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Sat 31 Jul, 2004 05:37 pm
Pardon my ignorance on this subject, I've only had one year of high school chemistry (my senior year this coming august will see my second) and I consider language arts to be my forte, despite finding the sciences very interesting.
This was one of the reactions in a lab activity:
Pb(NO[size=7]3[/size])[size=7]2[/size](aq) + 2NaI(aq) --> PbI[size=7]2[/size](s) + 2NaNO[size=7]3[/size](aq)
I mix two aqueous solutions, lead nitrate and sodium iodide and the double replacement reaction achieves a solid lead iodide and an aqueous solution of sodium nitrate.
According to my old textbook's solubility chart, lead is an insoluable element, which would explain why lead iodide forms as a solid, but why was lead nitrate aqueous to begin with?
Is it simply because lead formed a compound with a polyatomic ion (nitrate) that may be very soluable? Can any insoluable element become soluable this way?
My second question concerns precipitating metal salts from water. No matter what compounds are used to precipitate a metal from water as a solid x-carbonate or x-sulfide or what have you, no matter that the solid metal is then filtered by physical means, some other chemical will remain in the water as result of the double replacement reaction, correct?
What compounds are most commonly used for this precipitation? Can what remains be cleaned from water later, or are they chosen based on being harmless for human consumption?
Thanks for any answers and clarifications.
The nitrate ion (No3-) is quite bulky and has only a single charge. This makes it very easy for water to detach ions from solid nitrates. Generally speaking, all nitrates are soluble.
Molecular compounds have quite different characteristics from their components. The (in)solubility of Lead metal has nothing to do with the solubility of an ionic compound containing lead.
Lead nitrate is quite soluable.
In a double replacement reaction, why couldn't both products be insoluable?
You are thinking correctly that most lead compounds have very low soluability in water, but the nitrate and acetate and a few others are exceptions. Nearly all nitrates and nearly all sodium and potassium compounds are soluable in water.
The solid lead iodide can also be made from lead acetate and sodium iodide, or potassium iodide. Neil
In some of these double replacement reactions carbon dioxide, hydrogen sulphide H2S or chlorine is released as a gas or vapor Almost everything is toxic to humans in high concentrations. The chemical industry was quite irresponcable up to a few decades ago, and we likely still have some bad problems. Large scale it is often cost effective to crystalize the left over salt from the remaining solution, even though there are few uses for sodium sulphate, potassium sulphate and a few other left overs. Neil
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