I'm trying to understand the concepts of the reaction :delta: U and :delta: H, along with the first law of thermodynamics ( :delta: U = q + w), but I'm getting confused with the following thought experiment. I'm looking at U, since I find it easier to imagine, even though H is used more often. Imagine a reaction:
A + B --> C + D
has :delta: U = -500 kJ. (exothermic) Say they are all gases, and I put them in a constant volume, insulated container, so the process is adiabatic, and no work is done. (The reaction could be started with a tiny trigger, like a spark).
For the container as a system:
q = 0 [adiabatic, insulated container]
w = 0 [constant volume]
:delta: U = w + q = 0 + 0 = 0
Obviously the reaction is exothermic, and the temperature of this container would rise, but what exactly is the "system" for which the :delta: U = -500 kJ. Where is my thinking going astray?