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Name: Christina N.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 7/14/2003


Question:
In my Elementary Science Teaching Methods class, we did an experiment involving mixing three different temperatures of water with calcium chloride; the hotter the water before mixing in the calcium chloride, the hotter the mixture following the addition of the calcium chloride. I understand that it is an exothermic reaction, but why? It has been eons since I took chemistry...!


Replies:
First, if I could predict the water solubility of ionic salts, I would be on some tropical island enjoying the royalties such an algorithm would bring in!!! Second, regarding your inquiry: I assume that the experimental procedure involves the same amount of water at three different temperatures, and the rise in the temperature was measured. What is not evident from your description is whether the same amount of calcium chloride was added to each water sample, or was sufficient calcium chloride to saturate the solution was added. If it was the latter, calcium chloride is most likely more soluble in hot water than in cold, so that the more calcium chloride is added to the hot water sample than the cold water sample. If that is not the experimental setup, then the reaction CaCl2 (solid) -----> Ca(+2)aq + 2Cl(-1)aq must have a large negative change in heat capacity. The dH(T hot) = dH(T cold) + dCp(T hot - T cold).

We know already that dH, the heat of solution of calcium chloride is already very negative ( exothermic), if the change in heat capacity: dCp is also negative and large then dH(T hot) will become more negative, i.e. more exothermic as the temperature increases. The numbers are available to test this hypothesis, but usually the effect, though significant, is not large enough to show up in a simple "test tube" apparatus.

Vince Calder



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