Melting Point and BP of Organic Molecules

[gabe85]

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I know this has been discussed, but I just want to clarify because Kaplan pisses me off (I may be wrong though 😀).

Kaplan says "boiling point is much like melting point", I dunno about that one?

Okay, melting point vs boiling point.

High melting point is correlated with symmetrical structures that pack together, right? If it has hydrogen bonding does that increase melting point? Which has priority, branching or hydrogen bonding?

High boiling point is correlated with less molecule branching, large molecular weight, and number of hydrogen bonds, right?

Thanks everyone for your input. Any easy ways to memorize would be good too!

 

[cyrano]

You're right. Kaplan is very wrong on that issue. I brought it up in class and my teacher agreed with me that BP is quite different from MP. Sounds to me like you got it down.

 

[mx41]

Kaplan is kinda right

High boiling point/high melting point = similar cuz they take more heat to break up the molecules. they have stronger inter/intramolecular forces.

 

[doc toothache]

You are reading too much into the statement. The concept simply stated is that molecules with stronger intermolecular forces have higher boiling point/or melting points than those with weaker forces.

 

[tdkyun]

branching will 'sandwich' the liquid phase, it lowers boiling point while increasing melting point

 

[gabe85]

Thanks Toothache that's what I figured.

 

[doc3232]

I agree with dr t.
You really shouldn't think about this concept too much or you will get lost.
Keep it simple. (general rule)

 

[dvduplex]

I am a little confuse with that concept. According to kaplan, they say that a "increase in bp" = "increase in mp" and vise vesa. But there are some occasion where an "increase in mp" = "a decrease in bp", so when will I know that?

Also, could someone tell me about the vapor pressure too?

Thanks!

-dvduplex

 

[americanpierg]

I am a little confuse with that concept. According to kaplan, they say that a "increase in bp" = "increase in mp" and vise vesa. But there are some occasion where an "increase in mp" = "a decrease in bp", so when will I know that?

Also, could someone tell me about the vapor pressure too?

Thanks!

-dvduplex

increase in mp = decrease in bp only occurs when you're dealing with anything that stabilizes the liquid phase. i think this only happens when dealing with solutions. add a solute, and it decreases the vapor pressure of the pure solvent (assuming its at a low enough temp where the solute doesnt vaporize) = higher bp, while also stabalizing the liquid phase by disrupting the crystalization required to enter the solid phase = lower mp.