freezing point/boiling elevation

[dexadental]

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Are freezing point/boiling point problems common? They aren't hard, but just curious....thanks!

 

[cowsgomoo]

Yeah they are since most people don’t keep in mind that these properties are colligative properties. I would always get these wrong because I would forget that this property is proportional to the amount of a particular substance not the identity.

 

[dino_flagellate]

I don't know if they're common or not, but I do know one thing, there not as hard as they may appear. In fact, once you understand basic concepts (adding solute - what happens), you should understand it. In fact, I just finished studying this section a few days ago, so I'll do my best explaining it right here!

Do you know the various phase changes and the triple point diagram? If not, familiarize yourself with it, because it will help when visualizing some of the colligative changes I'm about two describe.

Basically, there are 'four' things in every chemistry chapter (correct me if I'm wrong), but they are, Raults Law, Boiling Point elevation, Freezing Point depression, and Osmosis.

Lets start with the basics:

What is it that is actually going on?

Bascially, imagine a beaker of water (open to the atmosphere). The water molecules are randomly associating/disassociating with one another. They are basically in a state of flux. However, some molecules (especially on the top layer of water), will escape as water vapor (gas) into the atmosphere (because of evaporation)...

What is it that keeps these molecules of water in the beaker? Essentially, it is the brief hydrogen bonding (is this right) attractions between the water molecules that opposes this tranformation into the gaseous phase...

NOW...What is the significance of adding solute (say NaCl)...?

Adding NaCl (a non volatile solute - which won't escape from solution), then the ions will dissociate into Na+ and Cl- ions. These ions will then be surrounded by water molecules (I hope you know what I'm talking about - remember - partially negativley charged oxygen aligned with sodium ion, and partially positively charged hydrogen with chloride ion....)

This attraction of water molecules within the beaker will INCREASE. Therefore, the heat (energy - or whatever causes a liquid into the gas phase) required has just increased - more energy is needed to input engergy that will release the attraction of water molecules (not only between each other, but also between water molecules and the corresponding ions).

From this, you should know what happens to the boiling point of water (100), when you add salt. And more importantly, Why? Any cooks here?

After understanding this, the same principles apply to freezing point depression, the answer is in the name of the topic itself! When adding solute, the freezing point will decrease (depression).

Now that you know the concepts, I would look up the equations (most important - and use them to solidify your understanding of colligative properties). If anyone needs help in that, then let me know.

 

[dexadental]

hey thanks! Thats a great explanation! Yes the equations are memorized, and quite simple...these problems really aren't so tough. If they ask for molecular weight (MW), you find it by (lets use freezing point depression as an example):

Tf= -(iKf(m)) where m=molality=moles/kg solvent=((grams/MW)/kg solvent)
So,
MW=grams(iKf)/(Tf(kg solvent))

Correct? (I know everyone knows this but it helps me write it out)
Hope this helps someone... 😳

 

[dino_flagellate]

hey thanks! Thats a great explanation! Yes the equations are memorized, and quite simple...these problems really aren't so tough. If they ask for molecular weight (MW), you find it by (lets use freezing point depression as an example):

Tf= -(iKf(m)) where m=molality=moles/kg solvent=((grams/MW)/kg solvent)
So,
MW=grams(iKf)/(Tf(kg solvent))

Correct? (I know everyone knows this but it helps me write it out)
Hope this helps someone... 😳

👍