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Why is the heat measured in a bomb calorimeter not equal to the enthalpy change of the reaction?
AThe sealed vessel prevents any expansion work
BThe sealed vessel absorbs none of the heat given out
CThe sealed vessel keeps the pressure exactly constant
DThe sealed vessel changes the reaction stoichiometry
Answer & Solution
Correct answer: A. The sealed vessel prevents any expansion work
1. Enthalpy change equals the heat flow only when the process runs at constant pressure.
2. That result comes from the expansion work term cancelling out of the derivation.
3. A bomb calorimeter is a closed metal container of fixed volume.
4. Fixed volume means no expansion work can happen, so the cancellation no longer applies.
5. The heat measured there is therefore not the enthalpy change.
6. A Bunsen burner flame burns at the constant pressure of the atmosphere, so there the heat does equal the enthalpy change.
7. The bomb absorbs a great deal of heat, so the second option states the opposite of what happens.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_
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