Chemistry Help on question from Old AP!?

Question

Its number 8 from the 2001 free response, which I have reproduced below:
Account for the following observations
a)Even though NH3 and CH4 have similar molecular masses, NH3 has a much higher boiling point (-33C), than CH4 (-164C).

b)At 25 C, and 1 atmosphere, ethane (C2H6) is a gas, and hexane (C6H14) is a liquid

c)Si melts at a much higher temperature (1410 C) than Cl2 (-101C)

d)MgO melts at a much higher temperature (2852C) than NaF (993C)

thanks

Answer 1

a.) Hydrogen bonding in ammonia, in which the slightly positively charged H's are attracted to the slightly negatively charged N's. The slight charges result from different electronegativities of N and H.

b.) The higher molecular weight of hexane, 86 versus 30. There is no hydrogen bonding in either compound.

c.) Silicon is not just single atoms sitting around, but a dense network of Si's, each of which is bonded to four other Si's. Cl2 has just two Cl's bonded together per molecule.

d.) Mg and O are +2 and -2 and attract one another more strongly than Na abnd F at +1 and -1.

Answer 2

AP Chemistry and AP Biology are perfect classes for the Health science careers. Physics not so much. I was, like you, trying to decide between those two classes. I had to ask myself: Which class is more related to my career? What class am I better at? And I chose to do AP Biology. Since you're a sophomore, is there any possibility you can take the other class (besides the one you chose) senior year? That will make the choice a bit easier. I suggest maybe doing AP Bio junior year and AP Chem (which uses both Bio and Chemistry) senior year. AP Biology is not quite as hard as AP Chemistry. Colleges typically put more pressure on the junior year of high school, so taking the easier alternative is a good option so that your GPA/class rank won't be messed up.

Answer 3

Having accounted for the observations, I am still confused as to what the question is. There is no question in there, only observations.

All thou, based on the observations I can see the point they where trying to get at. Electronegativity trends.

The stronger the Electronegativity difference between the bonds, more they will stick together, thus will have a higher boiling and melting points.

Get it?

Electronegativity trends listed and explained here:
http://en.wikipedia.org/wiki/Electronegativity#Electronegativity_trends

Now what is the question?


.