Schemp

drawing infinity
10+ Year Member
Mar 27, 2008
422
5
Seattle
Status
Pre-Medical
These are from Lecture 5, in the actual lecture, not the end of lecture 30 minute quizzes.

To keep it brief, #97 says that NaCl is poured into a coffee cup calorimeter and the temperature in the calorimeter drops by 1 degree Celsius. It then asks what the heat of solution is after giving you some necessary data.

I figured out the amount of heat that was transferred, but the sign was important too. The answer in the back of the book says "Since the temperature went down, the reaction is endothermic with positive enthalpy." Is this backwards, or do calorimeters have a different perspective regarding system and surroundings? I took the question to mean that the solution's temperature went down, and since the solution is the system (I thought), the reaction must be exothermic.

#116 is a similar question except that in the answer it says "the reaction is exothermic because the temperature increased."


Now, am I just misunderstanding this or is the book getting it backwards?
 

capn jazz

10+ Year Member
7+ Year Member
Aug 29, 2006
1,529
9
Status
Medical Student
If a reaction proceeds and the temperature goes down, the reaction is taking in energy and it's endothermic with a positive enthalpy.
 
OP
Schemp

Schemp

drawing infinity
10+ Year Member
Mar 27, 2008
422
5
Seattle
Status
Pre-Medical
Yeah, I actually found the answer to this in my chemistry textbook. I guess the solvent (usually water) is the surroundings, or part of it anyway, so when the temperature goes down that means the reaction itself is taking in heat.

Makes sense now, although it's kind of different from all the other exo/endothermic stuff. Usually if the solution decreases in temperature, it's exothermic. The setup is just a bit more confusing for calorimeters. Thanks for the help.

Oh, and I just realized a while ago that I posted this in the wrong place. Sorry! If someone wants to delete this that's fine, or moving it is okay too. My question is answered though.