.2 % of the total volume, while molecules in the liquid phase occupy ≈ 50% of the total volume.
• Molecules in the liquid are in permanent contact with each other, and undergo permanent collisions with each other.

Observation on liquid phase reactions:

Except if,

• The reaction occurs with the solvent,
• Strong interactions of the reactant molecule with the solvent (e.g. ionic reactions;  →  solvent shells),
• The reactions are diffusion controlled.

Qualitative model of liquid reactions

However, we can understand the required basis reaction steps as sketched in Fig. which are given below and a no. of limiting cases resulting from these steps.

Structure and dynamics of solution:

Solvent cages and encounter pairs

Formal kinetics

Above result is rate constant for liquid phase reaction.

Two Important Limiting Cases For Reaction in Solution:

Examples of diffusion-controlled reactions:

• The electron-transfer,

• Atom-recombination reactions in organic solvent,

• The proton-transfer.                                 

The kinetic study of reactions in solution phase is very complicated & is governed by many factors:

• Movement of ions in solution,
• Extent of salvation,
• Nature of reacting species.

       These effects are generally known as Salt Effects and these are of two types-

   (a). Primary salt effects.

   (b). Secondary salt effects.

Reactions of ions in solution:

The Reactions of ionic species in liquids are one exception to the rule that reactions in the liquid phase usually have similar rates as in the gas phase (the other exception being electron transfer reactions). The reason is that the charged species are very sensitive to their environment, in particular solvation. Changes of the charge distribution are accompanied by correspondingly large solvation shell rearrangements.

Effect of the ionic strength of the solution:

Equilibrium constant for [AB]:

Activity coefficient from Debye-Hückel theory:

Ionic Strength:

Dependence of the rate constant of the ionic strength:

Conclusion:

Kinetic salt effect for reactions of ions in solution:

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