Constant Pressure and how you get it in a calorimeter

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Payton Kammerer 2B
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Constant Pressure and how you get it in a calorimeter

Postby Payton Kammerer 2B » Sat Jan 25, 2020 11:55 pm

In the lecture where this was discussed, Dr. Lavelle said that delta h was equal to q at a constant pressure. When he was talking about this with an open vessel, I understood how that was possible, but aren't calorimeters sealed? If their volume can't change, then how do they maintain a constant pressure? Can they only be used when there are no gases involved?

Chem_Mod
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Re: Constant Pressure and how you get it in a calorimeter

Postby Chem_Mod » Sun Jan 26, 2020 12:29 am

Not all calorimeters are completely sealed. For instance, one of the best known examples of a constant-pressure calorimeter is a coffee cup calorimeter, which is simply two coffee cups stacked on top of each other and a lid on top with two holes for a stirrer and a thermometer. However, bomb calorimeters are constant-volume and are sealed, so the pressure inside the bomb calorimeter does change.

MariaJohn1D
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Re: Constant Pressure and how you get it in a calorimeter

Postby MariaJohn1D » Mon Jan 27, 2020 3:06 pm

I am still confused about the differences in pressure between the bomb calorimeter and coffee cup calorimeter. Why does the pressure change in a bomb calorimeter if its sealed?

Ruby Richter 2L
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Re: Constant Pressure and how you get it in a calorimeter

Postby Ruby Richter 2L » Mon Jan 27, 2020 3:11 pm

In a constant-volume calorimeter the system will be sealed and therefore isolated from its surroundings which is why the volume stays the same and there is no change in pressure. In a "bomb" calorimeter the process takes place at constant volume, so the reaction will have varying pressure due to combustion processes which can result in a confined explosion, hence the name "bomb" calorimeter


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