The discussion revolves around the relationship between change of enthalpy and latent heat (mL) during a phase change. Participants explore the underlying thermodynamic principles, definitions, and mathematical expressions relevant to this concept, focusing on the conditions of constant temperature and pressure during the phase change.
Participants express differing views on the derivation and interpretation of the equations involved, particularly regarding the treatment of dU and the implications of constant pressure and temperature. The discussion remains unresolved with multiple competing perspectives on the definitions and relationships involved.
Limitations include the dependence on the assumption of constant temperature and pressure during the phase change, as well as the need for clarity on the definitions and relationships among the variables involved.
How is L defined?Clara Chung said:Why is change of enthalpy equal to m L during a phase change?
Correct.Clara Chung said:H=U+PV
dH=dU+pdV+Vdp
That's not correct. How would you write dU in terms of temperature and pressure?Clara Chung said:If temperature and pressure is unchanged during a phase change,
dH=pdV [...]
dH= dU+pdV+Vdpkith said:How is L defined?
Correct.
That's not correct. How would you write dU in terms of temperature and pressure?
Yes, that's better. (dQ could also be written as dQ=TdS where dS is the change in entropy)Clara Chung said:dH=dQ+Vdp
Yes: L = Q/m (as a convention, pressure is assumed to stay constant throughout the phase change)Clara Chung said:L is defined as the energy need to vapourize/fuse 1kg of substance.
Thanks for the explanationkith said:Yes, that's better. (dQ could also be written as dQ=TdS where dS is the change in entropy)
In your first post, you stated that temperature and pressure should remain unchanged. This means that the change in pressure, dp, is equal to zero.
Therefore, dH = dQ.
Yes: L = Q/m (as a convention, pressure is assumed to stay constant throughout the phase change)