1. May 12, 2018

### Clara Chung

Why is change of enthalpy equal to m L during a phase change?
H=U+PV
dH=dU+pdV+Vdp
If temperature and pressure is unchanged during a phase change,
dH=pdV, how does it lead to m L ? Thank you

2. May 13, 2018

### kith

How is L defined?
Correct.
That's not correct. How would you write dU in terms of temperature and pressure?

3. May 13, 2018

### Clara Chung

dH= dU+pdV+Vdp
dH= dQ-pdV+pdV+Vdp
dH=dQ+Vdp
Is the pressure unchange?
L is defined as the energy need to vapourize/fuse 1kg of substance.

4. May 13, 2018

### kith

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)

Last edited: May 13, 2018
5. May 13, 2018

### Clara Chung

Thanks for the explanation
dH=dQ
so if I integrate it, I can get
ΔH=Q which is L for 1kg of substance