My textbook says that it is not practically feasible to make insoluble salts (say PbSO4) from other insoluble salts (say PbO), without explicitly mentioning the reason for the same.

My guess is: if the product is also an insoluble salt, it may deposit on the insoluble reactant, and stop further reaction. However, isn't this problem solved by simply stirring? (Is the constant stirring required the reason why such a method would not be "practically feasible"? But isn't stirring rather common for chemical reactions?)

So, what is the actual reason?

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    $\begingroup$ How would the counterions that constitute the product ever encounter each other if not dispersed as intermediates in a solvent? You could have interfacial reactions occur in solids but those would proceed slowly. $\endgroup$ – Buck Thorn Feb 15 '20 at 10:21
  • $\begingroup$ @BuckThorn I had guesses the same initially, but then I wondered : wouldn't this reasoning imply that you cannot make any salt from an insoluble salt (which isn't the case, as you can make lead nitrate from lead oxide by adding dilute HNO3)? $\endgroup$ – Arjun Feb 15 '20 at 11:05
  • $\begingroup$ @BuckThorn Another guess I have is: if the product is also insoluble, then it will form a layer on the surface of the insoluble reactant, ceasing further reaction. Only caveat: this must be easily solvable by just stirring the mixture? (maybe constant stirring is why it's not 'practically feasible'? But, then again, stirring is rather common for chemical reactions, isn't it?) $\endgroup$ – Arjun Feb 15 '20 at 11:09
  • $\begingroup$ I think the key point is that there are kinetic factors at play that might interfere with progress to thermodynamic equilibrium. As you yourself suggest, there are a lot of possible scenarios one can envision, some approximating what occurs in reality. The generalization is that insoluble means here also that you don't form reactive intermediates. Anything that encourages formation of reactive intermediates (here adding a particular acid to some solid) will encourage formation of the thermodynamically favored solid. $\endgroup$ – Buck Thorn Feb 15 '20 at 12:04
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    $\begingroup$ In few words,from thermodynamic point of view, it does not matter they are insoluble. From kinetic point of view, it is the major thing that matters. It would be very slow in the best . $\endgroup$ – Poutnik Feb 15 '20 at 14:24

The comment "My textbook says that it is not practically feasible to make insoluble salts (say PbSO4) from other insoluble salts" is not precisely accurate. Limited solubility in water does not necessarily imply a lack of solubility in an acidic medium.

For example, I have prepared Silver acetate (moderately water-soluble, see Table) from the action of dilute acetic acid and 3% H2O2 in the presence of an electrolyte (a small amount of KNO3). Jumpstart the reaction in a microwave. The created Silver acetate is somewhat soluble in acetic acid (see this), hence there is no issue in the creation reaction.

The addition of moderately soluble PbCl2 to Silver acetate/acetic acid likely creates insoluble AgCl and Lead acetate (where the Silver acetate is also moderately water-soluble with respect to water). An explanation of solubility properties, per reference above, relates to a possible creation of an acetic acid salt.


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