Freezing point trend (higher or lower)?

[agill786]

Hi,
I took AAMC 10 and was confused about one of the bio questions (Passage 2 and question 10). It says that the melting point is the same as the freezing point. But that just doesn't sink in with me.

The question asks which one has the greatest freezing point? From what I recall the equation for freezing point depression is deltaTf = -kf*m *i

Do they mean greatest freezing point as in what will give the least negative deltaTf or greatest as in what will give the most negative/higher negative number deltaTf?

BTW, it gives option choices as butane, 1-propanol, 1,2-ethanedithiol, 1,2-ethanediol and asks which one has the greatest freezing point?

Also, as a side note, what does deltaTf really mean? Does it mean the amount you have to lower your temperature compared to the actual/ambient temperature, to get your solution to freeze?

Sorry if this sounds dumb and poorly worded! My science GPA is low due to some awful Gchem grades!

Happy New Year!

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[loveoforganic]

Temperature doesn't change during phase transitions, correct? So melting ice must be at the same temperature as freezing water. With freezing point depression, you're looking for the largest decrease in freezing point, so the most negative deltaTf. DeltaTf is the change in degrees celsius in freezing point.

 

[agill786]

Temperature doesn't change during phase transitions, correct? So melting ice must be at the same temperature as freezing water. With freezing point depression, you're looking for the largest decrease in freezing point, so the most negative deltaTf. DeltaTf is the change in degrees celsius in freezing point.

Yes, temperature remains constant. This is due the kinetic energy of the molecules staying constant before the phase changes and afterwards, correct?

But deltaTf simply measures the change of freezing points/temperatures at all times between the pure solvent and the ionized solution you have, correct?

I looked up the definition of freezing point and it is the temperature below, the solvent will freeze. So does THIS temperature refer to the ambient temperature?

But the temperature that you were initially referring to is the temperature of the solution, which will always stay constant. So when going from liquid to solid (freezing water) we are releasing heat, but the temperature of the solution in the beaker will not change due to the kinetic energies not changing.

Sorry for being so nagging on this concept! BTW, loveoforganic, who is that in your profile pic? He looks very familiar.

Thanks in advance!

 

[loveoforganic]

I'll try to explain it the best I can think of. At any pressure,temperature coordinate along the phase change diagram lines, you're at some kind of equilibrium. If you're on the boundary of the solid/liquid phases, solid will stay solid, liquid will stay liquid. If you shift infintesimally off the line, you will begin having a phase change. I think you're overthinking ambient vs solution temp. Given enough time and an infinitely large environment, solution temp will eventually always get to ambient environment temp. If that's still unclear, someone else might need to try, but I'm happy to try again if I can think of a way 😛

My prof pic is Freddie Roach.

 

[LaurenMCAT]

Hi there

Your question has TWO parts, actually...
1) how to find the answer to Q #104
2) what's up with freezing point depression

The good news? Q#104 has nothing to do with freezing point depression. It's only testing your ability to interpret the table given in the passage 🙂 Yee Haw!

Let's break it down:

"I took AAMC 10 and was confused about one of the bio questions (Passage 2 and question 10)...The question asks which one has the greatest freezing point? It says that the melting point is the same as the freezing point...Do they mean greatest freezing point as in what will give the least negative deltaTf or greatest as in what will give the most negative/higher negative number deltaTf?"

First, they're not asking you about freezing point depression (a colligative property that we learned about in gen chem - not tested in BS), they're asking you to look at the data in the passage-based table and predict from that.

So...let's look at the table. We see that 1,2-ethanediol and butane have the highest and lowest melting points...they're on either end of the spectrum and our question asks us for some extreme. Right away, we can narrow our answer choices down to this...represented by A & C.

Then...we have to determine what freezing has to do with melting. So we think about phase changes and our phase change diagrams. How do we label the following phase changes? (Because you have to learn them all.)
s --> l
l --> s
s --> g
g --> s
l --> g
g --> l

Now...think about water as a sample compound. Ice melts to liquid water at 0'C and water freezes to form ice also at 0'C. Same reaction set up, same temperature, different directions:
H2O (s) <---> H2O (l)
forward: melting
reverse: freezing
(BTW...think about which direction is exothermic and which is endothermic.)
Point being: melting and freezing are different processes but occur at the same temperature! Yes! That gives us our answer:

If 1,2-ethanediol melts at -13'C then it also freezes at -13'C. That's the HIGHEST (or warmest) temperature in the table. WOO HOO!!



Moving on...

"From what I recall the equation for freezing point depression is: deltaTf = -kf*m *i
...what does deltaTf really mean? Does it mean the amount you have to lower your temperature compared to the actual/ambient temperature, to get your solution to freeze?"

Yep - that's the right equation 👍 but remember that m represents molality, not mass.

And the deltaTf indicates the change in the substance's freezing point as compared to a pure sample. DeltaTf isn't the NEW freezing temp, it only tells us by how much the old freezing temp changed. It leads us to the new freezing temp.

For example: Let's say Substance A freezes at -18'C. If, while Substance A is liquid, we dump in a bunch of X particles then drop the ambient temp to measure the new freezing point...those X particles will disrupt the orderly arrangement of Substance A molecules as it tries to condense into the solid state. So...X particles are thwarting the freezing process...and Substance A only becomes a solid at lower temperatures. Our deltaTf will be some negative value and our new Tf will be lower than -18'C. Hence, freezing point depression. 😎

 

[agill786]

Thank you Lauren and loveoforganic for taking the time and effort to go through this with me! I think it makes sense now. I am going to attempt some questions in the EK 1001 Chem book and we will see how it goes.

Gchem was not my favorite subject in school as you can tell.

 

[LaurenMCAT]

No worries! We all help each other 😉
It's just us little guys against the MCAT beast! hahahaha...