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Topic: Boiling point Elevation and Freezing point Depression  (Read 1146 times)

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Offline jkb2021

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Boiling point Elevation and Freezing point Depression
« on: June 30, 2021, 09:56:23 AM »
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In an experiment for BP elevation and FP depression:

We establish the boiling point of the solvent, water, before we find the boiling point of the solution.

We do not have to do this for the freezing point.

Why did we need to get a base line BP for water but not for its FP?


Information I know:

Both FP and BP are affected by atmospheric pressure.

BP = the temperature at which the vapor pressure of a liquid is equal to the external  pressure.

The addition of solute lowers FP by interfering with its ability to become ordered.

The addition of solute elevates BP by interfering at the interface between phases, lowering vapor pressure.

temperature of a substance does not immediately raise at the phase change because energy is going into breaking the bonds.

I don't see how any of this answers the question. The only data we are collecting is Delta Tb and Delta Tf. Do we need a baseline to know how much the BP changed because of atm not being 1 at the lab? How much does atm affect FP vs BP? The water is distilled, so there should not be any impurities. 

What am I missing?


Offline Corribus

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Re: Boiling point Elevation and Freezing point Depression
« Reply #1 on: June 30, 2021, 10:30:42 AM »
Can you think of which factor, unrelated to solute content, would influence the baseline boiling point much more than the baseline freezing point? 
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline jkb2021

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Re: Boiling point Elevation and Freezing point Depression
« Reply #2 on: June 30, 2021, 10:51:17 AM »
Can you think of which factor, unrelated to solute content, would influence the baseline boiling point much more than the baseline freezing point?

Well the three main factors that affect boiling point is temperature, atmospheric pressure, and vapor pressure. The only one that could affect one more than the other that is independent from solute is atmospheric pressure. But I can't find anything that confirms atmospheric pressure has a greater affect on BP than it does FP.

I suppose that if it did, then we would need a baseline to properly determine delta Tb. I'm just not confident that is the answer.

I imagine that not having the correct delta Tb would throw off our later calculation of molecular weight as well.

Offline Corribus

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Re: Boiling point Elevation and Freezing point Depression
« Reply #3 on: June 30, 2021, 11:12:09 AM »
Check out the phase diagram for water. You can find it here (right now, anyway):

https://en.wikipedia.org/wiki/Phase_diagram

Look at how the interface between solid and liquid changes as a function of pressure compared to the interface between liquid and gas.

An important point is that stating

Quote
We do not have to do this for the freezing point.

Is a little misleading to yourself. In principle, you should always do a baseline because the real values for freezing and boiling points of "pure water" can deviate from the idealized (0 and 100 C), and the degree of change when adding solute always is compared against the "pure water" frame of reference. In practice, the freezing point of pure water will only have small deviations from the theoretical value in most normal situations, whereas the boiling point of pure water can deviate by several degrees just based on your elevation... so it's much more important to take a baseline for the boiling point and then you can just assume the freezing point is 0 degrees. Your error by taking the latter assumption will usually be small. That's not the same as "we don't have to do this". As always, it depends on how accurate a measurement you need. But it can't be stressed enough: it's always better to take a baseline measurement than just assume a value, if only because a baseline also can give you confidence that your method is working properly.
« Last Edit: June 30, 2021, 11:31:43 AM by Corribus »
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline jkb2021

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Re: Boiling point Elevation and Freezing point Depression
« Reply #4 on: June 30, 2021, 11:36:20 AM »
If I am interpreting the diagram correctly, the linear relationship between solid and liquid phases does not have a sharp affect on freezing point at a given temperature/pressure. Compared to the more hyperbolic relationship between liquid and gas, which has a much more drastic effect.

Therefore, to get an accurate delta Tb, we need to establish the baseline because the lab is not at sea level and would have a lower atmospheric pressure which would lower our baseline boiling point.

Offline Corribus

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Re: Boiling point Elevation and Freezing point Depression
« Reply #5 on: June 30, 2021, 12:16:50 PM »
Yes, if you assumed a boiling point of 100 C, you would probably get a significant error unless you happened to be in a place that satisfied the conditions for which that assumption is valid. You will get an error as well by assuming a freezing point of 0 C, but as the freezing point does not vary as significantly as a function of atmospheric pressure, your error will be much smaller, possibly under the normal degree of error for the measurement method.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline jkb2021

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Re: Boiling point Elevation and Freezing point Depression
« Reply #6 on: June 30, 2021, 12:20:32 PM »
Thank you, I was able to wrap up my lab based on that help. I really wish that this stuff was clearly stated. It isn't that I can't interpret or synthesize information(I pretty much did), but without a source to check against, I lack confidence which makes my anxiety skyrocket.

Offline DrCMS

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Re: Boiling point Elevation and Freezing point Depression
« Reply #7 on: July 01, 2021, 04:20:44 AM »
I really wish that this stuff was clearly stated. It isn't that I can't interpret or synthesize information(I pretty much did), but without a source to check against, I lack confidence which makes my anxiety skyrocket.

That is the point of education.  Just giving you the correct answers to a test does teach you anything.

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