delta U within calorimeter

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Yasmine Razi 3L
Posts: 4
Joined: Tue Nov 25, 2014 3:00 am

delta U within calorimeter

Postby Yasmine Razi 3L » Wed Jan 28, 2015 6:13 pm

Just for clarification, in an bomb calorimeter (isolated system), if delta U (system)= q(inner system), then does that mean delta U(system)= -q (surrounding calorimeter) because within a calorimeter q(sys)=-q(surr)?

Megan White 2C
Posts: 18
Joined: Fri Sep 26, 2014 2:02 pm

Re: delta U within calorimeter

Postby Megan White 2C » Wed Jan 28, 2015 6:52 pm

Hi Yasmine!

To start, if you are dealing with an isolated system, delta U is always going to equal 0. Essentially, no heat is coming in or out of the system, nor is the volume/pressure of the system changing, so there would be no change in the internal heat (U) of an isolated system. This is the first law of thermodynamics.

Now, if a calorimeter is NOT isolated (i.e., open or closed), and there is a transfer of heat into or out of the system, then this will effect the surroundings. If heat is transferred into the system, then an equal and opposite amount of heat will be taken from the surroundings (and vice versa if heat is transferred out of a system).

^^This does not directly related to internal energy, however. Remember that internal energy is the sum of heat and work (q + w). So, all that said, you are correct in saying that qsys = -qsurr for a NON ISOLATED system, but that does not mean deta U (system) = -q (surroundings). You would need a work value for the system to make such claim.

Anyone reading, please add on to this if I missed something!

Hope this helps!

Justin Le 2I
Posts: 142
Joined: Fri Sep 26, 2014 2:02 pm

Re: delta U within calorimeter

Postby Justin Le 2I » Wed Jan 28, 2015 9:42 pm

Bomb calorimeters are isolated from the rest of the universe, which makes it easier to measure heat that is released in a reaction. Since volume is staying constant in a bomb calorimeter, there is no work being done because expansion or compression cannot take place. So w=0. But heat can be transferred to or from the calorimeter and the water that is surrounding the calorimeter. So delta U=q.

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