Why is the change of enthalpy during a phase change equal to mL?

[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

 

[kith]

Why is change of enthalpy equal to m L during a phase change?

How is L defined?

H=U+PV
dH=dU+pdV+Vdp

Correct.

If temperature and pressure is unchanged during a phase change,
dH=pdV [...]

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

 

[Clara Chung]

How is L defined?

Correct.

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

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.

 

[kith]

dH=dQ+Vdp

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.

L is defined as the energy need to vapourize/fuse 1kg of substance.

Yes: L = Q/m (as a convention, pressure is assumed to stay constant throughout the phase change)

 

[Clara Chung]

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)

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