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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: RobNk on October 21, 2010, 12:47:55 PM

Title: Help understanding delta U (or H) of reaction
Post by: RobNk on October 21, 2010, 12:47:55 PM
Hello,

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?

thanks,
Rob
Title: Re: Help understanding delta U (or H) of reaction
Post by: Juan R. on October 23, 2010, 01:35:04 PM
:delta:U is defined for a given process.

For a flow process :delta:Uflow gives the change of energy by flows through the boundaries of the system. In your case, your whole system is isolated and the change of energy with the environment is zero.

For a chemical reaction, :delta:Ureact gives the change of energy generated by the reaction going forward. In your case it is the energy of products minus the energy of reactants.
Title: Re: Help understanding delta U (or H) of reaction
Post by: Juan R. on October 24, 2010, 07:10:29 AM
Quote from: Juan R. on October 23, 2010, 01:35:04 PM
:delta:U is defined for a given process.

For a flow process :delta:Uflow gives the change of energy by flows through the boundaries of the system. In your case, your whole system is isolated and the change of energy with the environment is zero.

For a chemical reaction, :delta:Ureact gives the change of energy generated by the reaction going forward. In your case it is the energy of products minus the energy of reactants.

Sorry, I did not know that one may leave one space behind the delta code for correct rendering.


:delta: U is defined for a given process.

For a flow process :delta: Uflow gives the change of energy by flows through the boundaries of the system. In your case, your whole system is isolated and the change of energy with the environment is zero.

For a chemical reaction, :delta: Ureact gives the change of energy generated by the reaction going forward. In your case it is the energy of products minus the energy of reactants.