[lonehavoc]

Hello everyone, I have been very confused by this question. "A reaction in aqueous solution is carried out in a calorimeter and the temperature of the solution decreases. What is the sign of ΔH for the reaction."

My approach was assuming that the "system" was the solution and that the only way for the solution to decrease in temperature was by the heat being released to the surroundings (exothermic, ΔH=negative). I soon found out that the reaction was actually the "system" causing ΔH=positive (endothermic). How can the reaction absorb heat from the solution if the reaction is within the solution?

Hopefully, anyone who reads this can understand my thought process/confusion.

[Corribus]

If the solvent decreases in temperature during a reaction, it's because heat is lost from the solvent when reaction products are formed from the reactants. I.e., heat energy from the solvent (surroundings) is used to form the reaction products.

The solution in this case is your surroundings. The "system" is your reaction. The system is gaining energy because the products have higher energy than the reactants. Because energy is conserved, if the reaction system gains energy, the surroundings lose energy, which means the solvent decreases in temperature. Conversely, in an exothermic reaction, the products are lower energy than the reactants, so the reaction system loses energy, the surroundings gain energy, which means the surroundings (solvent) rise in temperature.

[lonehavoc]

Thank you for the response! Would it be true that in all cases the reaction is the system? I guess I assumed that the solvent itself was involved in the reaction. How can I be able to tell what is involved/not involved in the reaction itself?

[Corribus]

You have to think about what's going on. The calorimeter measures heat loss/gain during a chemical or physical process by monitoring temperature changes in the system (using the heat capacity to translate the temperature change into heat exchange). The system is the solvent plus the elements undergoing reaction - basically everything inside the (approximately perfectly) insulated calorimeter. But if the reaction components are dilute enough the system's heat exchange properties can be approximated as just the solvent plus some background correction due to the calorimeter itself. Note that the solvent DOES participate in the heat exchange with reaction components to some degree, because solubilization of the reaction components is also a thermodynamic process. This is why designations of, say, "aqueous" when writing chemical reactions is important, because these solubilization contributions are incorporated within reported thermodynamic values.