Boiling point elevation?

[frank51]

According to the colligative properties when you mix 2 solutions the Boiling Point increases because the number of particles increases, however it is also true that impurities lower the BP and MP, so how should we distinguish between these two concepts?

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[Drakensoul]

Originally posted by frank51
According to the colligative properties when you mix 2 solutions the Boiling Point increases because the number of particles increases, however it is also true that impurities lower the BP and MP, so how should we distinguish between these two concepts?

Are you sure increasing number of particles increases BP? 5 gallons of water versus 300 gallons, the water sitll boils at 100C, it just takes more energy to raise the temperature to 100C. I would think it would have a lot more to do with inter-mollecular forces of attraction (E.G., are solution 1 molecules more attracted to solution 2 molecules, or to other solution 1 molecules). These forces play a large factor in the volatility, and I would think would be the main concerns when dealing with BP/MP/FP changes with solutions. Other factors being whether or not the solutions are volatile, etc..

Could be wrong though, anyone else?

 

[Smitty3L]

This is how I understood boiling point elevation:

Colligative properties only depend on the number of solute particles, so intermolecular forces between solute and solvent are not taken into account for these properties.

The vapor pressure of the solvent is lowered when a nonvolatile solute is added. This is due to the fact that not as many solvent molecules are at the surface b/c some of the solute molecules are taking up that space. Because boiling point is the temp at which the vapor pressure = atmospheric pressure, the boiling point is increased due to the vapor pressure being decreased.

 

[frank51]

That's the way I understood it at first, but recently I've come across questions where I don't know which concept to apply, I guess it depends on the information they provide in the passage, what does every one else think?

 

[CH3CH2OH]

you can also look at BP elevation in terms of entropy.
the entropy increases by a certain amount when the liquid evaporates.
now, say you add a solute to that liquid, solvation increases entropy. so now, when your solvent evaporates the change in entropy in smaller than that for the evaporation of a pure liquid, so the process is less favorable and hence requires more energy, leading to elevation of the BP.


this is probably useless, but i find it pretty interesting (something i never heard about in gchem, but this is how my pchem book presents BP elevation)

 

[adennis]

This is taken straight from my textbook

The boiling point of a solution is always higher than the boiling point of the pure solvent. The amount by which the boiling point of the solvent is increased by the addition of a non-volatile solute is known as the boiling point elevation. The magnitude of the increase is proportional to the colligative molality of the solute.

Delta T(b) = k(b)m(c) = ik(b)m

where delta equals change, (b) denotes boiling point (not another variable) and (c) denotes colligative.
k(b) is the proportionality constant called the boiling point elevation constant of the solvent and has units of degrees C/molal


I hope this helps. I always found the colligative properties to be horribly confusing.. but this makes a little bit of sense I think.

 

[ASDIC]

This is the explanation for the boiling point elevation:

A non-volatile solute in a solvent will squeeze itself amongst the solvent molecules. Since the solute is non-volatile, it will have low or virtually no vapor pressure. Vapor pressure is the pressure of the solvent as it is converted into the gaseous phase from the liquid phase.

Vapor pressure is directly proportional to the surface area of the liquid. With the solute particles squeezing in between liquid particles, the surface area decreases. So less solvent is able to escape. As a result vapor pressure decreases.

Boiling point occurs when the vapor pressure just equals the atmospheric pressure. And the solvent will begin to escape as gas if the vapor pressure exceeds the opposing atmospheric pressure.

To cause such a high vapor pressure, the solution must be heated. More heat means more vapor pressure. So more heat must be supplied to reach the pressure when vapor pressure equals the atmospheric pressure (since the original vapor pressure was at a lower level due to the solute).

More heat means higher temperature. So if more heat is applied to reach the required vapor pressure, the temperature will increase.

Therefore, the boiling point temperature increases.

 

[Wahooali]

Thanks man, totally makes sense in that context.

 

[frank51]

All that makes sense but what about the Melting Point, I remember from organic lab that impurities lower the MP, does every one agree with this?

 

[Nuel]

Originally posted by frank51
All that makes sense but what about the Melting Point, I remember from organic lab that impurities lower the MP, does every one agree with this?

I think it is so. Compare it with freezing point, and you remember freezing point depression.

 

[ASDIC]

Heres the explanation for the depression of the melting point from the addition of a non-volatile solute:

Normally solvent has intermolecular attractive forces. These could be hydrogen bonds, dipole interactions and Van der Waals.

In order to become a solid, these interactive forces must be increased to very high levels. You must note that all of these forces become increasingly effective at short distances between the molecules. In order for the molecues to experience high levels of these forces, they must come together. What can make them come together?...removal of heat.

If heat is removed (temperature is lowered), the molecules will come together since kinetic energy is decreased. This will result in the formation of the solid as those intermolecular forces are increased.

When you add the solute, it intereferes with the forces since they sit between the solvent molecules. Therefore, more cooling has to be done (tempereture should be lowered more) so that the solvent particles come more and more closer to each other to form the solid.

 

[frank51]

makes sense, thanks