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Double displacement reactions?
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[QUOTE="Merlin3189, post: 6064809, member: 542077"] You're absolutely right. Except that this reaction does not usually happen! What happens is that both salts are dissolved in water, then the solutions are mixed. So Pb(NO[SUB]3[/SUB])[SUB]2[/SUB] + Aq = Pb[SUP]2+[/SUP][SUB](aq)[/SUB] + 2NO[SUB]3[/SUB][SUP]2-[/SUP] [SUB](aq)[/SUB] And MNI[SUB]4[/SUB] +Aq = Mn[SUP]4+[/SUP][SUB](aq)[/SUB] + 4 I[SUP]-[/SUP][SUB](aq)[/SUB] Now in solution you have a mixture of 4 type of ion (and H[SUP]+[/SUP], OH[SUP]-[/SUP] and maybe others in some cases.) You do not have either the initial salts or the final salts. In many cases these will happily stay like that, but here there is a salt, lead iodide, which has a very low solubility product, so that the lead and iodide ions cannot remain in solution together in large concentrations. Lead Iodide precipitates, leaving mainly manganese 4+ ions and iodide ions in solution with a very small concentration of lead 2+ and nitrate ions. So again you are absolutely right to wonder how you know whether the reaction occurs. Here, as I see Borek has now commented, it happens because one of the compounds is much less soluble than the others. That is a common reason for such a reaction to happen. If all of the possible salts are soluble, you can cause the reaction to happen to some extent if one salt has lower solubilty, by evaporating the solution to increase the concentration of ions until the lowest solubility salt starts to crystalise out. But you don't get 100% conversion (which you did not quite get with lead iodide either.) If one of the possible products is volatile (or unstable with volatile products) this can also priomote a reaction. So for example mixing ammonium sulphate solution and sodium carbonate solution, both fairly stable salts, you get a mixture of sodium, ammonium, sulphate and carbonate ions. The sodium and sulphate ions are goinig nowhere, but carbonate and ammonium ions have other reactions with water and the production od CO[SUB]2[/SUB] and NH[SUB]3[/SUB], both of which can diffuse into the air and be lost from solution. So if you evaporate this solution, you end up with sodium sulphate. IMO a similar situation exists with the single displacement reactions. For example when zinc metal displaces copper from copper sulphate solution, the sulphate is largely irrelevant. The actual reaction is, again as Borek pointed out, a redox reaction Zn + Cu[SUP]2+[/SUP] ⇒ Zn[SUP]2-[/SUP] + Cu. The zinc is replacing copper, not from copper sulphate, but zinc metal reacting with copper ions in aqueous solution. So, again IMO, these descriptions as displacement reactions are empirical views of what happens in some cases. They do not speak in any way about the reaction mechanism, which you need to seek through other ideas. [/QUOTE]
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Double displacement reactions?
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