## Constant Pressure and how you get it in a calorimeter

Payton Kammerer 2B
Posts: 49
Joined: Tue May 01, 2018 3:00 am

### Constant Pressure and how you get it in a calorimeter

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
Posts: 18879
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 714 times

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

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
Posts: 50
Joined: Wed Nov 08, 2017 3:01 am

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

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
Posts: 103
Joined: Thu Jul 25, 2019 12:17 am

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

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