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.

Filter by
Sorted by
Tagged with
179 votes
8 answers
163k 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 ...
user avatar
  • 1,907
119 votes
8 answers
28k 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 ...
user avatar
  • 18.1k
58 votes
7 answers
573k 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 ...
user avatar
  • 3,076
55 votes
6 answers
11k 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.
user avatar
52 votes
4 answers
373k 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? ...
user avatar
49 votes
3 answers
4k 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 ...
user avatar
48 votes
6 answers
10k 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 ...
user avatar
47 votes
2 answers
16k 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 ...
user avatar
  • 18.1k
41 votes
6 answers
10k 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 "...
user avatar
  • 1,599
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 ...
user avatar
  • 1,071
38 votes
10 answers
7k 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 ...
user avatar
  • 475
38 votes
3 answers
32k 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 ...
user avatar
  • 729
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 ...
user avatar
  • 505
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 ...
user avatar
35 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, ...
user avatar
35 votes
2 answers
28k 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 ...
user avatar
  • 1,113
34 votes
1 answer
9k 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 ...
user avatar
  • 2,687
34 votes
1 answer
653 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-...
user avatar
  • 35.8k
33 votes
2 answers
173k 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?
user avatar
  • 921
33 votes
5 answers
16k 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 ...
user avatar
  • 443
33 votes
1 answer
3k 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 ...
user avatar
  • 4,559
32 votes
3 answers
133k 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 ...
user avatar
31 votes
4 answers
30k 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 ...
user avatar
31 votes
2 answers
29k 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:...
user avatar
  • 538
30 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): $\...
user avatar
  • 577
30 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?
user avatar
30 votes
4 answers
54k 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 ...
user avatar
  • 3,076
29 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?...
user avatar
  • 3,000
29 votes
1 answer
18k 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)...
user avatar
  • 1,023
28 votes
3 answers
13k 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 ...
user avatar
28 votes
2 answers
4k 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{...
user avatar
28 votes
2 answers
8k 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 ...
user avatar
  • 483
27 votes
3 answers
25k 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 ...
user avatar
  • 14.9k
27 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 ...
user avatar
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 ...
user avatar
26 votes
4 answers
21k 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 ...
user avatar
  • 525
26 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?
user avatar
  • 6,364
26 votes
1 answer
6k 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 ...
user avatar
  • 896
26 votes
4 answers
7k 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 ...
user avatar
26 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 ...
user avatar
25 votes
4 answers
19k 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\...
user avatar
  • 1,413
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 ...
user avatar
  • 2,926
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 ...
user avatar
25 votes
5 answers
17k 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 ...
user avatar
25 votes
2 answers
56k 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 ...
user avatar
25 votes
5 answers
243k 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{...
user avatar
  • 2,256
25 votes
1 answer
2k 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)...
user avatar
  • 64.5k
25 votes
2 answers
409 views

Why does a mixture of siloxene and cerium(IV) sulfate luminesce?

I performed an experiment where siloxene and cerium(IV) sulphate were mixed together: when I did so the mixture produced an orange-yellow glow. Why does it glow? What is it about the two chemicals in ...
user avatar
24 votes
3 answers
4k views

How do they ensure there is one electron on an oil drop in Millikan's oil drop experiment?

Since air is ionized using X-rays, in the 'observation chamber', there must be several electrons in the chamber, so the oil drops are exposed to several electrons so it seems intuitive that a single ...
user avatar
24 votes
4 answers
28k views

How does oil on the surface of water prevent rust?

I distinctly remember a side-by-side comparison from a book where there are two nails submerged in water, in two beakers: one nail had a layer of oil on top of the water, and that nail didn't rust; ...
user avatar
  • 1,172

1
2 3 4 5
89