precipitation problems

[unsung]

Say you have 2 cmpds Pb(NO3)2 and Na2SO4 that are combined and they're supposed to react with each other. How do we know that they just react by having the cation-anion pairs switch places ?


I.e. how do we know the reaction is Pb(NO3)2 + Na2SO4 --> PbSO4 + NaNO3 ?

This is something I've kind of wondered about for a long time. It seems like all the problems like this assume the cation-anion pairs just switch places. But why would it do this?

Or is this something we should just memorize?

Thanks!

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[RPedigo]

Because opposites attract-- two anions will not form a compound, and two cations will not form a compound. Of course, the lead cation and two nitrate anions can recombine, but it would be a soluble compound (nitrates are always soluble) so it would just break up again. In solution everything is just kinda moving around, and if a cation and anion meet they have a chance to make a bond. When the lead meets a sulfate, it makes an insoluble compound and precipitates out of the solution. Hope this helps