lecsdg rtaert e et tryry be tyt y 5etr 5 (1).pptx

Presentation Topic

1. Kinetic Salt Effect (Ionic Strength Effect) The kinetic salt effect refers to the change in reaction rate caused by the addition of an inert salt , which alters the ionic strength of the solution. It mainly affects reactions involving charged species (ions) by changing the electrostatic interactions between them.

2. Importance Helps in determining reaction mechanisms (especially ionic ones). Indicates whether a reaction involves ions of same or opposite charges in the rate-determining step. Adding salt can either increase or decrease the rate of reaction depending on the nature of the reacting ions.

3. How It Works When an inert salt (e.g., NaCl, KNO₃) is added: It increases ionic strength (I) of the solution. The added ions shield the charges on reacting species. This reduces repulsion between like charges or reduces attraction between opposite charges. As a result, the rate of reaction changes .

4. Types of kinetic salt effects Primary Kinetic Salt Effect Positive salt effect Negative salt effect Secondary Kinetic Salt Effect

5. Primary Kinetic Salt Effect This is the direct effect of added salt on the rate-determining step of a reaction.

6. Positive Salt Effect Occurs when reacting ions have the same charge (e.g., two cations or two anions) or when the reaction forms ions . Adding salt reduces repulsion between like-charged ions. The ions can come closer more easily , so the reaction rate increases .

7. Examples Base hydrolysis of ester: CH₃COOC₂H₅  +  OH⁻  →  CH₃COO⁻  +  C₂H₅OH (Both reactants are negatively charged → rate increases with salt) Hydrolysis of t- BuCl (SN1 reaction): t- BuCl  +  H₂O  →  t- BuOH  +  H⁺  +   Cl ⁻ Reaction forms ions → adding salt stabilizes the developing charges in the transition state and lowers the energy barrier , increasing the rate.

8. Example I⁻  +   Cl ₂  →  ICl   +   Cl ⁻ Both reactants carry negative charge (I⁻ and Cl ⁻ in transition state) — adding salt reduces repulsion → rate increases .

9. Negative Salt Effect Occurs when reacting ions have opposite charges (e.g., a cation and an anion). Adding salt reduces (shields) the natural attraction between the oppositely charged ions. This makes it harder for them to come together → rate decreases .

10. Example RCOO⁻  +  R’NH₃⁺  →  RCOOH  +  R’NH₂ Adding salt (e.g., NaCl) reduces the attraction → reaction slows down .

11. Conclusion The kinetic salt effect shows how changing ionic strength influences reaction rate . Positive salt effect: rate ↑ (like-charged ions or ion formation). Negative salt effect: rate ↓ (opposite-charged ions). By studying salt effects, chemists can predict or confirm the mechanism of ionic reactions.