Understanding Enthalpy and Phase Changes: Exploring H=U+PV and dH=dU+pdV+Vdp

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In summary, the change in enthalpy during a phase change is equal to the latent heat, which is defined as the energy needed to vaporize or fuse 1kg of substance. This is because, for a constant temperature and pressure, the change in enthalpy is equal to the change in heat, and for a constant pressure, the change in heat is equal to the latent heat.
  • #1
Clara Chung
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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
 
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  • #2
Clara Chung said:
Why is change of enthalpy equal to m L during a phase change?
How is L defined?
Clara Chung said:
H=U+PV
dH=dU+pdV+Vdp
Correct.
Clara Chung said:
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?
 
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  • #3
kith said:
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.
 
  • #4
Clara Chung said:
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.
Clara Chung said:
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)
 
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  • #5
kith 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)
Thanks for the explanation
dH=dQ
so if I integrate it, I can get
ΔH=Q which is L for 1kg of substance
 

What is enthalpy?

Enthalpy is a thermodynamic quantity that represents the total energy of a system, including both its internal energy and the energy required to exert pressure on its surroundings.

How is enthalpy calculated?

Enthalpy can be calculated using the formula H = U + PV, where H is enthalpy, U is internal energy, P is pressure, and V is volume.

What is the significance of enthalpy in chemical reactions?

Enthalpy is important in chemical reactions because it indicates the amount of heat energy released or absorbed during the reaction. This can help determine whether a reaction is exothermic (releasing heat) or endothermic (absorbing heat).

How is enthalpy used in industry?

Enthalpy is used in industry for processes such as combustion, refrigeration, and power generation. It can also be used in the design of chemical reactions and processes.

How does enthalpy change during a phase transition?

During a phase transition, such as melting or boiling, the enthalpy remains constant. This is because the energy is used to break or form intermolecular bonds, rather than changing the temperature of the substance.

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