Gibbs free energy and melting

In summary, during a reversible phase change, such as slow melting of ice, the Gibbs free energy remains constant at 0. This is because the change in enthalpy and entropy are equal. However, for a non-reversible phase change, such as raising the temperature of water, the Gibbs free energy is not constant and can be greater than 0.
  • #1
AbsoluteZer0
125
1
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 30°C to 50°C.) How accurate is this assumption?

Thanks,
 
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  • #2
AbsoluteZer0 said:
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 30°C to 50°C.) 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...
 
  • #3
sankalpmittal said:
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.
 
  • #4
AbsoluteZer0 said:
<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-luento/soft-2-phase.html
 
  • #5
Andy Resnick said:
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-luento/soft-2-phase.html

Thanks
 

What is Gibbs free energy?

Gibbs free energy is a thermodynamic quantity that measures the amount of energy available to do useful work in a system at a constant temperature and pressure.

How does Gibbs free energy relate to melting?

Gibbs free energy is a crucial factor in determining whether a substance will melt or not. When the Gibbs free energy of a substance is lower than its melting point, the substance will spontaneously melt.

What is the significance of negative Gibbs free energy in melting?

A negative Gibbs free energy for a substance at its melting point indicates that the substance is in a stable state and will melt spontaneously under the given conditions.

Can Gibbs free energy be used to predict the melting point of a substance?

Yes, Gibbs free energy can be used to predict the melting point of a substance. The melting point can be calculated by finding the point at which the Gibbs free energy change is zero.

How does temperature affect Gibbs free energy and melting?

The temperature of a substance affects both its Gibbs free energy and its melting point. As temperature increases, the Gibbs free energy of a substance also increases, making it less likely to melt. However, if the temperature rises above the melting point, the substance will melt regardless of its Gibbs free energy.

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