Question

how does hot water separate silver (1) chloride from lead (2) chloride?

Answer 1

To understand the logic behind seperation of Silver Chloride (AgCl) from Lead Chloride using hot water we have to take into consideration following factors

• The solubility product, K_sp, is defined as the mathematical product of dissolved ion concentrations raised to the power of their stoichiometric coefficients.
• Solubility products are relevant when a sparingly soluble ionic compound releases ions into solution.
• Solubility products changes with temperature.
• The solubility products (Ksp) of silver chloride and lead chloride are 1.8 x 10^-10 and 1.7 x 10^-5, respectively. (You can find these values in literature.)
• From the values of solubility product we can analyse that neither of the salts (AgCl or PbCl₂₎ are completely soluble in water. Further, solubility of AgCl is less than that of PbCl₂ because of its value. (# note more the negative power raised to base 10 lesser is the solubility Compre 10 ^-10 and 10 ^-5 ).
• So, solubility products changes with temperature when you use cold water the solubilty of both the salts is slight soluble but when the hot water is used lead chloride is almost three times more soluble than that of AgCl.
• This can be explained using Le Chatelier's principle When any system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to (partially) counteract the effect of the applied change and a new equilibrium is established.
• Now Consider reaction of PbCl₂ and AgCl at equilibrium. this is an endothermic reaction (delta H solvation > 0)

PbCl₂ ⇌  Pb ²⁺ + 2Cl^-

AgCl ⇌ Ag⁺ + Cl^-   

So When Hot water is added to the system the temperature of overall system increases and reaction is favoured more towards right that is PbCl2 dissociates more.

• So when hot water is poured that means we are applying heat to the PbCl₂ and AgCl the equilibrium shifts more towards right that is more of Pb ²⁺ and Cl^- are produced in order to maintain equilibrium state. But we already know Ksp values of lead chloride and AgCl so when hot water is added PbCl2 solubility increases that that of AgCl and hence hot water is used to seperate thr silver chloride from lead chloride.