Phase Change Diagrams

[MedPR]

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So on an enthalpy vs temp graph, the flat-ish horizontal lines represent heating but not phase change or bond breaking and the vertical lines represent the breaking of intermolecular bonds and phase change.

If the enthalpy equation is H=U+PV, what is changing that makes Enthalpy increase so rapidly without a change in the temperature?


Wikipremed says that the internal energy is increasing because there is greater electrostatic potential energy for the h-bonds in water than the h-bonds in ice.

Why?

U=kqq/r. The charges in water are further apart than they are in ice, so shouldn't U be higher for ice than water?

 

[whatsway]

Enthalpy increase so rapidly without a change in the temperature?

I think you may be confused because you are used to a traditional graph with t=time as the x-axis. for all we know, these phase changes could take days.

but somebody feel free to correct me if i'm wrong.

 

[chiddler]

"what is changing that makes Enthalpy increase so rapidly without a change in the temperature?"

Enthalpy, at constant pressure, is equal to q (heat). So the amount of heat in the molecules increases. Where does this energy go? Into breaking the bonds of the molecules in order to change the phase.

I thought i had an answer to your second question, wrote a lot, did some test values and it came up wrong. so i'm figuring that out.

 

[MedPR]

I think you may be confused because you are used to a traditional graph with t=time as the x-axis. for all we know, these phase changes could take days.

but somebody feel free to correct me if i'm wrong.

No, these types of graphs are typically enthalpy vs temp.

"what is changing that makes Enthalpy increase so rapidly without a change in the temperature?"

Enthalpy, at constant pressure, is equal to q (heat). So the amount of heat in the molecules increases. Where does this energy go? Into breaking the bonds of the molecules in order to change the phase.

I thought i had an answer to your second question, wrote a lot, did some test values and it came up wrong. so i'm figuring that out.

E=Q+PV, so constant pressure means E=Q. Ok that makes sense. But there's constant pressure the entire time since it is open to the atmosphere, so what is the difference between the period where the heat is being used to increase kinetic energy of the molecules and the period where heat is being used to break the bonds?

 

[chiddler]

We're looking at a positive and negative charge of the polar regions of water.

First, a generalization. Logically, a positive change in PE means that potential energy is gained. For a ++ system, their distances are closer together. For a +- system, their distances are further apart.

So we should see a positive change in potential energy as their distances increase.

Lets assume both are one coulomb and are 0.0001 meters apart as liquid form and then become 1 meter apart in gas form. Our frame will be the positive charge staying in place and the negative charge moving away:

(so the q in qV is the positive, the changing potential is the negative charge)

ΔPE = qV = qΔ(phi)

q[kq/r(final) - kq/r(initial)]

(1) * [(k(-1)/1) - (k(-1)/0.0001)] = -k + k/0.0001 = -k + 10000k = 9999k. A positive value! Potential energy is increasing.

I hope this was clear. I hope more that I understood your question lol.

 

[MedPR]

We're looking at a positive and negative charge of the polar regions of water.

First, a generalization. Logically, a positive change in PE means that potential energy is gained. For a ++ system, their distances are closer together. For a +- system, their distances are further apart.

So we should see a positive change in potential energy as their distances increase.

Lets assume both are one coulomb and are 0.0001 meters apart as liquid form and then become 1 meter apart in gas form. Our frame will be the positive charge staying in place and the negative charge moving away:

(so the q in qV is the positive, the changing potential is the negative charge)

ΔPE = qV = qΔ(phi)

q[kq/r(final) - kq/r(initial)]

(1) * [(k(-1)/1) - (k(-1)/0.0001)] = -k + k/0.0001 = -k + 10000k = 9999k. A positive value! Potential energy is increasing.

I hope this was clear. I hope more that I understood your question lol.

Wouldn't we have to use E=kqq/r? And since the charges are the same but r is increasing, E should go down? 🙁

 

[chiddler]

No, these types of graphs are typically enthalpy vs temp.



E=Q+PV, so constant pressure means E=Q. Ok that makes sense. But there's constant pressure the entire time since it is open to the atmosphere, so what is the difference between the period where the heat is being used to increase kinetic energy of the molecules and the period where heat is being used to break the bonds?

I have not read this in a book, per se, but this is how I understand it and I think it makes sense to me. This is just to let you know that I may be prone to error. More than usual, anyway 😛

The difference is the distribution of absorbed energy. For a cold molecule, the energy that is absorbed increases the average kinetic energy of the molecule and thus the temperature. Until it reaches a certain threshold.

Why is this threshold reached? Because up to a certain point, it cannot vibrate any more due to its phase. It's like trying to dance with shackles on your arms and legs. If you try to dance hard enough (lol), then you'll break the shackles on your arms. Now you're in liquid form! But still have shackles on your legs. Notice that the additional heat that was added was not increasing how much you are dancing, but just went into breaking the shackles.

So you start dancing even harder. The shackles that are off your arms allow for greater movement so you can increase your temperature until you reach that second threshold. Now your dancing is limited by the shackles on your legs. Adding more temperature causes you to break those bonds as well leaving you free and gaseous.

 

[chiddler]

Wouldn't we have to use E=kqq/r? And since the charges are the same but r is increasing, E should go down? 🙁

Yes charges are the same, but opposite in sign. This is why bringing two charges close together that are the same charge will increase potential energy. Doing the same thing with opposite charges does the opposite, it decreases potential energy.

E = kqq/r = q (kq/r) = q(phi).

What I did was E = q(Δphi) to compare the change in potential.

hold on let me draw an image that helped me...

 

[chiddler]

do i have the right idea of what your question is? or is what i wrote not exactly what you were asking.

well here's hoping i was on track. i'll be really sad if i wasn't!

looking again, i don't think i was clear enough...sorry.

Try to prove it to yourself. Do some test values, make a table, determine what you expect to happen, and figure out what positive and negative values mean.

 

[MedPR]

Yes charges are the same, but opposite in sign. This is why bringing two charges close together that are the same charge will increase potential energy. Doing the same thing with opposite charges does the opposite, it decreases potential energy.

E = kqq/r = q (kq/r) = q(phi).

What I did was E = q(Δphi) to compare the change in potential.

hold on let me draw an image that helped me...

do i have the right idea of what your question is? or is what i wrote not exactly what you were asking.

well here's hoping i was on track. i'll be really sad if i wasn't!

looking again, i don't think i was clear enough...sorry.

Try to prove it to yourself. Do some test values, make a table, determine what you expect to happen, and figure out what positive and negative values mean.

imgur is blocked at work, but I think I know what you're saying (and yes, it is exactly what I was asking). I did some quick math with easy numbers and came out with this.

q1=10
q2=(-10)
r=1

So E = -100k

If you increase r, like you do when solid changes to liquid, to something like 100 for simplicity...

q1=10
q2=(-10)
r=100

So E = -1k

So E becomes less negative = increasing. That makes sense.

The graph, however, shows that the enthalpy initially is negative, then increases to a pretty large positive number. How is that possible?

 

[chiddler]

i'm not sure what negative enthalpy means. negative change in enthalpy, sure. but negative enthalpy i don't know.

according to wiki, it's the "measure of the total energy of a thermodynamic system." Perhaps the negative was an error.

assuming it's an error, as energy of system increases, temperature increases. the gaps in temperature tells that energy is increasing but temperature is not because that energy is going into breaking the phase-changing intermolecular bonds.

 

[MedPR]

i'm not sure what negative enthalpy means. negative change in enthalpy, sure. but negative enthalpy i don't know.

according to wiki, it's the "measure of the total energy of a thermodynamic system." Perhaps the negative was an error.

assuming it's an error, as energy of system increases, temperature increases. the gaps in temperature tells that energy is increasing but temperature is not because that energy is going into breaking the phase-changing intermolecular bonds.

Ok that makes sense. At what point does the energy go from increasing KE to breaking bonds? I know that the point at which bond breaking ends minus the point at which bond breaking begins is the heat of "name of phase change here" (e.g heat of fusion for solid to liquid, heat of vaporization for liquid to gas) but is there a significant qualitative event that occurs?

 

[chiddler]

i'm not sure about what kind of qualitative change occurs.