41

Physics, Astronomy & Cosmology / Re: What is the weakest sound that can be detected?

« on: 07/10/2019 20:57:20 »

Thanks but I didn't say by human ear.

Ok, then I think the most sensitive detectors we have for vibrations are those that are looking for gravitational waves (LIGO and VIRGO), which can detect minuscule vibrations. (I can't find the numbers now, and gotta run--but I will say that they are the equivalent of using a seismometer to hear a mouse fart on the other side of the globe.)

Found it: One example is 10⁴⁹ Watts at the source, but the source is 410 Mpc away, or 1.26x10²⁵ meters away. (so not as faint as I thought...)

42

Chemistry / Re: Why are most molecules in singlet state if the triplet state is lower energy?

« on: 01/10/2019 16:46:56 »

Molecules adopt the lowest energy state they can, subject to laws of thermodynamics (zero point energy and boltzmann distribution) and QM (Pauli exclusion principle). (sometimes it can take a while for a metastable state to decay into the most stable state--especially when a change in spin state is involved)

The energy of an electronic state involves contributions from electron-nucleus interactions (energy shell, n), electron angular momenta (L and m_L) and electron-electron interactions (electrostatic repulsion and exchange energy).

For molecules with an odd number of electrons, there will always be at least one unpaired electron (doublet).

For molecules with an even number of electrons the choice beetween singlet and triplet (or higher) will be determined by the interplay of the aforementioned contributions. Typically differences in orbital energy due to n, L, and m_L are sufficiently large to favor the singlet state. However, if there are multiple orbitals with the same energy (degenerate) or very close, then the energy due to exchange can lead to triplet (or higher) states being favored. (essentially, if there are multiple electrons in degenerate orbitals, it is slightly favorable for their spins to align, leading to a high spin state)

Subtle effects in molecular geometry or composition (breaking or forming degeneracies) can thus lead to more stable high spin states, or more stable low spin states.

O₂ is an example of a simple molecule that is more stable in the triplet state (having two electrons in two . degenerate π* orbitals--singlet oxygen can be produced by either chemical or photochemical methods, and is incredibly reactive).

Ni(II) atoms have 8 d electrons, which favor a singlet state when the ligands around the Ni ion are in a square planar arrangement, but favor a triplet state when the ligands adopt a tetrahedral or octahedral arrangement. Elements near the middle of their respective rows in the periodic table an form very high spin states because they have half-filled orbitals (like Fe(III) which can adopt a sextet state with five parallel unpaired electrons, or Gd(III) which can have an octet state with seven parallel unpaired electrons)

43

Physiology & Medicine / Re: Why Is It Easier To Complete A Puzzle After A "time-out" ?

« on: 13/08/2019 15:56:29 »

It is also quite possible that you are still working on solving the problem subconsciously.

44

Chemistry / Re: What is the proton number in NH3 and NH4+?

« on: 24/07/2019 17:31:14 »

If you add a proton to the actual nucleus of a chloride ion (which has 17 protons and 18 electrons), it becomes an argon atom (with 18 protons and 18 electrons). However, if you don't combine nuclei, and just "stick" the proton "in" the electron cloud of the chloride it forms a molecules of hydrogen chloride (which has 18 electrons surrounding a nucleus with 17 protons and a nucleus with 1 proton.)

Cl^– + H⁺ → HCl (chemical reaction)
Cl^– + H⁺ → Ar (nuclear reaction)

45

Chemistry / Re: What is the proton number in NH3 and NH4+?

« on: 24/07/2019 03:49:05 »

Like I said in another of your threads, the number of protons in a given element never changes. Nitrogen always has 7 protons, regardless of what molecule it is in or what its electric charge is. The number of protons is what defines the element. If the number of protons was different, it would no longer be nitrogen.

I also agree with you but I am confused about why N got positivie charge on it. As I know positive charge means proton. one positive charge means one extra proton. could you explain it, please?

Yes, protons have positive charge. But, as has been pointed out many times, changing the number of protons in the nucleus changes the identity of the element. Instead, electrons are exchanged between atoms. Any atom (or molecule) with an equal number of protons and electrons in it will be neutral, any atom (or molecule) with more electrons than protons will be negatively charged (magnitude of charge scales linearly with the difference is), and any atom (or molecule) with fewer electrons than protons will be positively charged (again magnitude of charge scales linearly with the difference is.)

Note that it is also possible to add one or more protons to an atom (or molecule) without changing the element. each proton just has to be its own nucleus, ie a H⁺ ion. Chemists and biologists will often talk about the "protonation state" of a species, meaning how many of theoretically exchangeable protons are still on there.

46

Chemistry / Re: How to find sigma and pi bond?

« on: 23/07/2019 12:09:55 »

The COOH part has a pi bond between the C atom and one of the O atoms. That is the fourth.

47

Chemistry / Re: In this reaction How many electrons carbon anion has?

« on: 22/07/2019 16:53:53 »

H⁺ has no electrons (just one proton--this reaction is a "protonation")
There are a total of 10 electrons in CH₃^– (6 from the C atom, 1 for each of the 3 H atoms, and 1 extra electron for the negative charge.) Two of these electrons form the inner "core" of the carbon atom (a full 1s orbital), and the remaining 8 electrons are all in the "valence." We can think of there bing 2 electrons in each of the 3 C-H bonds, and another 2 electrons as a lone pair on C.

When the H⁺ comes close to the electrons in the lone pair, it will be attracted electrostatically, and will get closer and closer until it is trapped and "bonded." The product molecule, CH₄ is neutral and has 10 electrons (the proton didn't bring any electrons with it.) The mechanism can be represented by a single curved and double-headed arrow, pointing from the lone pair of the methyl anion (CH₃^– and going to the proton (H⁺).

48

Chemistry / Re: How many protons and electrons of C and Cl are in this compound?

« on: 22/07/2019 15:38:18 »

Every H atom has 1 proton and 1 electron
Every C atom has 6 protons and 6 electrons
Every Cl atom has 17 protons and 17 electrons

These are all conserved when the atoms come together to form (neutral) molecules.

49

New Theories / Re: How will buckyballs fired at a double slit behave?

« on: 17/06/2019 21:09:23 »

Not everyone on this forum is an expert, certainly not in all topics discussed here. However, some of the members posting in this thread (including myself) are very well educated in QM. You would do well to try to understand what we are saying. Based on the types of assertions and confusions you (pittsburghjoe) appear to be making, and the great degree of confidence you appear to be displaying, I suspect that you currently reside somewhere near the top of "Mt. Stupid" (https://www.theengineeringmanager.com/growth/mount-stupid/)


Screen Shot 2019-06-17 at 4.06.15 PM.png (127.11 kB . 990x690 - viewed 4190 times)

Don't worry, it is possible to scale back down (and to the right side), but it's a very disconcerting climb...

50

Physics, Astronomy & Cosmology / Re: Can tunneling work with gravitational potential wells?

« on: 15/06/2019 12:34:43 »

Tunneling amplitude is proportional to e^{–width of barrier}

https://en.wikipedia.org/wiki/Quantum_tunnelling

even empty space can be viewed as a barrier through which to tunnel.

Electric potentials within molecule-scale systems can have very high gradients, so the distances involved are small.

For gravitational wells, gradients are typically not very strong (except in the hypothetical cases mentioned by evan_au), so the distances are likely to be such that tunneling can only happen once every million universe lifetimes or so (not actually calculated, but you get the idea)

51

New Theories / Re: How will buckyballs fired at a double slit behave?

« on: 14/06/2019 03:53:18 »

Mass has little to do with how big a slit something needs to get through... (so the proposed experiment can neither confirm nor challenge the notion that waves can have mass--spoiler alert, waves can have mass)

...but there is a somewhat nifty way to see that mass is important for calculating tunneling barriers for particles (which depends on the wave function treatment of the particles):

We can measure the rates of chemical reactions, and see how much they change when hydrogen atoms are switched for deuterium (chemically identical isotope, but with twice the mass). This is called the kinetic isotope effect (or KIE). Sometimes a hydrogen atom has to tunnel from one place to another for the reaction to occur, and sometimes (especially when the reaction is very cold) this is the rate determining step, which means that we can directly measure how long (on average) it takes for protons to tunnel from x to y, and compare with hoe long it takes deuterons to tunnel from x to y. When this tunneling is the rate determining step, the KIE is very large, sometimes even slowing down by a factor of 100 when doubling the mass of the tunneling particle. This means that mass is an important property of a quantum object, even when considering it as a wave.

https://en.wikipedia.org/wiki/Kinetic_isotope_effect#Tunneling

52

General Science / Re: Can you make a huge prism, and disperse light in different colors?

« on: 04/06/2019 02:45:57 »

I just found this reference online: https://www.nnin.org/sites/default/files/files/Karen_Rama_USING_CDs_AND_DVDs_AS_DIFFRACTION_GRATINGS_0.pdf

The CD peeling methods might be of interest to those, likeJMerrill, who wish to incorporate rainbows in artwork, and the equations shown might be of interest to those, like Colin2B, who wish to examine the mechanism of forming those rainbows--it turns out that one can even calculate the spacing of the ridges with some accuracy using a laser and a ruler (I am certain the error bars would drop if one used multiple lasers, each with a different wavelength...

53

Chemistry / Re: Does magnesium sulphate react with hydrochloric acid?

« on: 01/06/2019 21:48:41 »

And thanks @chris and @Colin2B 

54

Chemistry / Re: Does magnesium sulphate react with hydrochloric acid?

« on: 31/05/2019 15:10:50 »

In aqueous solution, there is no difference between a solution containing magnesium chloride and sulfuric acid, and a solution containing magnesium sulfate and hydrochloric acid, each just contains Mg²⁺ ions, H⁺ ions, Cl^– ions, and a mix of SO₄^2– and HSO₄^– ions (depending on pH).

Trying to precipitate out MgCl₂ selectively from this solution would be difficult, but not impossible (though yield and purity will be traded off--ie high purity probably only possible with very small yield, and high yield only possible with low purity).

With a sufficiently high ratio of chloride to sulfate, the solution could be cooled to selectively precipitate out MgCl₂. The ratios needed can be derived from looking at their respective K_sp values:
MgCl_2(s) Mg²⁺ + 2 Cl^– K_sp = 738 M³
MgSO_4(s) Mg²⁺ + SO₄^2–  K_sp = 4.7 M²

So if a starting solution has a 1:1 molar ratio of Mg²⁺ SO₄^2–, one can calculate just how much chloride would need to be added to favor precipitation of the chloride salt over the sulfate salt.

Based on the K_sp, saturated MgSO₄ is about 2.1 M. Plug that in to the MgCl₂ K_sp expression, and you will find that the concentration of chloride needed is about 18.7 M. The solubility of HCl in water maxes out at about 12 M, so you would need to cool and/or pressurize the system to accomplish this (I don't recommend actually trying this!)

Alternatively, you could add 1 equivalent of CaCl₂ to the solution, which will precipitate out as CaSO₄, which can be filtered off, and then evaporation of the water from the solution will leave MgCl₂.


I hope this isn't homework...

55

General Science / Re: Can you make a huge prism, and disperse light in different colors?

« on: 29/05/2019 20:15:39 »

The dispersion obtained from a CD (or similar) is due to diffraction. This can be verified by shining light with different spectral makeup on it and looking at the reflected patterns. If one has access to red and/or green laser pointers, one can easily see that a single laser point generates a whole diffraction pattern (multiple bright spots), and that the spacing of the diffraction pattern is different for red (typically 600-700 nm, depending on source) and green (typically 532 nm doubled Nd:YAG). One can also look at the "rainbows" produced from light from an incandescent source (continuous), and from fluorescent or LED lighting (a few narrow bands).

56

Physics, Astronomy & Cosmology / Re: Why don't atmospheric gases separate according to specific gravities?

« on: 29/03/2019 18:58:12 »

There is a very small amount of separation, but this is largely due to the fact that most of the gases that make up the atmosphere have very similar molecular masses (N₂ 28, O₂ 32, Ar 40).

The difference in gravitational potential energy that would occur by stratifying would be less than the energy equivalent of the decrease in entropy (due to unmixing) at the temperature that the atmosphere is. Same reason that vodka doesn't spontaneously separate into water and ethanol (which is about 20% less dense than water).

57

Chemistry / Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?

« on: 07/03/2019 21:12:53 »

As far as I can tell evan_au's objection stands.

It doesn't really matter what is done with the "coal." I have worked on a few carbon sequestration projects myself--it really only works out if (a) the energy being used to capture and convert the CO₂ is 100% carbon free, and (b) if the product has value. There may be use in products from electrocatalytically reduced CO₂, but if you think of the carbon footprint required to generate the raw materials to build a CO₂-reducing plant on the scale needed to have any effect on the atmospheric composition, it is unlikely to pay for itself (in terms of net CO₂) before it breaks!

Photosynthetic organisms already capture and convert CO₂ far faster than we could hope to do it (in the next few decades, at least). Just look at the slope of the jags on the Keeling curve compared to the slope of the overall trend:

maunaloa_2004.gif (23.92 kB . 540x404 - viewed 4836 times)
We just need to focus on not burning the fossil fuels in the first place, and allowing the photoautotrophs to do the work (ie preventing deforestation, promoting reforestation, and promoting healthy oceans). As long as we can allow the downward parts of the jags to extend a little bit, and the upward parts of the hags to be a little less intense, we can return to a reasonable equilibrium pretty quickly.

There might be a tiny little nudge we can do technologically to speed the process along, but focusing money and attention on these sorts of "band-aid fixes" merely diverts it from the "severed jugular" that the actual problem is.

58

General Science / Re: Is there any science behind divining water or oil?

« on: 04/03/2019 14:59:11 »

Indeed--there is no reasonable physical mechanism to facilitate divining. There are no properly controlled studies to indicate that it works better than chance. Major corporations each spend (many) millions of dollars on devices that they use to find water and oil with--why would they opt to do that if a dude with a stick could do the same?

Divining/dowsing is fakery.

https://xkcd.com/808/

the_economic_argument.png (37.62 kB . 356x476 - viewed 3560 times)

59

Technology / Re: Why are Some Fridges Hard To Open Again Once Just Closed?

« on: 04/03/2019 14:52:23 »

I have noticed this before as well. I suspect it has to do with pressure:

Fridges don't have a perfect airtight seal, but they are pretty good.

Let's say that the fridge interior is kept at 8 °C (281 K), and the air in your kitchen is 24 °C (297 K), so there is a fairly significant difference between the two (otherwise you wouldn't need a fridge!). When you open the door, the air mixes--for the sake of argument let's say that the air inside reaches 22 °C (295 K) by the time you are ready to close it. Then when the door is closed, the mixing no longer happens, and the air inside begins to col rapidly (the walls of the fridge and all the stuff in it is still at 8 °C, and has a larger heat capacity than the newly replaced air. If the air inside then cools down to 10 °C (283 K) from 22 °C (295 K), then the pressure would drops to 96% of atmospheric pressure (283/295). Standard atmospheric pressure is 101.3 kPa, so the difference in pressure between the inside and outside would be about 4 kPa, or 4 Newtons per square meter. Thus if your fridge door has an area of 2.5 m², you would need to supply 10 Newtons of additional force to open the door (about the same as lifting a 1 kg weight). This isn't much, but it is significantly more than would be required to open the door in the first place. (There may be more at play, as this number does seem low--even increasing the temperature difference within reason only gains a factor of 2. Perhaps condensation of humidity in the air compounds the effect--the vapor pressure of water falls from about 3 kPa at 24 °C to about 1 kPa at 8 °C, so this would add another 50% to the 24-8 difference...)

If , however, instead of trying to open the door again right away, you give it a few minutes, there is enough time for the pressure to fall, and then more air is able to leak in, re-equalizing the pressures.

60

Chemistry / Re: What happens during electrolysis of salt water with gold and copper electrodes?

« on: 04/03/2019 02:50:38 »

30 V is really high for a single electrochemical cell! At these potentials you're likely going to see many different competing reactions:
• Oxidation of Cl^– to Cl₂ or ClO^– (which I now know can then react with gold metal in water--though if the chlorine gas leaves solution before reacting with gold, it won't be much use...) or even up to ClO₃^– and/or ClO₄^–
• Oxidation of water to oxygen gas or hydrogen peroxide (which will then decompose into oxygen gas)

I suspect you would have better luck using a more conductive electrolyte. Maybe try bumping up the concentration of salt by a factor of 10 (1.2 g per 250 mL). Then see what voltage you need to get the same current--I doubt you need more than 4 V to force the electrochemical reactions to occur, and the rest is just to overcome the resistivity of the solution.

If your goal is to isolate gold chloride (or chloroauric acid), you might have better luck using hydrochloric acid as your electrolyte--Even low concentrations of HCl are very conductive, and increasing the concentration of H⁺ ions means that hydrogen generation will be very fast at the copper electrode (outcompeting gold reduction). Finally, you can evaporate or boil off all of the remaining acid (carefully!), rather than trying to separate from NaCl and NaOH, (and possibly NaOCl, NaClO₃ and/or NaClO₄).

Stirring likely won't have much effect at these currents and electrode separation (trying to run high current with large electrode separation is made easier when there is mixing--otherwise current is diffusion limited)

Room temp is probably best--increasing the temperature will make it much easier to drive off chlorine gas and oxygen gas (which you don't want), and will only marginally facilitate gold dissolution.