July 18, 2024, 12:59:43 AM
Forum Rules: Read This Before Posting


Topic: Colligative properties  (Read 2612 times)

0 Members and 1 Guest are viewing this topic.

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Colligative properties
« on: January 08, 2023, 08:11:50 AM »
If adding a solute to a solvent raises the boiling temperature of the solution, why does adding alcohol (solute) to water (solvent) lower the boiling temperature of the solution?
I suppose a distinction needs to be made between volatile and non-volatile solutes. However, even adding a volatile solute to a volatile solvent, in theory the vapor pressure of the solvent tends to lower, so the boiling temperature should still rise (in the phase diagram the solid-liquid curve lowers).

Furthermore, if one considers the eutectic freezing graphs of mixtures, a cryoscopic lowering is always observed regardless of whether the solute is volatile or not, at least as far as I understand.
So I wanted to understand if the colligative properties always refer to non-volatile solutes. If yes, why is cryoscopic lowering observed regardless of the solutes used, whether they are volatile or non-volatile?

Offline jeffmoonchop

  • Full Member
  • ****
  • Posts: 334
  • Mole Snacks: +37/-5
  • Gender: Male
Re: Colligative properties
« Reply #1 on: January 08, 2023, 10:42:43 AM »
water and ethanol are both solvents, or else if not, why do you choose water as the solvent and ethanol as the solute? Besides, is that actually a rule?

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Re: Colligative properties
« Reply #2 on: January 08, 2023, 10:48:05 AM »
water and ethanol are both solvents, or else if not, why do you choose water as the solvent and ethanol as the solute? Besides, is that actually a rule?

No, It was Just an example. We can consider also alcohol as a solvent and water as a solute. It depends on quantity. But my doubt still remains.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27744
  • Mole Snacks: +1804/-411
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Colligative properties
« Reply #3 on: January 08, 2023, 03:45:29 PM »
If adding a solute to a solvent raises the boiling temperature of the solution

You are on the right track. This statement is based on the assumption solute is not volatile. The problem is it is typically poorly spelled out in textbooks. But there is really no need to make a special distinction, it would be enough to teach the thing correctly, without omitting important parts.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Re: Colligative properties
« Reply #4 on: January 08, 2023, 04:44:50 PM »
If adding a solute to a solvent raises the boiling temperature of the solution

You are on the right track. This statement is based on the assumption solute is not volatile. The problem is it is typically poorly spelled out in textbooks. But there is really no need to make a special distinction, it would be enough to teach the thing correctly, without omitting important parts.

So this only works for non-volatile solutes, right?
So eutectic mixtures and the corresponding graphs are associated ONLY with the presence of a volatile solute? That is, if for example a small amount of ethanol (solute) is added to a mass of water, would there be no lowering of freezing point, since ethanol is volatile?

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Re: Colligative properties
« Reply #5 on: January 09, 2023, 09:25:55 AM »
If adding a solute to a solvent raises the boiling temperature of the solution

You are on the right track. This statement is based on the assumption solute is not volatile. The problem is it is typically poorly spelled out in textbooks. But there is really no need to make a special distinction, it would be enough to teach the thing correctly, without omitting important parts.

So this only works for non-volatile solutes, right?
So eutectic mixtures and the corresponding graphs are associated ONLY with the presence of a volatile solute? That is, if for example a small amount of ethanol (solute) is added to a mass of water, would there be no lowering of freezing point, since ethanol is volatile?

I meant 'so eutectic mixtures and the corresponding Graphs are associated ONLY with the presence of a *non-volatile* solute'

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27744
  • Mole Snacks: +1804/-411
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Colligative properties
« Reply #6 on: January 09, 2023, 10:36:03 AM »
Do you know Raoult's law? You can use it to derive most of what is being taught as "colligative properties" (hint: at temperatures involved salts are virtually non volatile).
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Re: Colligative properties
« Reply #7 on: January 09, 2023, 01:42:26 PM »
Do you know Raoult's law? You can use it to derive most of what is being taught as "colligative properties" (hint: at temperatures involved salts are virtually non volatile).

Of course. Raoult's law states that P solution = P solvent + P solute = X solvent * P° solvent + X solute * P° solute.
If the solute is non-volatile, P solute is negligible, so Psolution = P solvent.
Given the presence of the solute, X solvent <1, therefore P solution < P° solvent. This is the vapor pressure drop, one of the colligative properties. Now, if the vapor pressure of the solution is reduced, in the phase diagram of the solvent the liquid-vapour curve is "lowered", so that, consequently, the boiling point rises and the freezing point lowers (other colligative properties).

My doubt arises right here: it seems to me that cryoscopic lowering and ebullioscopic raising don't ALWAYS go hand in hand. Here is an example: considering a solution of ethanol in water (therefore where ethanol acts as a solute and IS VOLATILE), an increase in boiling point is not observed, on the contrary, the boiling temperature is lower than that of pure water, this because even ethanol participates to vapor pressure.
However, this does not exclude the occurrence of cryoscopic lowering: in fact, a solution of water in ethanol or ethanol in water reaches an eutectic.
Therefore, to obtain the freezing point lowering it is not necessary for the solute to be non-volatile, but it is necessary for it to be insoluble in the solid.
Conversely, boiling point elevation should necessarily require a non-volatile solute. If the solute is volatile, then the boiling temperature of the solution will be between that of the two compounds, so in the phase diagram the liquid-vapor curve does not "go down", but the solid-liquid curve can still shift to the left.

Please give me feedback, thank you.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27744
  • Mole Snacks: +1804/-411
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Colligative properties
« Reply #8 on: January 10, 2023, 06:25:38 AM »
Raoult's law states that P solution = P solvent + P solute = X solvent * P° solvent + X solute * P° solute.

Not exactly. It doesn't describe solution, it describes MIXTURE:

[tex]P_{total} = \sum_{components} P^0_i x_i[/tex]

which makes the distinction between solvent and solute a moot.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Re: Colligative properties
« Reply #9 on: January 10, 2023, 06:38:06 AM »
Raoult's law states that P solution = P solvent + P solute = X solvent * P° solvent + X solute * P° solute.

Not exactly. It doesn't describe solution, it describes MIXTURE:

[tex]P_{total} = \sum_{components} P^0_i x_i[/tex]

which makes the distinction between solvent and solute a moot.

Yes, that's actually more correct. I was considering a mixture of One solvent+One solute as an example.

Anyway, does the rest of the reasoning about boiling point elevation and cryoscopic lowering fit?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27744
  • Mole Snacks: +1804/-411
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Colligative properties
« Reply #10 on: January 10, 2023, 07:55:06 AM »
Anyway, does the rest of the reasoning about boiling point elevation and cryoscopic lowering fit?

Elevation requires some point of reference. If we have a solution that reference is the solvent BP. Once we move to mixtures there is no solvent as such, so we have no reference - and we can't speak about BP elevation. We can speak about BP of a mixture being different from BPs of its components.

Same about freezing.

In general: BP elevation and FP lowering are specific cases that apply to mixtures with a main component (called "solvent") set as a reference and other components being non volatile. Logic and wording used for this case don't apply universally to more general case of mixtures of any type.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Re: Colligative properties
« Reply #11 on: January 10, 2023, 08:27:52 AM »
Anyway, does the rest of the reasoning about boiling point elevation and cryoscopic lowering fit?

Other components being non volatile.




but non-volatility is not a necessary condition for cryoscopic lowering: just think of the water + ethanol solution, in which both substances are volatile, and reach an eutectic at around -95°C. To obtain cryoscopic lowering it is sufficient that the solute is not soluble in the solid solvent being formed, while the temperature is lowered, and that it is contextually soluble in the liquid solvent in equilibrium with the solid.

Conversely, non-volatility is necessary for the boiling point to rise, so much so that this property cannot be associated with the water-ethanol mixture.

I seem to have figured this out by sifting through various sources, but I'm not entirely sure.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27744
  • Mole Snacks: +1804/-411
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Colligative properties
« Reply #12 on: January 10, 2023, 03:28:08 PM »
but non-volatility is not a necessary condition for cryoscopic lowering: just think of the water + ethanol solution, in which both substances are volatile

I feel like you are still trying to use limited logic instead of seeing the larger picture. Yes, temperature goes down if you look from the water point of view, but goes up if you look from the ethanol pov (why not assume ethanol to be a solvent and water solute?). What happens to the freezing point when you add acetic acid to water? Does it go up, or down?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Luckenberg99

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +0/-0
Re: Colligative properties
« Reply #13 on: January 11, 2023, 06:46:42 AM »
but non-volatility is not a necessary condition for cryoscopic lowering: just think of the water + ethanol solution, in which both substances are volatile

I feel like you are still trying to use limited logic instead of seeing the larger picture. Yes, temperature goes down if you look from the water point of view, but goes up if you look from the ethanol pov (why not assume ethanol to be a solvent and water solute?). What happens to the freezing point when you add acetic acid to water? Does it go up, or down?

mmh, acetic acid has a higher boiling point than water, it is less volatile, which means that, at the same temperature, acetic acid will have a lower vapor pressure than water . Consequently, the total vapor pressure of the solution will tend to decrease, so that the boiling temperature tends to increase as acetic acid is added, so that the boiling point is raised even if the acetic acid can be considered overall volatile .

For freezing point lowering, acetic acid is soluble in water, so it should lower the freezing point.

but, in theory, also adding water to acetic acid causes freezing point lowering, reaching an eutectic, while the boiling point of the solution, in this case, should increase

where do you want to take me?

Offline Vidya

  • Full Member
  • ****
  • Posts: 839
  • Mole Snacks: +46/-62
  • Gender: Female
  • Online Chemistry and Organic Chemistry tutor
    • Online Chemistry Tutor
Re: Colligative properties
« Reply #14 on: January 20, 2023, 07:24:20 AM »
If adding a solute to a solvent raises the boiling temperature of the solution, why does adding alcohol (solute) to water (solvent) lower the boiling temperature of the solution?
I suppose a distinction needs to be made between volatile and non-volatile solutes. However, even adding a volatile solute to a volatile solvent, in theory the vapor pressure of the solvent tends to lower, so the boiling temperature should still rise (in the phase diagram the solid-liquid curve lowers).

Furthermore, if one considers the eutectic freezing graphs of mixtures, a cryoscopic lowering is always observed regardless of whether the solute is volatile or not, at least as far as I understand.
So I wanted to understand if the colligative properties always refer to non-volatile solutes. If yes, why is cryoscopic lowering observed regardless of the solutes used, whether they are volatile or non-volatile?
You are right that both solute and solvent is volatile here.
Now you need to understand deviation in Raoult's law in non ideal solution.
Ethanol and water are making new attractive forces which are weaker than pure water. Ethanol has C2H5 group which is hydrophobic nature and due to this solution has less stronger H-bonding than pure water.
This increases the vapor pressure and lower the boiling point.
In non volatile solute, vapor pressure of the solution is less than that of the pure solvent and hence there is elevation in boiling point.

Sponsored Links