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Name: danna c griffiths
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999 


Question:
Some solids have a negative heat of solution, yet, their solubility increases when the temperature is increased. This is contradictory to Le Chaltelier's Principle. Sodioum Hydroxide is an example. Could you explain why?



Replies:
This is an incredibly good question!

All right...see if you can follow this: When the enthalpy is positive, solid will absorb heat when it goes into solution. At high temps, more heat is available so more solid will dissolve. A negative enthalpy means the solid gives off heat when it dissolves and subsequently absorbs heat when it precipitates. At high temps then, precipitation is favored. Here's the catch: The heat of solution (enthalpy) AT THE SATURATION POINT determines the temperature effect. _Chemistry_ by Radel & Navidi says: "When NaOH dissolves in nearly pure water, the solution process is strongly exothermic. When it dissolves in an almost saturated solution, however, the process is endothermic. The solubility of NaOH increases with temperature even though the overall process is exothermic." So the overall enthalpy does not determine the how the temperature will affect solubility. Think about that for awhile...

-Joe Schultz


In thermodynamics, the dependence of the equilibrium constant on temperature is given by the Gibbs-Helmholtz equation... it says that ln K_eq will change in temperature in proportion to Delta H for the reaction (which is, itself, a function of temperature) and inversely in proportion to T^2. This equation, when applied to chemistry, gives rise to LeChatelier's principle.

The reason I point this oout is that Joe's explanation of the phenomenon shows why LeChatelier's principle is obeyed by this reaction, at least in its most general from (from thermodynamics). Le Chatelier's principle is never wrong for chemical equilibria.

-topper



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