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Author Topic: Trieste's Chemistry Conundrum  (Read 1661 times)

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Offline TriesteTopic starter

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Trieste's Chemistry Conundrum
« on: September 16, 2008, 07:51:51 PM »
It's no secret that I struggle with chemistry, despite the fact that I adore science and find these things fascinating.

Lucky for me that there are so many chem geeks here. ^_^

Been working on bonding. "Which of the following compounds would you expect to be ionic, and which nonionic? Give a simple electronic structure for each, showing only valence electrons (Lewis dot structure)."

The first few I have done are as follows ... I just want to make sure I'm on the right track. The prof will give the answers out later, but I'm impatient and don't have time to waste waiting for him.

a) KBr       . .
           K : Br : covalent (the underlining is 2 electrons)


b) H2S     . .
          H : S : H  covalent

c) NF3       . . . . . .
               :F: N :F: ionic
                   :F:

d) CHCl3    . .  H  . .
               :Cl :C: Cl:       ionic between C-Cl, covalent between H-C
                   :Cl:

... do I have that about right? My guide is that when sharing the valence electrons would bring it to noble configuration, they do that, but if snatching will give both atoms noble configuration, they do that instead. That's ... what I've managed to infer, anyway.

Offline Vekseid

Re: Trieste's Chemistry Conundrum
« Reply #1 on: September 16, 2008, 10:44:18 PM »
I'm pretty sure Potassium Bromide and Hydrogen Sulfide are ionic. Both are entirely composed of simple anion-cation pairs. CHCl3 is completely covalent, I believe.

The idea behind an ionic bond versus a covalent bond is that in an ionic bond, the anion rips one or more electrons from the cation, rather than sharing them in a covalent orbital.

Carbon on its own, for example, does not tend to form ionic bonds, though it can be part of larger molecules that can form ionic bonds.

Offline TriesteTopic starter

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Re: Trieste's Chemistry Conundrum
« Reply #2 on: September 17, 2008, 11:23:03 AM »
I understand the difference between an ionic and a covalent bond... What I'm not understanding is how to determine what compounds form ionic bonds and what compounds form covalent bonds. Why?

Like, NaCl is ... ionic or covalent? And how can I predict that if I'm looking at the lewis structures of the atoms? It's a concept I was never able to get a handle on last year, and I'm trying to do so now.

Offline Storiwyr

Re: Trieste's Chemistry Conundrum
« Reply #3 on: September 17, 2008, 11:37:52 AM »
Trieste--

Ionic bond vs. Covalent bond has to do with electronegativity. Both kinds of bonds involve the valence electrons of the atoms involved, the question is, which one wants those electrons more? The greater the difference between the electronegativities of the atoms, the more likely it is that the bond will be Ionic rather than Covalent.

The best way to compare and decide if a bond is Ionic or Covalent (for me anyway) is to compare the positions of the bonded atoms on the periodic table. The further apart two atoms are on the periodic table, the more likely they are to be ionic. So looking at your examples, KBr is ionic, as potassium and bromide are on opposite sides of the periodic table. Bromine needs an electron to get to noble gas configuration and is very electronegative, potassium has an extra above noble gas configuration and thus is not very electronegative at all. Bromine will be able to strip that electron from potassium.

Hydrogen Sulfide actually kind of breaks this rule, it's compositional atoms are on opposite sides of the periodic table, but it is considered covalent rather than ionic--similar to water. I don't know how much you've learned about acids? But Hydrogen Sulfide is a weak acid. It does not dissociate entirely in water, while the strong acids do, and most are considered ionic.

NF3 contains two atoms from the same side of the periodic table. It's going to be covalent, since the difference in electronegativities will be rather negligible.

As Veks said, Carbon is one thing you'll essentially never see as an ion. It will exist as an ion for brief periods ONLY as a reaction intermediate, and you'll be seeing that later in the year. But ionic bonds will dissociate from each other in solution, and carbon just won't do that. Not when it would be left with nothing but hydrogens to balance its charge, anyway. Methyl chloride is covalent.

To be honest, though ... that's a pretty mean question. There's not really any set easy formula for figuring out ionic vs. covalent ... it's sort of a gut instinct based on experience with chemistry.

Basically, if you know it'll dissolve in water, conduct electricity, and is present in a solid form ... it's probably an ionic compound.

Offline Cherri Tart

Re: Trieste's Chemistry Conundrum
« Reply #4 on: September 17, 2008, 01:11:21 PM »
*accidently reads chem questions, has terrifying flashbacks to HS, hyperventilates and passes out in the hall*

Offline Vekseid

Re: Trieste's Chemistry Conundrum
« Reply #5 on: September 18, 2008, 09:13:29 AM »
Hydrogen Sulfide actually kind of breaks this rule, it's compositional atoms are on opposite sides of the periodic table, but it is considered covalent rather than ionic--similar to water. I don't know how much you've learned about acids? But Hydrogen Sulfide is a weak acid. It does not dissociate entirely in water, while the strong acids do, and most are considered ionic.

Bah, I can't be right all the time : /

Anyway, it's important to remember that Hydrogen is not a true alkali metal, though it's considered to tend to that over being a halogen, especially at high temperatures. It's position on the periodic table is sometimes displayed on its own.

Offline TriesteTopic starter

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Re: Trieste's Chemistry Conundrum
« Reply #6 on: September 18, 2008, 11:15:05 AM »
I just went ahead and made an appointment with a school tutor on this one. Nothing against you guys, but I'm getting all sorts of answers and it's only confusing me more...

I'm going to be taking calc next fall... and physics... I'mma bug the hell out of you people then, too. *dies*

For now, what's stumping me is bond angles...

Offline Storiwyr

Re: Trieste's Chemistry Conundrum
« Reply #7 on: September 18, 2008, 11:26:16 AM »
I just went ahead and made an appointment with a school tutor on this one. Nothing against you guys, but I'm getting all sorts of answers and it's only confusing me more...

I'm going to be taking calc next fall... and physics... I'mma bug the hell out of you people then, too. *dies*

For now, what's stumping me is bond angles...

*snuggles* I'm sorry, Trieste. :( It's been a long time since Calc, but I CAN help with Physics for sure. :) Hell, I'll scan and e-mail you my notesheets I used for the exams if you want. XD

And I promise, as you study this year for Organic, you'll start to recognize what's ionic and what's not just by looking at the molecular formula. Like ... I would have known without a periodic table that KBr was ionic, because I know that K+ and Br- are pretty common ions to see. It will get easier!

What's got you about bond angles?

Offline TriesteTopic starter

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Re: Trieste's Chemistry Conundrum
« Reply #8 on: September 18, 2008, 12:24:29 PM »
Just need to memorize them, I think. Like, I have a structure... H3C-O-H with the lone pairs shown above and below the O, asking what the expected COH angle is in degrees... and then asking the hybridization of Oxygen, which is probably sp3 but I'd have to double check my notes on that one.

On the plus side, I actually UNDERSTAND orbitals, pi/sigma bonding, and hybrids this semester, which I never, never did even with several HOURS of tutoring last fall. Thank god.

Offline Neroon

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Re: Trieste's Chemistry Conundrum
« Reply #9 on: September 18, 2008, 02:24:16 PM »
Just poking my nose in here (a bit of professional interest, as it were) I'd note the following rules of thumb.

Groups  I and II never (as close as dammit) make covalent compounds.

Group VII tends to make ionic compounds, though it can make covalent ones.

Hydrogen is correctly on its own in the periodic table, being unique in its ability to get a stable ec by either gaining one electron or losing one. 

Any compound that is liquid or gaseous at room temperature is covalent.

When in doubt look up the electronegativities: if they are similar, it's probably covalent.

Covalent bonds with large dipole moments (as that in hydrogen chloride) can ionise in the presence of a suitable polar solvent.  So HCl dissolves in water and ionises to become hydrochloric acid.

So applying these rules, I'd say that:

KBr     Ionic, K+ and Br- (classic example used to determine the charge on an ion by electrolysis of the molten salt)

H2S    Covalent (smelly gas)

NF3    Covalent (similar electronegativities, plus it's a gas)

CHCl3  Covalent (Chloroform is a volatile liquid)

In methanol, I'm fairly sure the hybridisation of the orbitals in the oxygen is sp3, giving a COH angle of 109.5 degrees, but it is about five years since I had to teach any of this stuff, so I could be wrong there.

Hope this helps,

N.

Offline TriesteTopic starter

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Re: Trieste's Chemistry Conundrum
« Reply #10 on: September 18, 2008, 06:29:41 PM »
*looks left*

*looks right*

*packs up Neroon and tucks him into her backpack to take with her to orgo tomorrow*

If my tutor and/or professor confirms (I've been trying to understand this stuff for about 2.5 semesters now) what you're talking about, I'm going to keep you as a study tool... right next to my tinker-chemicals (what I call my organic modeling set).

Just lettin' you know.

Offline Neroon

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Re: Trieste's Chemistry Conundrum
« Reply #11 on: September 19, 2008, 05:20:14 PM »
*peers out from within the backpack*

Glad to be of help, though to be honest, I'm better at Physical Chemistry.

Best,

N.

Offline Storiwyr

Re: Trieste's Chemistry Conundrum
« Reply #12 on: September 19, 2008, 09:25:02 PM »
*peers out from within the backpack*

Glad to be of help, though to be honest, I'm better at Physical Chemistry.

Best,

N.

*grins* Oh great. If I ever need to know P. Chem again, I'll bother you.

I far prefer Organic.

Offline Oniya

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Re: Trieste's Chemistry Conundrum
« Reply #13 on: September 20, 2008, 12:24:51 AM »
I'll poke in with the advanced maths.  I still have all of my college math text books.

Offline Storiwyr

Re: Trieste's Chemistry Conundrum
« Reply #14 on: September 20, 2008, 02:29:27 AM »
Yay Calculus! I actually really liked Calculus, but I took it ages ago. :(

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Re: Trieste's Chemistry Conundrum
« Reply #15 on: September 20, 2008, 09:51:47 AM »
<_<;  Actually, since I took Calc I in high school (and then again my freshman year by accident), it's been *cough* quite a while for me too.  I'm just one of those supposedly rare ladies with a knack for the numbers.

Offline TriesteTopic starter

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Re: Trieste's Chemistry Conundrum
« Reply #16 on: October 24, 2008, 07:27:55 PM »
Grah. Dienes are apparently my achilles heel, along with resonance structures, which tend to confuse the crap out of me unless I write it all out.

So if I have 1,3-dimethylcyclohexa-1,3-diene that I'm reacting with HCl, where's it go? Markovnikov has failed me. ;_; How the heck do I figure out what's stable? They all look stable.

Offline Neroon

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Re: Trieste's Chemistry Conundrum
« Reply #17 on: October 25, 2008, 06:34:36 PM »
I believe that the structure is this:



The trick is- and don't quote me, I'm a crap organic chemist- to treat each double bond in isolation and then I think Markovnikov's rule should work well.  So carbons 1 and 3 have the fewest hydrogens attached (none as far as I can see) so the chlorine from the HCl will attach there and the hydrogens should go to carbons 2 and 4.

Thus, the result should be 1,3-dichloro-1,3-dimethylcyclohexane (with some 1-chloro-1,3-dimethylcyclohexane mixed in if the reaction is not taken to completion).

Of course, I could be wrong.

Offline TriesteTopic starter

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Re: Trieste's Chemistry Conundrum
« Reply #18 on: October 25, 2008, 07:39:06 PM »
Nein, the double bonds go in the direction of the locant numbers. I had the original structure; I just translated it from picture to IUPAC. I don't know why that site calls that'un 1,3-dimethylhexa-1,3-diene. Under how I've been taught, that'd be 2,4-dimethylhexa-1,3-diene.

Odd.

That said, I believe I was able to figure it out; resonance does a crazy thing to dienes - conjugated ones in specific - such that it has a tendency to pull a 1,2 addition on one of the double bonds and leave the other one intact, or it pulls a 1,4 addition and the remaining double bond moves to C2-C3. Note that the locants I just gave were from the perspective of the diene itself, so if you have something like 2,4-hexadiene, it would actually be a 2, 3 addition with the remaining double bond between C4 and C5 and a 2, 5 addition with the double bond moving to C3-C4, respectively. I think; I didn't draw it out so hopefully my locations are correct.

Hopefully that did not make your eyes cross.

Have you ever looked at your desk and thought with quite a bit of exasperation, "I need more goddamn desk space"?

Offline Neroon

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Re: Trieste's Chemistry Conundrum
« Reply #19 on: October 26, 2008, 05:40:00 AM »
I said I was crap at organic! *grins*

One of the reasons I dislike it is that I see no underlying unified theory upon which I can hang the mass of knowledge onto.  The one thing that I can remember is that the IUPAC names for cyclic compounds are a nightmare, with the decision having to be made, do you go clockwise or anticlockwise around the ring?

Give me Physical Chemistry any day!

Offline TriesteTopic starter

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Re: Trieste's Chemistry Conundrum
« Reply #20 on: October 26, 2008, 10:46:23 AM »
Clockwise or anticlockwise are so relative that I've never been too worried about it; after all, 1,3-cyclohexadiene is 1,3-cyclohexadiene whether you go clockwise or counter clockwise. It's just a matter of turning the ring around and looking at it from the back or the front (or top or bottom). That said, yes, there are an awful lot of rules... just like there are an awful lot of organic compounds.

I like organic better than inorganic because organic is largely concerned with electrons. The orbitals, the resonance structures, the mechanisms, all of the absorbance testing  ... it's all about finding where those electrons are. So in a sense, it's all probabilities and as long as you have an idea of how electrons behave, you can make a guess as to where it'll be. If you're right, blammo. If you're wrong, then you get to learn about a new interaction. I find that it's becoming a little on the intuitive side.

In inorganic, I made As, but the only thing I really liked was thermodynamics. Because, really, who doesn't like burning stuff?

Offline Storiwyr

Re: Trieste's Chemistry Conundrum
« Reply #21 on: October 26, 2008, 11:16:09 AM »
I said I was crap at organic! *grins*

One of the reasons I dislike it is that I see no underlying unified theory upon which I can hang the mass of knowledge onto.  The one thing that I can remember is that the IUPAC names for cyclic compounds are a nightmare, with the decision having to be made, do you go clockwise or anticlockwise around the ring?

Give me Physical Chemistry any day!

If there's more than one substituent, you go whichever direction puts the second one on the lowest numbered carbon. :)

Clockwise or anticlockwise are so relative that I've never been too worried about it; after all, 1,3-cyclohexadiene is 1,3-cyclohexadiene whether you go clockwise or counter clockwise. It's just a matter of turning the ring around and looking at it from the back or the front (or top or bottom). That said, yes, there are an awful lot of rules... just like there are an awful lot of organic compounds.

I like organic better than inorganic because organic is largely concerned with electrons. The orbitals, the resonance structures, the mechanisms, all of the absorbance testing  ... it's all about finding where those electrons are. So in a sense, it's all probabilities and as long as you have an idea of how electrons behave, you can make a guess as to where it'll be. If you're right, blammo. If you're wrong, then you get to learn about a new interaction. I find that it's becoming a little on the intuitive side.

In inorganic, I made As, but the only thing I really liked was thermodynamics. Because, really, who doesn't like burning stuff?

I agree. I'm totally an Organic Chemistry person. It became pretty intuitive, and I love drawing out reaction mechanisms. They remind me of one of few things I really loved doing in math ... proofs. They're logical progressions with lots of arrows and pretty looking molecules that take you from reactant(s) to product(s).

P. Chem and I did not get along. Too many numbers I didn't care about. ;)

Offline Neroon

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Re: Trieste's Chemistry Conundrum
« Reply #22 on: October 26, 2008, 11:25:18 AM »
How things have changed: for me the role of electrons in bonding and thermodynamics were both part of phys chem.  I still remember going to on my first ever lecture at university and being presented with the derivation of Schrodinger's wave equation in three dimensions. That I could cope with, but the endless cauldron bubbling of organic never did it for me.

Give me the certainty of the calculations of physical chemistry, even if they can seem arcane, and I'm a happy little weasel.