General Chemistry Questions

[ssjsike]

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Hello, as I am going through the general chemistry section of my TPR book, I have come across a few bits of information that I do not understand.


1. Covalent bonds- according to the book, a covalent bond is created for each unpaired valence electron. A halogen has 1 unpaired valence electron, group VI has 2 unpaired electrons, group V has 3 and so on. This made sense to me until I saw the lewis dot structure for SO2 (sulfur dioxide). Its lewis structure consisted of [O-S=O <-> O=S-O]. This does not make sense to me because I was under the impression that Sulfur is a group VI atom that only has two unpaired valence electrons. How then does it make three bonds?

2. Of the following, which one will have the lowest melting point?
A. MgO
B. CCL4
C. Cr
D. HF

According to our TPR instructor, melting point and boiling point are attributed to three factors:
-branching on a chain decreases the melting point and boiling point
-increased molecular weight increases the melting point and boiling point
-hydrogen bonding increases the melting point and boiling point

The answer says that it would be B. This does not make sense to me because it has such a high molecular weight (152). I understand that HF would exhibit hydrogen bonding. However, my inclination was that MgO would have the lowest melting point due to its molecular weight of 40.

That is all for now. Thank You.

 

[puravida85]

Sulfur in sulfur dioxide is hypervalent, meaning it does not obey the octet rule and can therefore surpass it for various reasons. For the MCAT, I'd stick with the idea that it has two unpaired electrons.

MgO is an ionic solid at room temperature (used in ochem lab to soak up H2O), while CCl4 is a non polar liquid at room temperature (also used in ochem lab as a non polar solvent). If you hadn't come across them in lab, then think about this:

The intermolecular forces between MgO are dipole dipole interactions because of the polarity differences between Mg2+ and O2-. On the other hand, CCl4 is a non polar molecule because of its geometry, and it only experiences london dispersion forces. In most cases and for the MCAT:

Hydrogen Bond> Dipole Dipole > London Dispersion

Edit: And BTW, it's good to consider molecular weight but I believe it is more efficient in cases where the intermolecular forces are equal (both have permanent dipoles or both are nonpolar).

 

[addo]

The intermolecular forces between MgO are dipole dipole interactions because of the polarity differences between Mg2+ and O2-. On the other hand, CCl4 is a non polar molecule because of its geometry, and it only experiences london dispersion forces. In most cases and for the MCAT:

👍
Also Cr is a metal at room temp so thats out of the question, and HF also has dipole dipole interactions, and hydrogen bonding.

 

[ssjsike]

hello, thank you all for your help. Would an oxygen on a molecule (other than hydrogen) automatically cause one of you to suspect dipole dipole interactions?

 

[Phantastic]

hello, thank you all for your help. Would an oxygen on a molecule (other than hydrogen) automatically cause one of you to suspect dipole dipole interactions?

Oxygen is a very electronegative element. Any time you see Hydrogen on the phone (get it? FON, flourine, oxygen, nitrogen), think hydrogen bonds, which are the strongest intermolecular interaction, and will contribute most to the MP/BP increase.

Most of the time with MCAT, when a MP/BP question comes up (even in disguise), the underlying science concept will involve distinguishing polarities (much more so than chain length, etc.)

 

[DrknoSDN]

Found this 4 year old thread because I was looking to find confirmed Errata. Apparently everyone 3 years ago did not decide the book was wrong.

Just to confirm. TPR GenChem Example 4-23:
Of the following, which one will have the lowest melting point?
a) MgO
b) CCl4
c) Cr
d) HF

Book explanation:
"Almost all ionic compounds are solids at room temperature. Therefore, choice A is eliminated. Similarly, all metals except for mercury (Hg) are solids at room temperature so eliminate choice C. Both answers B and D will be molecular solids. Hydrogen fluoride is able to hydrogen bond and will therefore have stronger intermolecular interactions than the nonpolar solvent carbon tetrachloride. Since choice B has the weakest intermolecular forces (London dispersion), it will be easiest to melt."

I'm just going to say this is blatantly wrong, and anyone who comes across this problem down the road, I hope you find this thread and pat on the back yourself for questioning things that seem wrong.
CCl4 MP = -22.9 C
HF MP = -83.6 C <== Lowest MP, molecule is very small.

Whoever wrote this doesn't know enough about chemistry. CCl4 was a very commonly used nonpolar liquid solvent (BP = 76 C), before they realized it caused cancer. Hope this helps future MCATers. GL

Sources:
http://en.wikipedia.org/wiki/Carbon_tetrachloride
http://en.wikipedia.org/wiki/Hydrogen_fluoride
Tags: Princeton General Chemistry page 82 pg82 Example 4 23

 

[Teleologist]

Sulfur in sulfur dioxide is hypervalent, meaning it does not obey the octet rule and can therefore surpass it for various reasons.

Sulfur generally doesn't expand its octet (it's not generally hypervalent).

 

[Teleologist]

Just to confirm. TPR GenChem Example 4-23:
Of the following, which one will have the lowest melting point?
a) MgO
b) CCl4
c) Cr
d) HF

Book explanation:
"Almost all ionic compounds are solids at room temperature. Therefore, choice A is eliminated. Similarly, all metals except for mercury (Hg) are solids at room temperature so eliminate choice C. Both answers B and D will be molecular solids. Hydrogen fluoride is able to hydrogen bond and will therefore have stronger intermolecular interactions than the nonpolar solvent carbon tetrachloride. Since choice B has the weakest intermolecular forces (London dispersion), it will be easiest to melt."

I'm just going to say this is blatantly wrong, and anyone who comes across this problem down the road, I hope you find this thread and pat on the back yourself for questioning things that seem wrong.
CCl4 MP = -22.9 C
HF MP = -83.6 C <== Lowest MP, molecule is very small.

This is just a bad question in general. If you're gonna compare intermolecular forces and relate it to boiling point then don't throw so many curveballs at the test-taker. The explanation is also crap. Every single choice listed above exhibits London dispersion forces; it's not just choice B exhibiting London dispersion forces. Using the fact that London dispersion forces tend to be weaker than hydrogen "bonding" among molecules, then one would likely choose CCl4 as the answer. Unfortunately the answer is instead HF. Size may have something to do with this; London dispersion forces tend to increase with surface area (and therefore generally molecular mass). E.g. CH4 has a much lower BP than CCl4.