Gibbs free energy and melting


by AbsoluteZer0
Tags: energy, free, gibbs, melting
AbsoluteZer0
AbsoluteZer0 is offline
#1
Feb5-13, 01:16 PM
P: 126
Hi,

As I understand, during the process of phase change from a liquid to solid (or any phase change for that matter,) the temperature of the substance remains constant as the energy being applied to the substance is used in changing phase.

How does this relate to Gibbs free energy? I read that [itex]\Delta G[/itex] during melting is zero. Enthalpy and entropy, however, increase. Does this have any relation to the uniformity of the temperature of the substance during the change of phase?

I'm led to believe that [itex] \Delta G > 0[/itex] when phase change isn't taking place because of the changing temperature. (For example, when the temperature of water is raised from 30C to 50C.) How accurate is this assumption?

Thanks,
Phys.Org News Partner Physics news on Phys.org
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Mapping the road to quantum gravity
Chameleon crystals could enable active camouflage (w/ video)
sankalpmittal
sankalpmittal is offline
#2
Feb8-13, 12:42 PM
sankalpmittal's Avatar
P: 700
Quote Quote by AbsoluteZer0 View Post
Hi,

As I understand, during the process of phase change from a liquid to solid (or any phase change for that matter,) the temperature of the substance remains constant as the energy being applied to the substance is used in changing phase.

How does this relate to Gibbs free energy? I read that [itex]\Delta G[/itex] during melting is zero. Enthalpy and entropy, however, increase. Does this have any relation to the uniformity of the temperature of the substance during the change of phase?

I'm led to believe that [itex] \Delta G > 0[/itex] when phase change isn't taking place because of the changing temperature. (For example, when the temperature of water is raised from 30C to 50C.) How accurate is this assumption?

Thanks,
Gibbs free energy is not constant during phase change! Who told you this ?

Clearly ΔG=ΔH-TΔS

Entropy factor also changes though...
DrDu
DrDu is offline
#3
Feb8-13, 12:53 PM
Sci Advisor
P: 3,378
Quote Quote by sankalpmittal View Post
Gibbs free energy is not constant during phase change! Who told you this ?
During a reversible phase change, e.g. slow melting of ice, G does not change.
##dG=VdP-SdT+\mu_l dN_l +\mu_s dN_S.##
P and T are constant and ##dN_l=-dN_s##. ##\mu_l=\mu_s## is the condition for equilibrium of the two phases, so dG=0.
ΔH is positive for melting (endothemal process), ΔS, too, as the entropy of the liquid is higher than that of the solid.

Andy Resnick
Andy Resnick is offline
#4
Feb8-13, 02:02 PM
Sci Advisor
P: 5,468

Gibbs free energy and melting


Quote Quote by AbsoluteZer0 View Post
<snip>

How does this relate to Gibbs free energy? I read that [itex]\Delta G[/itex] during melting is zero. Enthalpy and entropy, however, increase. Does this have any relation to the uniformity of the temperature of the substance during the change of phase?
<snip>
The sign of ΔG tells you if the process is spontaneous (ΔG < 0) or not. As for phase transitions, there are at least 2 kinds: a 'first order phase transition' is accompanied by a discontinuous change in the derivative of the free energy and are associated with freezing/melting/etc. ΔG = 0 for a first order phase transition (ΔH = TΔS).

Second-order phase transitions are associated with discontinuous changes to the second derivative of the free energy. IIRC, ΔG = 0 for a second order phase transition as well.

http://www.helsinki.fi/~serimaa/soft...t-2-phase.html
AbsoluteZer0
AbsoluteZer0 is offline
#5
Feb12-13, 01:54 PM
P: 126
Quote Quote by Andy Resnick View Post
The sign of ΔG tells you if the process is spontaneous (ΔG < 0) or not. As for phase transitions, there are at least 2 kinds: a 'first order phase transition' is accompanied by a discontinuous change in the derivative of the free energy and are associated with freezing/melting/etc. ΔG = 0 for a first order phase transition (ΔH = TΔS).

Second-order phase transitions are associated with discontinuous changes to the second derivative of the free energy. IIRC, ΔG = 0 for a second order phase transition as well.

http://www.helsinki.fi/~serimaa/soft...t-2-phase.html
Thanks


Register to reply

Related Discussions
Gibbs Free Energy Introductory Physics Homework 0
Gibbs Free Energy help Biology, Chemistry & Other Homework 5
Gibbs Free energy Biology, Chemistry & Other Homework 3
Gibbs Free Energy Biology, Chemistry & Other Homework 2
Gibbs Free energy Introductory Physics Homework 2