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Today I created $1~\mathrm{M}$ aqueous solution of $\ce{CaCl2}$ and realized that the water before the mixture had a temperature of 23.4 Celsius degrees. Then I added the $\ce{CaCl2}$ and the temperature started growing until it reached 27.5 Celsius degrees. So we have an exothermic reaction. What does that actually mean and how does it work in my example? By the book, an exothermic reaction occurs, when the energy stored in reactants is smaller than the energy stored in products.

Here we have (with my opinion):

$$\ce{CaCl2 + H2O <=> Ca^{2+} + 2Cl^{-} + H2O}$$

From where did that energy come from? Is it just released from the fission of $\ce{CaCl2}$ reactant or from the fusion of some molecules of $\ce{CaCl2}$ again in products? Also, is it possible to have in the products side $\ce{CaCl2}$ molecules or only ions of $\ce{Ca}$ and $\ce{Cl}$?

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Actually, exothermic means that energy is released so that the initial energy is higher than the final one. That's say the contrary of your assumption. This energy is contained in the atoms states themself and in the bondings. So when you break the bonds to form another compound, you change the energy state of your atoms, releasing energy if the final state is more stable (less energetic). Since in your reaction it's the case, the reaction happens then spontaneously.

Since you should keep in mind that nothing is lost, everything is transformed, this extra energy remaining is transformed in heat.

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    $\begingroup$ Actually, exothermic just means that the overall change in the enthalpy is negative. That usually means heat is transferred to the system. When you are talking about the spontaneous reaction, you are talking about an exergonic process, wich can still be endotherm. End exothermic reactions could be endergonic, but this is just a hypothetical case. $\endgroup$ – Martin - マーチン Dec 11 '14 at 11:39

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