1

Chemistry / How does paint alter the colour of a surface?

« on: 14/10/2009 18:14:12 »

I don't think the object itself changes how it reflects back light, but rather you apply a surface which reflects light differently (i.e. absorbs different wavelengths). Since you don't alter the original object chemically, then it doesn't make sense that it interacts with light in a different way.

2

Chemistry / What determines how many bubbles form in a liquid?

« on: 11/10/2009 17:03:07 »

It's been a while since I last posted, but I hope you guys can help me out!

Does anyone have any idea on how heat or concentration or how vigorous whisking might affect the formation of bubbles in a liquid? I'm trying to make the MOST bubbles with as small a size as possible - i.e. I just want foam.

So far, I've established that very hot water prevents the small foam from forming - in fact I get really few big bubbles and almost no small bubbles this way.


[MOD EDIT - GOOD TO HAVE YOU BACK - BUT PLEASE CAN YOU ENSURE THAT YOUR THREAD TITLES ARE PHRASED AS QUESTIONS, IN LINE WITH OUR FORUM POLICY. THANKS. CHRIS]

3

Chemistry / Why does water behave oddly compared to alcohols?

« on: 20/09/2009 05:08:15 »

For melting point, it might be worthwhile to consider that Water arranges itself in a lattice and, because of extensive hydrogen bonding, it should have a higher melting point than methanol which cannot form an equally nice lattice because it's missing the other hydrogen atom.

There's a uniform increase of boiling point probably because of the fact that alcohols can only form one hydrogen bond and the importance of ID-ID interactions of the alkyl chain in keeping the alcohol liquid. Having two hydrogen bonds should clearly give water an advantage over those which can only form one, so it starts out higher. However, alkyl groups contribute to what is known as ID-ID interactions which, if the molcule is large enough, can actually play a greater role than the hydrogen bond, or even overpower two hydrogen bonds. This could also explain why, after hexanol (if I remember right), alcohols are no longer miscible in water.

As for the 'kink' in the melting point curve probably has something to do with the ability of the smaller alcohols to form lattices like water but because they're missing one hydrogen bond they're considerably weaker lattices. After propanol, the lattice is replaced by packing of the side-chains which is largely an ID-ID interaction which, as the chain gets longer, the interactions get stronger.

4

Physics, Astronomy & Cosmology / Does the frequency of light change when it refracts?

« on: 13/05/2009 12:23:52 »

I was just wondering...

Refractive Index, n, is defined by

n = c / v

where c is the speed of light in a vacuum, and v is the speed in the medium.

Now, since light is a wave, we look at v = fλ.

So when light refracts, it slows down but then is it the frequency that changes, the wavelength that changes or both?

5

Chemistry / What is tautomerism ?

« on: 26/03/2009 12:10:07 »

Tautomerism is an equilibrium between two forms of the same molecule. For example:

Keto-Enol Tautomerism, which involves a Ketone interconverting into its enol (Alkene with Alcohol: ene + ol) form readily.

Wikipedia also has a decent set of examples with explanations here:

http://en.wikipedia.org/wiki/Tautomer

6

Chemistry / What are the principles of electrolysis?

« on: 23/03/2009 12:51:21 »

Since I am new to this, I do not know the details. How much voltage to apply for a certain amount of time for a measured thickness of deposit on the cathode, or what would be the best salt solution of a particular metal to use? Or would applying heat to the electrolyte solution help or hurt the process?   

The current is more important than the voltage for this, and while I'm sure you can convert that voltage into current via the equation V = IR, I'm not too sure at the moment.

Anyway, if we take silver for example:

Ag+ + e- → Ag

So basically the rate at which I deposit Silver depends on how fast this reaction happens, i.e. how much current is driven through the system. To work out how big a current I need given the amount of time I have, I need to know the following:

1. Density of Silver (ρ, 10.5 g cm^-3)
2. Surface Area to be plated (A)
3. A_r of Silver (107.9 g mol^-1)

So we could do this the long way...

Q = It (Charge = Current x Time)
N = Q/e = It/e (Number of Electrons or Silver Atoms formed = Charge / Elementary Charge)
N = Aρ/A_r (Number of Silver Atoms Required per cm = Surface Area x Density of Silver

So per cm of thickness, I need to deliver:
It/e = Aρ
I = (Aρe/A_rt) Amperes of Current.

The short way would involve using Faraday's Constant, 96500 Coulombs per Mole. The maths is similar.

7

Chemistry / Organic Synthesis.. I bet you can't explain why?!!!!!!

« on: 22/03/2009 16:08:42 »

The Thiol Group is electron donating, so it pushes electron density into the aromatic ring and more so into the Para and Ortho positions.

For the S_NAr reaction, you need a Strong Withdrawing Group Para or Ortho (1,4 or 1,2) to the leaving group to form a strong partial positive charge. So the presence of a Strong Activating Group (-SH) might actually counteract that.

But then again Wikipedia says diazonium compounds substitute following a different (S_N1) mechanism. So maybe the positive charge in the stabilized intermediate isn't enough to attract SH^-. The added stability could also be used to explain a lower E_A and thus more favourable forward reaction, so it's not really anywhere near a nail in the coffin. Alternatively, it may actually make the C-N bond stronger than it should be preventing it from leaving in the way that it should under the S_N1 mechanism.

8

Chemistry / BF3 and octet rule

« on: 20/03/2009 09:44:21 »

Also, I just realized that Aluminium Chloride has a similar 'problem' of trivalency. However, we could argue that because it is in a plane and the p-Orbitals perpendicular to this plane can all overlap, there is a degree of 'delocalization'. This may also be applicable for Boron Trifluoride but less likely because Fluorine is highly electronegative.

9

Chemistry / BF3 and octet rule

« on: 20/03/2009 09:26:43 »

The Nitrate Ion is arranged in a plane at all times as it is sp² hybridized. In that state, it is just like Boron Trifluoride. In which case Chem4Me's first answer and mine should work as well.

10

Chemistry / Organic Synthesis.. I bet you can't explain why?!!!!!!

« on: 20/03/2009 09:21:28 »

It looks to me like a Nucleophilic Aromatic Substitution Reaction. (I could be wrong about the second step because I've no idea what the steps/intermediates look like.)

It would be nice to know what steps you tried, since that says a lot about where the reaction is going or if there's something you overlooked.

What I can say is that Thiophenol is an activating group so it stabilizes positive charges at the para and ortho positions. The Diazonium ion is reactive because it his highly nucleophilic.

Unfortunately, the diazonium at the para-position is in a sense 'deactivated' leading to no further substitution by SH^-. I have no idea how you get the quinone from this, but I suspect the HNO₂ to be responsible

The Meta Position is unaffected substantially by this and so reaction can go. (if you go draw the canonical structures)

The Ortho Position I find interesting. The presence of such an electron-rich S atom (which is huge) and the bulky N₂⁺ group in such close proximity makes me believe that there should be a lot of steric strain around this area. I would go so far as to think that an S-N bond will form but obviously that doesn't happen since we're supposed to get our 1,2. At this point I'm guessing. Electrons don't stabilize the 2-position and thus still allow for a nucleophilic attack at the point? The steps would really help in drawing mechanisms to see what might actually be going on.

11

Chemistry / Re: what is Super Cooling?

« on: 20/03/2009 05:23:56 »

I wonder if this has applications in cryogenic stasis.

12

Chemistry / Why does salt added to hand-cleaner produce a gum?

« on: 20/03/2009 04:13:37 »

I would say polymerization as a result of introducing a good nucleophile (probably sodium salt of one of those things)

13

Chemistry / BF3 and octet rule

« on: 19/03/2009 08:17:35 »

Violation of the Octet Rule for Period 3 and up (or is it down based on the Periodic Table) is because of the presence of d-Orbitals, and usually the valency is in excess of 8 if I remember correctly.

Boron Trifluoride and Beryllium Chloride are both violations of this octet rule. Exactly why, I do not know but I assume it has something to do with stability. >.<

14

Chemistry / Re: To molecules or atoms have colour?

« on: 18/03/2009 11:43:41 »

The phenomenon of colour is simply the receptors on our eyes sending electrical impulses to the brain when they are struck with a certain wavelength of light. The brain simply maps this out into images and colours.

As to whether a single atom has colour, that depends on whether it absorbs any particular wavelength of energy. Read: Absorption Spectrum and Emission Spectrum

15

Physiology & Medicine / Why can we blow out hot air and cold air?

« on: 17/03/2009 03:54:56 »

The highest pressure than can be created by blowing is aproximately nothing. The chance in pressure and volume are very small. The effect of these changes on temperature are also small.
Even if you could produce a significant change in pressure the change in temperature isn't big and it depends on the gas. For air it's generally a cooling effect. For hydrogen it heats up when you let it expand ( this has been  considereed as the cause of hydrogen fires, though I'm not convinced). For an ideal gas the temperature change is zero.
Have a look here.
http://www.chemistry.mcmaster.ca/~ayers/chem2PA3/labs/2PA36.pdf
and here
http://home.sou.edu/~chapman/ch444/joulet.htm
where they point out that the temperature difference produced by a drop from 4 bar to 1 bar is 3 degrees or so.
PV=nRT isn't the right equation in this case.

Maybe it's not, but what I understand from the Joule-Thompson Effect is that it is under closed-system conditions with very good insulation. I haven't been able to find any numbers on what the maximum air pressure of blowing is, but I'm pretty sure it's not a closed system, though I think it does sound reasonable to assume that it is.

If you blow the air out as a fast stream theough pursed lips  it entrains more of the air round it. That air is generally cold.
Breathing gently onto something means that the air that reaches the object is pretty much the same air as you breathed out which is warm (and wet).
This is also why you can steam up your glassed by breathing on them then blow on them and evaporate off the moisture again.

If this is true, then if I blow fast really really close to my glasses it should fog up right? I shall go try this as soon as I get the opportunity, but I think it is indeed the case. Someone else try in the mean time?

16

Physics, Astronomy & Cosmology / How much power can I generate with a small wind turbine?

« on: 17/03/2009 03:03:51 »

What I've never understand about wind transferring energy to a turbine is this:

If the wind in front of the turbine is travelling at 30mph then the wind behind the turbine must also be travelling at the same speed or it would back up and have nowhere to go. If the wind behind the turbine is travelling as fast as the wind in front then the turbine must have taken no energy from it.

How does the energy picked up by the turbine translate to a change in wind movement?
 

I've had trouble with this too, but I think the answer goes something along the lines of the wind doesn't have the same speed before and after the turbine. After all, as the wind collides with the blade, it strikes it at an angle so that part of the energy goes into turning the blade in a certain direction and deflecting the air in another. So it's more of a conservation of momentum thing than speed.

17

Chemistry / Can A Diamond Melt ?

« on: 17/03/2009 02:51:14 »

... I have the strangest inkling he messed "isotope" up with "allotrope".

18

Chemistry / Re: Is hydrogen a good coolant?

« on: 13/03/2009 18:07:51 »

I didn't think H2 would be a good coolant, especially since it can be combusted fairly easily.

But for the sake of discussion, it may have something to do with how it is quite close to an ideal gas.

19

Chemistry / Re: Can I lower temperature by reducing volume?

« on: 13/03/2009 18:05:12 »

This is not necessarily true.

Unit mass of water at 4 degrees centigrade has a smaller volume than unit mass of water at 1 degree centigrade, if I remember right. Water is densest at 4 degrees I believe.

However, using the ideal gas law,

pV=nRT

or Charles Law (I believe)

V₁/T₁=V₂/T₂

We find that compression should actually lead to an increase in temperature. Hence to cool a gas, you'd probably want to rapidly expand it.

20

Chemistry / What dissolves in mineral oil?

« on: 13/03/2009 18:01:05 »

What have you done with Chemistry4Me?!

XD

There's actually a lot to say, but it's just an elaboration of how that rule came to be.