Hot answers tagged physical-chemistry
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No, the elements of the periodic table don't form any representation of a group or, more precisely, any irreducible representation. Even more precisely, the real insights by Mendeleev – that the reactivity etc. is a repeating function of the atomic number – doesn't follow from any property of a representation that could be derived by group theory.
The ...
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I found two web pages that explain the phenomenon quite well, and even looks into the misconceptions people have.
The candle flame heats the air in the vase, and this hot air expands. Some of the expanding air escapes out from under the vase — you might see some bubbles. When the flame goes out, the air in the vase cools down and the cooler air ...
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Bonds - lots of bonds
Chemical reactions take in energy to break bonds and give off energy when they make bonds.
Big organic molecules like those in petrol have lots of weak carbon-carbon and carbon-hydrogen bonds which don't take a lot of energy to break.
But when it burns the combustion products make lots of very strong Carbon-Oxygen and Hydrogen-Oxygen ...
8
This is actually a more complex question than you might think, because the distinction between mass and energy kind of disappears once you start talking about small particles.
So what is mass exactly? There are two common definitions:
The quantity that determines an object's resistance to a change in motion, the $m$ in $\sum F = ma$
The quantity that ...
8
Although there is no known group representation which encapsulates all the properties of the periodic table, there are, however, attempts to gain a representation theoretical understanding of the periodic table at least qualitatively and there are recent works
mainly by M. Kibler in this direction, please see the following two articles ...
7
235U is a nonrenewable resource in that when we use it in weapons or power plants it is not regenerated (and eventually after a long series of decay steps, mostly ends up as lead). However, there is a not entirely negligible amount being extruded by mid-oceanic rifts and volcanos, so there will continue to be a small supply that becomes physically reachable ...
7
A simple material will not undergo a liquid to solid transition as the temperature is raised. When you see this it means somthing more complicated than a simple phase transition is going on.
In the example of egg white, what you are seeing is denaturation of the protein albumin. The heat causes the protein to lose its tertiary structure then form cross ...
6
It is an interesting question, I will take a stab at it. Hopefully my answer will be generic enough to remain plausible.
First of all the ISM (Interstellar Medium) which collapsed to form the sun would not likely have been well mixed. It would be some sort of turbulently mixed mixture of recent SN remnants, and the molecular cloud with which it presumably ...
6
Dear Wade, your good question is easily answered if you consider "pressure" to be a derived quantity, and let us derive it.
An average molecule (or atom) of an ideal gas - and your proposition only holds for an ideal gas - has kinetic energy equal to
$$mv^2/2=3kT/2$$
It's because every degree of freedom carries $kT/2$ and there are three degrees of fredom ...
6
The first estimate of Avogadro's number was made by a monk named Chrysostomus Magnenus in 1646. He burned a grain of incense in an abandoned church and assumed that there was one 'atom' of incense in his nose at soon as he could faintly smell it; He then compared the volume of the cavity of his nose with the volume of the church. In modern language, the ...
6
The divide is actually not between covalent and ionic, but rather a spectrum between localised and delocalised electrons. The history of all this is actually quite fascinating, and Phil Anderson in his book "More and Different" has a nice chapter on this. Essentially, around the time that people started doing quantum mechanics on molecules seriously, there ...
6
There does seem to be a lot of mythology around about the "grape in a microwave" experiment. I have never see any publications on the subject in a respectable journal, however from chatting to other scientists there seems to be a consensus about what happens.
It's all rather boring really. The grape is the right size (about a quarter wavelength) and shape ...
5
To deduce this, You have to specify the kind of decay and the nature of the "compund"
is it a crystal, a small molecule in gas phase, a organic material?
Beta decay shifts the nucleus one position upward in PSE,
thus any "compound" will be transformed into a cation by
loss of an electron, and whre say a iodide Ion had been,
there will be an Xe atom. ...
5
Of course they are related, the Fermi-Dirac chemical potential is defined to enforce the exclusion principle inside a thermodynamic framework rather than having to apply it with a separate mechanism.
Consider three systems, an ideal gas, a van der Waals type gas, and a Fermi gas. At a fixed volume and energy, ask "How much energy is required to add another ...
5
The photons of light, especially ultra violet photons from sunlight, carry energy that can break chemical bonds.
The bonds in chemicals that make up colours, ie. dye molecules, are most affected - since they create the colour effect by absorbing some photons (of one colour) and not others.
So they have chemical bonds of the correct energy to absorb ...
5
a great question. First, a detail: a person needs something like 2,000 kilocalories a day. At least that's how the unit is used today; in the past, the modern kilocalorie used to be called a calorie. Today, 1 kcal equals 4.182 kilojoules.
Ideally, one could measure the kilocalories by direct calorimetry, i.e. in a bomb calorimeter. However, that would also ...
5
Although the question has been partially answered, there is a superb reference on this topic which will certainly give you some of the deep, and not so deep insights needed to understand the answer to this question.
http://pubs.acs.org/doi/abs/10.1021/ed068p110
Nevertheless, both the contraction of the s(1/2) orbitals predicted by the Dirac equation, and ...
5
Some more misconceptions:
The ChemGuide website quoted above might be a useful reference for "UK-based exam purposes" as stated there, but it certainly does not help in solving the question. The arguments given above that followed the comments on ChemGuide are inaccurate. A simple quantum chemistry calculation of gold in its ground state will give you that ...
5
A bound system will indeed have a lower mass than it's constituent free parts. The binding energy is usually understood to be the energy it would take to separate the bound system to free constituent parts. Then we would say $M_{tot} = M_1 + M_2 - E_{bind}/c^2$ Or we could by convention make the binding energy a negative number and say it "increases" mass ...
5
I will add an interesting flavor of physics to spice up Martin's excellent chemistry answer and which is in line with your original reasoning on how to trace energy content.
When chemical reactions take place and bonds form, break, and reshape, the atom nuclei don't change in any way. What happens is that the electrons jump between atoms or groups of atoms ...
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The Cabrera-Frank theory is strictly speaking valid for crystals only. Quasicrystals – which were awarded by the Nobel prize – are strictly speaking not crystals. Crystals, by definition, have to be periodic while quasicrystals are not. So details of the theory for crystals of course don't have to hold for quasicrystals. But there are still many similarities ...
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There is an earlier question on this site that addresses essentially the same issue: Conversion of mass to energy in chemical/nuclear reactions. As written in the answers there, the amount of energy that is lost or gained in a chemical reaction is roughly 10 (or more) orders of magnitude smaller than the mass of the participating molecules.
I took a look ...
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Yes, quantum mechanics – even non-relativistic quantum mechanics for several electrons orbiting nuclei – fully, quantitatively, and comprehensively explains all of chemistry (including biochemistry and, in fact, biology). This fact has been known since the late 1920s.
To understand the periodic character of the properties of the elements, one must realize ...
5
While quantum mechanics explains the gross features of the periodic system, many fine details of the periodic table of elements are computable numerically from various approximations to QED, but are conceptually ill understood. See, e.g.,
Eric R. Scerri,
How Good Is the Quantum Mechanical Explanation
of the Periodic System?
Journal of Chemical Education ...
5
According to http://ga.water.usgs.gov/edu/earthhowmuch.html the total volume of water on the earth is $1.386\times 10^9$km$^3$, which is about $1.4 \times 10^{21}$kg (I'm rounding because I don't know the average temperature and therefore density of the water).
According to http://en.wikipedia.org/wiki/Biomass_(ecology)#Global_rate_of_production the annual ...
5
A battery is basically just a chemical reaction. At the negative (cathode) end of the battery the reaction releases electrons while at the positive (anode) end of the battery the reaction consumes electrons. As long as the external circuit allows electrons to flow from the cathode to the anode the reaction goes and the battery generates power.
If you break ...
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It's not often that dmckee and I differ (mainly because he's usually right :-) but we differ on this on. Or at least we differ if I've correctly understood what you're asking.
In a hydrogen atom the 1s, 2s, etc wavefunctions are (subject to various approximations) good descriptions of the single electron and have well defined angular momentums. In ...
4
Gold, as a metal, is, like an noble gas, not very reactive. But if you take a single gold atom it is very reactive. It combines with other gold atoms to form gold metal, and it reacts chemically with other atoms. For example, the Colorado, US town of Telluride is named after a gold-tellurium mineral found there, see Calaverite:
...
4
good theory
how about a test
my niece just did 3 trials each on 2 cups of water and varied the number of tablespoons of salt
0 and 1 tablespoons boiled at about 10.5 minutes
2 tablespoons boiled at about 9.3 minutes
3 about 7.5 minutes
and 4 boiled at about 6 minutes.
and now she wants to know why she got those results
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More like the latter than the former, although you've got one detail wrong. An $s$ orbital is $\ell=0$, so it corresponds to $L=0$, not $L=\hbar$. A $p$ orbital is $\ell=1$, corresponding to $L=\hbar\sqrt{2}$. If the electron is in a state that is a superposition of $s$ and $p$, then a measurement of angular momentum will yield either 0 or $\hbar\sqrt 2$, ...
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