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1500 questions
27
votes
3 answers
There's an absolute zero, is there an "absolute hot"?
This might be more of a physics question, but is there a ceiling on how hot things can get?
What happens at this temperature?
Melanie Shebel
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27
votes
1 answer
Why do people often capitalize element names?
According to IUPAC rules, names of chemical elements should not be capitalized. See Wikipedia’s take on the issue:
According to IUPAC, chemical elements are not proper nouns in English; consequently, the full name of an element is not routinely…
F'x
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27
votes
1 answer
How can there be decimal subscripts in a molecular formula?
While learning about how batteries work I have encountered the following notation for a Li-ion cathode: $\ce{Li_{0.5}FePO4}$.[1]
According to Wikipedia, the subscript number in a reaction equation represents the number of atoms in a molecule. But…
Physther
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27
votes
1 answer
Addition of hydrogen bromide to 1,3-butadiene: thermodynamic and kinetic control
The reaction of one equivalent of hydrogen bromide with 1,3-butadiene gives different products at under different conditions:1
The addition of hydrogen chloride also gives rise to similar products.2 I can see that the low-temperature product is…
orthocresol
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27
votes
3 answers
A comprehensive list of theoretical approximations that are used in computational chemistry
I am having trouble recognizing all the approximations that are used in computational chemistry. I would like to start an answer list (similar as to the list of resources for learning chemistry) that addresses this question.
I will try to answer…
CoffeeIsLife
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27
votes
2 answers
What is antisymmetric exchange? What is J-strain? Where does it come from?
I'm reading a paper1 by Sanakis, et al. that characterises the magnetic coupling in the $\ce{Fe3S4}$ clusters present in bacterial ferredoxin II and beef heart aconitase as arising through something called 'antisymmetric exchange'. This looks like…
Richard Terrett
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27
votes
2 answers
What does this lambda–sigma notation mean?
I am currently reading the CRC Handbook of Phosphorus-31 Nuclear Magnetic Resonance Data by John C. Tebby (CRC Press, 1991) and on several figures (ex. pages 9 to 14) there is a caption that reads along these lines
$\ce{^{31}P}$ NMR chemical shifts…
Canageek
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27
votes
4 answers
Why don't equivalent hydrogens cause splitting in NMR?
When doing NMR spectroscopy, it is an observed fact that equivalent hydrogens do not split one another. Why don't equivalent hydrogens split each other's signals? For example, why is the NMR spectrum for ethane a singlet instead of a quartet or even…
Niels Kornerup
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27
votes
6 answers
Is it true that all of the physical laws and mathematics needed to describe chemistry are known?
This question comes from a rather famous quote by Paul Dirac which goes like this,
The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the…
jheindel
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27
votes
2 answers
Why does benzaldehyde not respond to Fehling's test?
All aldehydes are supposed to respond to Fehling's test. So why doesn't benzaldehyde respond to it?
Does it have something to do with the fact that benzaldehyde is an aromatic compound? If so, then how?
user25546
27
votes
2 answers
What is the difference between D and L configuration, and + and −?
What is the difference between D and L configuration, and + and −?
My textbook says they are two different things.
It also says that the correct way to name glucose is D(+)-glucose.
Could someone please explain what D, L and +, − represent, and…
Mahathi Vempati
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27
votes
2 answers
How to rationalise the resonance structures and hybridisation of the nitrogen in a conjugated amine?
I was given the first structure, and then drew the other 5 resonance structures:
First of all, are they correct? ChemBioDraw had some complaints, but as far as I can see there's the same number of electrons, and no valence orbitals exceeding…
user1160
27
votes
3 answers
Why are the C-C bonds in benzene closer in length to double bonds than single bonds?
The $\ce{C-C}$ bonds in benzene are $\mathrm{140~pm}$ long which puts them in between single bonds and double bonds (given at $\mathrm{147-154~pm}$ and $\ce{134-135~pm}$ respectively, sourced from here and here) as expected. However, since the two…
bon
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27
votes
1 answer
Are long oxygen molecules possible?
Are large oxygen containing molecules possible? Either large rings, or chains with hydrogen atoms at the ends. Like this:
$\ce{H-O-O-O-O-O-O-O-O-O-$\cdots$-O-O-O-O-O-H}$
Cano64
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27
votes
2 answers
Molecular orbital (MO) diagram for N2 and N2^-
I have been taught that the MO diagram is different for molecules with 14 or less electrons than the one used for molecules with 15 or more electrons.
For $\ce{N2}$ the orbitals in increasing energy are:
$$\sigma_\mathrm{1s} < \sigma^*_\mathrm{1s}…
user17987
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