BP and MP

[DrRoyal Pains]

Just a quick check, but branching DECREASES BP but INCREASES MP? Because I have usually seen that branching decreases melting point because it has less surface area. I've seen it so many ways that Im confusing myself..ahh

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[Demps]

Just a quick check, but branching DECREASES BP but INCREASES MP? Because I have usually seen that branching decreases melting point because it has less surface area. I've seen it so many ways that Im confusing myself..ahh

Yes

 

[DrRoyal Pains]

I ask because on one of Chad's problems he says that branching decreases the MP because it decreases the surface area? But I thought branching increases the MP because it is more able to form a crystal lattice?

 

[Adamsswag]

I ask because on one of Chad's problems he says that branching decreases the MP because it decreases the surface area? But I thought branching increases the MP because it is more able to form a crystal lattice?

That is correct. A molecule more branched resembles that of a crystal lattice and so there would be a higher melting point (crystal lattice --> liquid). A higher boiling point would be seen in a less branched molecule where surface area is greater.

 

[eyeball87]

Just a quick check, but branching DECREASES BP but INCREASES MP? Because I have usually seen that branching decreases melting point because it has less surface area. I've seen it so many ways that Im confusing myself..ahh

Branching reduces attractive forces between molecules, and thus it lowers both melting point and boiling point.

I'm taking this from the MCAT Princeton review... so this is correct

 

[Adamsswag]

http://en.wikipedia.org/wiki/Alkane#Branched_alkanes

"A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules."

MP has some variation, but for the purpose of solving DAT problems stick with the general rule: The more branching, the greater the MP.

I'd trust wikipedia over any PR course

 

[eyeball87]

http://en.wikipedia.org/wiki/Alkane#Branched_alkanes

"A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules."

MP has some variation, but for the purpose of solving DAT problems stick with the general rule: The more branching, the greater the MP.

I'd trust wikipedia over any PR course

The quote taken from wikipedia says the same thing as my post from from the princeton review book. The quote given says nothing about melting point.

Let's see if somebody else can jump in

 

[DrRoyal Pains]

Alright, so straight chain alkane = Higher BP due to greater surface area and lower MP.
Branched alkane = lower BP due to less surface area and higher MP because of it's ability to form a crystal lattice?