SALT EFFECT - PRIMARY SALT AND SECONDARY SALT EFFECT

For the reaction in solution, if the rate is getting affected on changing the ionic strength, it's called salt effect.

PRIMARY SALT EFFECT :-

                             For a chemical reaction, if the activities of the reactant and the rate constants are affected due to added electrolyte, it's called primary salt effect.

SECONDARY SALT EFFECT :-

                             If the reactivities of reactant species alone getting affected due to added electrolyte, it's is called secondary salt effect.

APPLICATIONS :-

1) Let us consider

[(NH₃) Co Br ]²⁺ + Hg²⁺ → [(NH₃)₅ Co H₂O]³⁺

         + Hg Br

ZA = - +2 ; ZB = 2

ZA - ZB = 4

Therefore, 

log kz = log k₀ + 1.02 (4) √μ

⇒kz = k₀

∴ Added salt will increase the rate of constant.

(2)  Let us consider,

S₂ O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂

ZA = - 2 ; ZB = - 1

ZA. ZB = +2

log kz = log k₀ + (1.02) (+2) √μ

⇒ kz > k₀ added salt will increase rate constant.

(3) Let us consider,

[(NH₃)₅ Co Br] ²⁺ + OH⁻ →[(NH₃)₅ Co  OH]²⁺ + Br⁻

ZA = +2 ; ZB = - 1

ZA.ZB = - 2

log kz = log k₀ + 1.02 (−2) √μ

⇒ kz < k₀

∴ added salt will decrease the rate constant.

(4) Let us consider,

[(NH₃)₅ Co Br ]²⁺ + H₂O →(dipole) [(NH₃)₅ Co H₂O] ²⁺ + Br ⁻

ZA = +2 ; ZB = 0

ZA.ZB = 0

∴ loɡ kz = loɡ k₀ + (1.02) (0) √ μ

⇒kz = k₀

∴ The added salt will not affect the rate constant.