Question
Question: Which of the following statements are true about AgI sol prepared by peptization of Agl with AgNO₃....
Which of the following statements are true about AgI sol prepared by peptization of Agl with AgNO₃.
In electrophoresis, the dispered phase will move towards cathode.
AlCl₃ is more effective than Na₃PO₄ for bringing about coagulation of sol.
Na₃PO₄ is more effective than AlCl₃ for bringing about coagulation of sol.
On persistant dialysis of sol, precipitation of coloidal sol take place (coagulation)
(A), (C), (D)
Solution
The AgI sol is prepared by peptization of AgI with AgNO₃. In this process, AgI precipitate is dispersed in a medium containing AgNO₃. The AgI particles preferentially adsorb common ions from the solution. Since AgNO₃ provides Ag⁺ ions, which are common with the AgI lattice, Ag⁺ ions are adsorbed onto the surface of the AgI particles. This gives the AgI colloidal particles a positive charge.
(A) In electrophoresis, the dispersed phase will move towards cathode. Since the AgI sol is positively charged, the colloidal particles will move towards the negatively charged electrode, which is the cathode, under the influence of an electric field. This statement is true.
(B) AlCl₃ is more effective than Na₃PO₄ for bringing about coagulation of sol. (C) Na₃PO₄ is more effective than AlCl₃ for bringing about coagulation of sol. Coagulation of a positively charged sol is brought about by negatively charged ions (anions) from the added electrolyte. According to the Hardy-Schulze rule, the coagulating power of an electrolyte increases with the valency of the coagulating ion.
In AlCl₃, the ions are Al³⁺ and Cl⁻. The coagulating ion is Cl⁻, with a valency of 1. In Na₃PO₄, the ions are Na⁺ and PO₄³⁻. The coagulating ion is PO₄³⁻, with a valency of 3.
Since the valency of PO₄³⁻ (3) is greater than the valency of Cl⁻ (1), PO₄³⁻ is a more effective coagulating agent than Cl⁻. Therefore, Na₃PO₄ is more effective than AlCl₃ in coagulating the positively charged AgI sol. Statement (B) is false, and statement (C) is true.
(D) On persistent dialysis of sol, precipitation of coloidal sol take place (coagulation). Colloidal sols are stabilized by the presence of electrolytes which form the electrical double layer. This double layer provides electrostatic repulsion between the colloidal particles, preventing them from aggregating. Dialysis is a process used to remove excess electrolytes from the sol. If dialysis is carried out for a prolonged period (persistent dialysis), most of the electrolytes are removed. The removal of electrolytes reduces the stability of the sol by decreasing the charge on the particles or the thickness of the diffuse layer, leading to reduced electrostatic repulsion. This allows the van der Waals attractive forces to dominate, causing the particles to aggregate and precipitate, i.e., coagulation occurs. This statement is true.
Based on the analysis, statements (A), (C), and (D) are true.