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Name: tom hammargren
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999 


Question:
Most salts are _more_ soluble in water at higher temperatures. Some salts, like cerium(III) sulfate are actually less soluble at 40C than at 5C. Why does this sulfate behave differently than most salts?



Replies:
This depends on whether heat is liberated or absorbed upon addition of a pinch of the salt. The solution of common table salt is endothermic (energy is absorbed into the solution from the environment when this process happens). All endothermic processes are driven towards making more products when you add heat; thus, when you heat the system, the process tends more towards products and the salt becomes easier to dissolve. Exothermic processes, conversely, are driven towards reactants when heat is added.

The above is usually called "Le Chatelier's principle," but in reality it is a consequence of thermodynamics; it is a qualitative description of the van't Hoff equation:

d (ln K) /d (1/T) = -(delta H) /R

which shows that a plot of ln K (K=equilibrium constant) vs 1/T (temperature) has an instataneous slope of (-deltaH /R) (R=gas constant, deltaH = exo-or endo-thermicity of the process).

I could go into more detail but it would require a more detailed explanation of entropy than I have time to do here... fundamentally, if you want a molecular-level model to explain why some salts dissolve endothermically and others do so exothermically, there are some models that work well for very very dilute solutions but no good model exists for concentrated solutions...sorry. Hope this helps

- topper



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