Questions tagged [physical-chemistry]
The study of chemical systems using the laws and concepts of physics. This usually requires the techniques of thermodynamics, statistical mechanics and quantum mechanics.
4,742
questions
183
votes
8
answers
171k
views
Can an atom have more than 8 valence electrons? If not, why is 8 the limit?
According to some chemistry textbooks, the maximum number of valence electrons for an atom is 8, but the reason for this is not explained.
So, can an atom have more than 8 valence electrons?
If ...
- 1,947
125
votes
8
answers
31k
views
Why is absolute zero unattainable?
We were dealing with the Third Law of Thermodynamics in class, and my teacher mentioned something that we found quite fascinating:
It is physically impossible to attain a temperature of zero ...
- 18.6k
62
votes
7
answers
593k
views
Positive or Negative Anode/Cathode in Electrolytic/Galvanic Cell
In a galvanic (voltaic) cell, the anode is considered negative and the cathode is considered positive. This seems reasonable as the anode is the source of electrons and cathode is where the electrons ...
- 3,206
56
votes
4
answers
382k
views
Why is it important to use a salt bridge in a voltaic cell? Can a wire be used?
I was learning about voltaic cells and came across salt bridges. If the purpose of the salt bridge is only to move electrons from an electrolyte solution to the other, then why can I not use a wire?
...
- 787
56
votes
6
answers
12k
views
The last element's atomic number
I was just thinking what can be the last atomic number that can exist within the range of permissible radioactivity limit and considering all other factors in quantum physics and chemical factors.
- 1,254
51
votes
3
answers
5k
views
Unit of the equilibrium constant: contradiction of Bridgman's theorem?
The following equation is standard in thermodynamics:
$$
\Delta G^\circ=-RT\log(K)
$$
where $K$ is the equilibrium constant. In dimensional analysis, Bridgman's theorem tells us that the argument ...
- 613
49
votes
6
answers
11k
views
Why does wood burn but not sugar?
Fundamentally, they're both carbohydrates, although the cellulose in wood is essentially polymerized glucose, which combined with its isomer fructose forms sucrose.
So why does wood readily burn ...
- 614
48
votes
2
answers
18k
views
What volume does one mole of an ideal gas occupy?
This has been bugging me for a while now...
Obviously, to calculate the volume/space occupied by a mole of (an ideal) gas, you'll have to specify temperature ($T$) and pressure ($P$), find the gas ...
- 18.6k
43
votes
10
answers
8k
views
Why is chemistry unpredictable?
Disclaimer: I am not a chemist by any means, and I only have knowledge limited to what I learned in my university's Chemistry III course. Basic understanding of everything up to valence electron ...
- 533
40
votes
6
answers
11k
views
How does ammonium nitrate explode on its own?
I thought ammonium nitrate was an oxidizer that needed to be mixed with fuel to form a high explosive (e.g., ANFO). But apparently there have been accidental explosions involving just the "...
- 1,623
40
votes
3
answers
5k
views
Why is fresh ice sticky?
Fresh ice cubes are almost instantly sticky and easily cling on to fabric and other similarly rough surfaces.
A few minutes later, however, the effect almost completely disappears.
What is the cause ...
- 1,091
39
votes
3
answers
40k
views
What is known about liquid carbon?
The wiki tells me that if you heat carbon at atmospheric pressure it eventually turns directly into a gas without being liquid first. At what pressure can you make liquid carbon? Has anyone actually ...
- 739
38
votes
4
answers
9k
views
What exactly is happening when sodium, potassium, or other alkali metal explodes in water?
There are lots of videos on YouTube showing sodium, potassium, etc. exploding when dropped into water (this, for example).
I understand that when an alkali metal is exposed to water, a violent ...
- 513
38
votes
2
answers
29k
views
What exactly is hydrogen bonding (and does it really need fluorine, oxygen, or nitrogen)?
I'm not satisfied with the rationale for the intermolecular attraction known as hydrogen bonding. In my book, it states that
Hydrogen bonding is a special type of intermolecular attraction between ...
- 1,193
37
votes
5
answers
19k
views
Why don't the electrons move through the electrolyte (instead of the circuit) in a galvanic cell?
I was learning about galvanic cells and I had a problem understanding why electrons do not travel through the electrolyte solutions themselves, instead preferring to travel through metals. Can ...
- 483
37
votes
1
answer
782
views
Single-molecule magnet with electrically-controlled permeability: How does the Titan Shield from Deus Ex work?
In a recent installment of Deus Ex game series there is an augmentation called "Titan Shield" (it has nothing to do with $\ce{Ti}$ element):
A neodymium skin underlay matrix built of nano-...
- 37.4k
36
votes
3
answers
1.2m
views
What are the maximum number of electrons in each shell?
In my textbook, it says that the maximum number of electrons that can fit in any given shell is given by 2n². This would mean 2 electrons could fit in the first shell, 8 could fit in the second shell, ...
- 597
36
votes
4
answers
4k
views
Does any known substance ignite on cooling?
As the title says, I'm interested in knowing if there is any substance — or combination of substances — that ignites (or even increases its chance of spontaneous ignition) when cooled.
I've never ...
- 463
35
votes
1
answer
4k
views
Is there an energy cost associated with flipping the spin of an electron?
THE STORY:
A common example used to illustrate the limitations of restricted Hartree-Fock (RHF) theory is the H$_2$ dissociation energy ($D_e$) curves. RHF enforces electrons to be paired into spin ...
- 4,609
34
votes
2
answers
186k
views
What would be the effect of the addition of an inert gas to a reaction at equilibrium?
Why does a dissociation reaction shift to the right with the addition of an inert gas?
- 979
34
votes
1
answer
10k
views
When is it true that more nodes equals higher energy?
Consider all the MOs of some isolated molecule. (It could be a single
atom too; I'll use MO to refer to AOs as well.) Number them in
increasing order of the number of nodes (node = surface where the ...
- 2,717
32
votes
4
answers
65k
views
Why are solids and liquids not included in the equilibrium constant? What about in a reaction rate calculation?
Take for instance the reaction
$$\ce{H2(g) + I2(s) <=> 2HI(g)}$$
The equilibrium constant would not include the solid $\ce{I2}$, but why is this? I have read that its concentration is a ...
- 3,206
32
votes
2
answers
34k
views
What is the pKa of the hydronium, or oxonium, ion (H3O+)?
Although the wikipedia page on Hydronium indicates a $\mathrm{p}K_\text{a}$ of −1.74, I noticed in the discussion of this page that the subject seems debated (cf. http://en.wikipedia.org/wiki/Talk:...
- 587
31
votes
3
answers
140k
views
What are angular and radial nodes?
Nodes are the points in space around a nucleus where the probability of finding an electron is zero. However, I heard that there are two kinds of nodes, radial nodes and angular nodes. What are they ...
- 651
31
votes
4
answers
33k
views
What roles do neutrons play in an atom?
An atom typically consists of electrons, protons, and neutrons. Electrons are negatively charged and participate in bonding to stabilize the atom.
Conversely, protons are positively charged and ...
- 401
31
votes
2
answers
55k
views
What is the unit of pH?
I'm making some graphs and I have to label the axes. I want to be extra careful and put the units in even though the meaning of $\text{pH}$ is well known. But I have a problem (though a simple one): $\...
- 587
30
votes
1
answer
20k
views
Electrical conductivity of graphite
On this Wikipedia page, the electrical conductivity of various materials are given in the third column ($\sigma \text{ (S/m) at 20}^\circ \text{C}$). I am interested in the entry for Carbon (graphite)...
- 1,043
29
votes
3
answers
5k
views
Why don't gases escape Earth's atmosphere?
Some gases are lighter than others and rise. Why don't they continue going up, leave the atmosphere, and then enter outer space?
- 523
28
votes
3
answers
15k
views
Why isn't the American nickel magnetic?
I never thought that modern American nickels actually contained nickel anymore. However, according to this wiki article, the coins actually do contain 25% nickel, the rest being copper. And yet, no US ...
- 1,298
28
votes
2
answers
5k
views
How to find the second order perturbation to wave function?
Today, I'm looking for how to find the 2nd perturbation to the base in Rayleigh Schrödinger Perturbation Theory (RSPT).
SETUP
Starting from the 2nd order perturbation in Dirac's notation:
\begin{...
- 2,580
28
votes
2
answers
2k
views
Why is snow white?
I know that this is a rather ambiguous question; but my question is, whenever we take water and freeze it in the freezer, it still tends to stay clear. Since snow is just frozen water, why is it white?...
- 3,050
28
votes
2
answers
13k
views
Confusion about direction of dipole arrow in alpha-helices and other molecules
I understand that molecular dipoles are electric dipoles. And electric dipole moment vectors point from the negative to the positive charge.
In class we learned to draw these special molecular dipole ...
- 627
27
votes
5
answers
21k
views
Why does water evaporate spontaneously at room temperature despite ΔG > 0?
Standard Gibbs free energy of formation of liquid water at $\pu{298 K}$ is $\pu{−237.17 kJ mol-1}$ and that of water vapour is $\pu{−228.57 kJ mol-1}$ therefore,
$$\ce{H2O (l) -> H2O (g)}\qquad\...
- 1,483
27
votes
3
answers
2k
views
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?
- 6,490
27
votes
4
answers
8k
views
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 ...
- 2,338
27
votes
3
answers
27k
views
Why does ice cream make soda fizz?
I've noticed that adding a chunk of ice cream to soda makes the soda fizz slightly near the soda-ice cream interface. I thought it was a physical effect due to the temperature, but adding ice has no ...
- 15.1k
27
votes
2
answers
9k
views
Can magnetic fields affect a chemical reaction?
This question was asked recently in an interview and I just said that "Yeah, like if the reaction involves ions or paramagnetic species". But the interviewer went on and asked me to elaborate on HOW ...
- 493
27
votes
1
answer
2k
views
While filling electrons, we follow Aufbau principle, but not while removing them. Why is this so?
I recently came across a question Why is the vanadium(3+) ion paramagnetic?, where the asker is wondering how $\ce{V^{3+}}$ is paramagnetic (he used Aufbau in reverse to remove the electrons), while ...
27
votes
2
answers
1k
views
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 ...
- 10.8k
26
votes
4
answers
22k
views
Hypothetical: What happens to water as pressure increases to infinity? [duplicate]
I've asked a similar question here but the answer given shows the behaviour of water under general conditions.
I'd like to know what the behaviour of water is like as pressures increase towards ...
- 535
26
votes
1
answer
7k
views
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 ...
- 904
26
votes
4
answers
4k
views
Relation between chemical kinetics and chemical equilibrium
In my chemistry book, the law of chemical equilibrium is derived from the law of mass action:
For a reversible chemical reaction $$\ce{aA +bB\rightleftharpoons cC + dD}$$ where $a$, $b$, $c$ and $d$ ...
- 5,637
26
votes
2
answers
62k
views
How does a lone pair of a central atom affect the dipole moment?
Dipole moment is the degree of polarity, i.e. the seperation of positive and negative charges. But I am not getting the intuition why and how lone pairs affect the polarity and dipole moment. I cannot ...
user5764
26
votes
5
answers
254k
views
Why do branched chain compounds have lower boiling points than the corresponding straight chain isomers?
The branched chain compounds have lower boiling points than the corresponding straight chain isomers. For example,
$\ce{CH_3CH_2CH_2CH_2CH_3}$ -
No branching-Pentane (n-pentane) ($\mathrm{...
- 2,326
26
votes
1
answer
3k
views
When can a molecule be considered freely rotating at room temperature?
This question sparked from a long discussion in chat about the nature of $\ce{H2O2}$ and whether that molecule can be considered to rotate around the $\ce{O-O}$ axis (and hence display axial chirality)...
- 67.3k
26
votes
3
answers
11k
views
Solutions of Group 1 and Group 2 metals in Ammonia
When we add Group-1 and Group-2 metals to liquid ammonia, they dissolve to form metal cations and solvated electrons.
$$\ce{M + NH3(liq) -> M+ + e-}$$
Now, when the G-1 solutions evaporate, we get ...
- 1,132
25
votes
6
answers
1k
views
Why are equations of state for a non-ideal gas so elusive?
The ideal gas equation (daresay "law") is a fascinating combination of the work of dozens of scientists over a long period of time.
I encountered Van der Waals interpretation for non-ideal gases ...
- 2,957
25
votes
3
answers
8k
views
Why shouldn't Uranium come in contact with water?
In the documentary about Chernobyl, it was mentioned that Uranium should not come in contact with water, otherwise an explosion occurs. What is the reason for that? What kind of reaction makes it ...
- 353
25
votes
5
answers
18k
views
Why does radium have a higher first ionisation energy than barium?
I'm wondering why radium appears to buck the general trend that first ionisation energies decrease as you move down a group in the periodic table: barium (the group 2 element preceding it) has a first ...
- 383
25
votes
1
answer
3k
views
What is the mathematical basis behind the Jahn-Teller effect?
Both first-order and second-order Jahn-Teller distortions play a very important role in chemistry.
It is often said that the Jahn-Teller effect is based on symmetry arguments, and hence nothing can ...
- 71.1k