Effects of H-Bonding and Impurities on BP/MP

[sunflower18]

This always confuses me a little bit and I get mixed up, so I want to make sure I have it straight.

Impurities:
- lower melting point (and broaden range)
- raise boiling point (and broaden range??)
- reduce vapor pressure of each component??

Hydrogen Bonding:
- lowers melting point
- raises boiling point

Larger Molecular Mass:
- lowers melting point
- raises boiling point

So all three of these conditions serve to keep the solution in liquid phase for a longer range of temperatures? Is there anything that you can manipulate in a solution to make it have a lower boiling point and higher melting point, and cause the liquid phase have a small temperature range? Also, how is vapor pressure affected by hydrogen bonding and molecular mass?

Thanks!!

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[tdod]

Impurities: you are correct
- lower MP (b/c the impurity ****s up the solid's lattice)
- raise BP (b/c the impurity reduces water's partial vapor pressure, b/c the impurity itself contributes to the vapor pressure of the system; higher vapor pressure in the system causes higher BP)

H-bonding, however, increases MP and BP; h-bonded molecules are held together strongly, and therefore are less likely to separate (separation IS what happens in melting and boiling)


I had to look up the effect of molecular mass http://www.chem.sc.edu/faculty/shimizu/333/Chem_333/2b.i.html . What I can tell you from reading that is that molecular mass itself does NOT affect MP or BP. However, higher mass molecules typically have higher MP and BP, b/c they have more surface area for h-bonding and Van Der Waals forces. However, if the increased mass increases the kinks, making a shape that is non-ideal for these forces, then it can also lower MP and BP.

I hope this helps.

 

[sunflower18]

Impurities: you are correct
- lower MP (b/c the impurity ****s up the solid's lattice)
- raise BP (b/c the impurity reduces water's partial vapor pressure, b/c the impurity itself contributes to the vapor pressure of the system; higher vapor pressure in the system causes higher BP)

H-bonding, however, increases MP and BP; h-bonded molecules are held together strongly, and therefore are less likely to separate (separation IS what happens in melting and boiling)


I had to look up the effect of molecular mass http://www.chem.sc.edu/faculty/shimizu/333/Chem_333/2b.i.html . What I can tell you from reading that is that molecular mass itself does NOT affect MP or BP. However, higher mass molecules typically have higher MP and BP, b/c they have more surface area for h-bonding and Van Der Waals forces. However, if the increased mass increases the kinks, making a shape that is non-ideal for these forces, then it can also lower MP and BP.

I hope this helps.

Aaaahh yes it does!!! Thank you so much! This makes a lot of sense, and I was having a hard time rationalising the trends.

So unbranched, unkinked chains have higher MP and BP?

 

[tdod]

Aaaahh yes it does!!! Thank you so much! This makes a lot of sense, and I was having a hard time rationalising the trends.

So unbranched, unkinked chains have higher MP and BP?

yup

a nice graphical representation can be found here: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/lipids.htm

 

[gettheleadout]

I had to look up the effect of molecular mass http://www.chem.sc.edu/faculty/shimizu/333/Chem_333/2b.i.html . What I can tell you from reading that is that molecular mass itself does NOT affect MP or BP. However, higher mass molecules typically have higher MP and BP, b/c they have more surface area for h-bonding and Van Der Waals forces. However, if the increased mass increases the kinks, making a shape that is non-ideal for these forces, then it can also lower MP and BP.
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I hope this helps.

Incorrect, increasing molecular mass alone increases boiling point. http://adsabs.harvard.edu/abs/2000JChEd..77..732B

It's intuitive when you consider that particles of greater mass will have greater inertia and thus require more energy to attain gaseous behavior.

 

[sunflower18]

Incorrect, increasing molecular mass alone increases boiling point. http://adsabs.harvard.edu/abs/2000JChEd..77..732B

It's intuitive when you consider that particles of greater mass will have greater inertia and thus require more energy to attain gaseous behavior.

Does higher molec mass also mean higher MP?

 

[gettheleadout]

Does higher molec mass also mean higher MP?

Should, the molecular inertia issue still applies to liberating molecules from the solid phase into liquid.