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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?
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.
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?
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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