Solubility of solids with - heat of solution
Name: danna c griffiths
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?
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...
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.
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Update: June 2012