Energy balance -- Physical interpretation of Q

[MexChemE]

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!

 

[DrClaude]

exothermic reaction

That should tell you all you need to know.

 

[Chestermiller]

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

 

[MexChemE]

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?

 

[Chestermiller]

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