Enthalpy and Internal Energy

[Karina Kong 2H]

Why is enthalpy in terms of constant pressure, but internal energy is in terms of constant volume?

[SnehinRajkumar1L]

Since enthalpy involves reactions, volumes are bound to change.

[005206171]

Enthalpy doesn't have to be at constant pressure. It's just that at constant pressure, enthalpy will just be the heat released or absorbed by the reaction. All of the work will be done will be to change the volume of the system. We just use constant pressure to relate heat to enthalpy. Most reactions occur at constant pressure, so we use enthalpy to measure heat given off or absorbed during a reaction.

At constant volume, measuring heat will give you the change in internal energy. Usually reactions are done in open beaker, allowing gases to escape into the air. This means the volume has changed. This is where measuring enthalpy will be helpful.