Talk:Freezing-point depression

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[edit] Confused

I am confused. How does this explain the mixture/solution of water and alcohol whose freezing temperature is only lower than water, but not alcohol? I am sure I am wrong, but why? Can we not deem alcohol as the solvent and water the solute?

Racoon.zip (talk) 00:15, 28 November 2007 (UTC)

[edit] Mistake???

From the article:

ΔTf = i · Kf · activity

   * activity is in units of mol/kg, and is equal to an activity coefficient times the molality

...

   * i is the i factor or the van 't Hoff factor (see van 't Hoff), accounts for the number of individual ions formed by a compound in solution.

Eehm, isn't i the activity coefficient? I'm a bit puzzled, but it seems to me that it is used twice here.

—The preceding unsigned comment was added by 83.98.253.62 (talk) 09:05, 6 February 2007 (UTC).

No, i isn't the activity factor, and neither the activity factor or the activity of the solute is really applicable in the case of colligative properties, where the solute plays the passive role of diluting the solvent (which is why molality is the relevant concentration measure). The standard phrase "concentration, or to be more precise activity..." does not apply to colligative properties! The article has now been changed to reflect this. The activity (or better, the chemical potential) of the solvent is another matter, but the ΔTf formula is the final stage in a derivation where the chemical potential has been expressed in those other terms on the right hand of the equation. Tomas e 20:59, 20 October 2007 (UTC)

[edit] Osmometry

a good addition to this article would be the clinical uses for freezing point depression, such as osmometry!


From the article:

Freezing-point depression is the difference between the freezing points of a pure solvent and a solution of a nonelectrolyte in that solvent.

It is a well-known fact that salt can cause freezing-point depression. The electrolyte article mentions that sodium and chloride are both electrolytes. Maybe the article should mention electrolytes instead of nonelectrolytes?

Freezing-point depression is the difference between the freezing points of the pure solvent and the solution. It occurs in both electrolyte and non electrolyte solutions. However, the van't Hoff factor kicks in when you have an electroyte solution because it takes into account how the ions disassociate. ha ha

[edit] Table of constants?

Anyone willing to compile a table of freezing point depression constants for a variety of solvents? I can do it if no one else does, but right now I'm busy.Quaxmonster 23:23, 22 May 2006 (UTC)

I compiled one from Atkins Physical Chemistry & German Wikipedia. Feel free to add more solvents! Tomas e 20:59, 20 October 2007 (UTC)

[edit] Theory

I think it would be wise to include a section on what exactly causes freezing point depressions. I would also be glad to add this section if there are no objections--Helgers7 05:57, 15 May 2007 (UTC)

I've added a section on this, based on a diagram of chemical potential against temperature. It's correct, but it can probably be made more pedagogical. However, this is a phenomenon where I've often seen erroneous explanations (and I just removed one from the end of the previous version of the article). Many people are tempted to find "direct" molecular explanations, which tend to be nonsense in the case of colligative properties (and in many other cases where more subtle entropic phenomena are to be explained). Tomas e 20:59, 20 October 2007 (UTC)

[edit] Pointless entry

"In Cohen's Practical Organic Chemistry of 1910 [1] the molar mass of napthalene is determined in a so-called Beckmann freezing apparatus at 128 g/mole."

I see absolutely no point in this entry. I will delete it unless someone objects —The preceding unsigned comment was added by Helgers7 (talk • contribs).

  • The statement is vital: nowadays nobody uses freezing point depression as an analytical tool but in 1910 it was pretty standard stuff. Please do not delete, See also User:V8rik#On_deletions V8rik 15:16, 28 May 2007 (UTC)
  • Actually, depending on the equiptment you have available, some chemists do use it as an analytical tool, I know I have. However, this still doesn't justify this statement. It is simply a pointless fact that has no real right to be on this page. It would be acceptable to add "however, this technique is not as common as it once was" to the statement "or to determine molar mass of the solute." —The preceding unsigned comment was added by Helgers7 (talk • contribs).
I have no problem with it being there (I tried to integrate it a little better), but V8rik, please don't call things "vandalism" in your edit summaries unless they're clearly vandalism. That word has a specific meaning and doesn't apply to Helgers7's edits because they were made in good faith. —Keenan Pepper 18:14, 31 May 2007 (UTC)

[edit] Typo

"This is due to solute molecules disrupting the ability of the solute to form crystals during the freezing process."

Since it has been a while since first year chem, I thought I would check...

The second "solute" should be "solvent"... right?

—carnage_joe 6:35, 8 July 2007 (UTC)

Difficult to say, since it is totally wrong as an explanation of why the freezing point depression takes place. This sentence is now removed. Tomas e 20:59, 20 October 2007 (UTC)