Calorimeter differences

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Calorimeter differences

What is the difference between a constant volume and constant pressure calorimeter and what is the significance of each?

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Re: Calorimeter differences

In a constant pressure calorimeter, the pressure is kept constant, but the volume of solution is allowed to change, so work can be done in addition to heat exchange. When you measure the enthalpy of reaction, you get changeH=qp. But ChangeU=changeH-pchangeV.

In a constant volume calorimeter, no work of expansion can be done. The only energy leaving the "bomb" is heat energy. In this case, we can fully understand the internal energy of the system simply by measuring the enthalpy of reaction. ChangeU=qv. The reason for this is changeU=q+w, and no expansion work can be done so w=0.