[CHM 112] chapter 11 questions

[Andrew J. Pounds]

On 01/26/14 14:37,
> Dr. Pounds,
> I've been reading the chapter and working the checkpoints and I got 
> most of them correct. I started working on the homework and I'm still 
> confused on a few things. I understand that compounds that have low 
> boiling points will have high vapor pressures. But I don't understand 
> how you can tell one compound has a higher boiling point than another 
> compound. I am also confused about the concepts you have to understand 
> to be able to answer number 136.
> Thank you for your help,
>

Problem 136 from Burdge and problem 14 from the chapter 11 additional 
problems are on my list to do tomorrow in class.

As far as how to tell which molecule has the higher boiling, it is ALL 
tied to the intermolecular forces involved (section 11.1 in Burdge).   
Except for blatantly obvious questions, like which has the higher 
boiling point, H$_2$ or H$_2$O, you really have to conceptualize the 
three dimensional molecular structure.  In other words, you have to 
think about the Lewis diagram and how that translates to a 3D molecular 
shape (VSEPR theory, section 9.1 in Burdge, CHM 111 stuff).  Once you do 
that then you should determine if there is a dipole moment present 
(section 9.2 in Burdge).

Here are some simple rules that I follow when comparing two molecules:

1. the molecule with the most intermolecular forces will have the higher 
boiling point.

2. if two molecules have the same number of intermolecular forces
     a) the one with the stronger intermolecular forces will have the 
higher boiling point
     b) if comparing polarity, the molecule with the more 
electronegative atom will have the higher boiling point
     c) if all of the forces are the same, then the heavier molecule 
will have the higher boiling point


I just took a look at the book and sample problem 11.1 on page 481, the 
subsequent three practice problems, and numerous end of chapter review 
problems (11.1 - 11.21) do a good job driving these points home.

-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627
http://faculty.mercer.edu/pounds_aj

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