Energy balance -- Physical interpretation of Q

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Discussion Overview

The discussion revolves around the energy balance in an isothermal reactor undergoing an exothermic reaction at 450 °C. Participants explore the implications of the energy balance equation Q = ΔH, specifically addressing whether heat must be added to maintain the temperature or if the system can simply release heat to achieve equilibrium.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether maintaining the reactor at 450 °C requires allowing the reactor to release 150 kcal of heat or adding 150 kcal while the reaction occurs.
  • Another participant asserts that the reactor must be in contact with a reservoir at 450 °C to maintain temperature, indicating that running the reactor adiabatically would lead to a temperature rise.
  • A follow-up inquiry challenges the assumption about the reservoir's temperature, asking if it would increase due to the heat released by the reaction and whether it behaves like a heat sink that does not significantly change temperature.
  • One participant clarifies that the reservoir in question is indeed a heat sink, implying it can absorb heat without a significant temperature increase.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the heat exchange required to maintain the reactor's temperature, indicating that multiple competing perspectives exist regarding the role of the reservoir and the energy balance in the system.

Contextual Notes

The discussion does not resolve the assumptions about the behavior of the reservoir or the specifics of the energy balance in this context, leaving these aspects open for further exploration.

MexChemE
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Hello PF! I have a simple question on energy balances. Suppose you have an isothermal reactor where you want to have an exothermic reaction happening at 450 °C. The simplified energy balance for the reactor is Q = ΔH. Supposing ΔH = -150 kcal, then Q = -150 kcal. Does this mean that, a) in order for the system to stay at 450 °C, we just have to "let" the reactor release the 150 kcal of heat, or b) in order for the system to stay at 450 °C, we have to add 150 kcal of heat while the reaction releases the same amount of heat?

Thanks in advance for any input!
 
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MexChemE said:
exothermic reaction
That should tell you all you need to know.
 
You don't "let" the reactor release that amount of heat. You need to contact the reactor with a reservoir at 450 C in order for the final temperature to equilibrate at 450 C. If you run the reactor adiabatically, the temperature will rise.

Chet
 
Chestermiller said:
You don't "let" the reactor release that amount of heat. You need to contact the reactor with a reservoir at 450 C in order for the final temperature to equilibrate at 450 C. If you run the reactor adiabatically, the temperature will rise.

Chet

But wouldn't the temperature of the reservoir increase because of the heat released by the reaction, therefore displacing the equilibrium temperature? Or, are we talking about the same kind of reservoir we talk about when discussing heat engines, i.e. a heat sink? Meaning its temperature will not rise significantly, no matter how much heat it absorbs. Is that how cooling jackets in reactors/tanks behave?
 
MexChemE said:
But wouldn't the temperature of the reservoir increase because of the heat released by the reaction, therefore displacing the equilibrium temperature? Or, are we talking about the same kind of reservoir we talk about when discussing heat engines, i.e. a heat sink? Meaning its temperature will not rise significantly, no matter how much heat it absorbs. Is that how cooling jackets in reactors/tanks behave?
The latter. A heat sink.

Chet
 
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