Sciencemadness Discussion Board

Reactions of touching solids

CyrusGrey - 21-3-2008 at 21:37

When I was ballmilling dry CuO with chrome plated ball bearings, I noticed that the balls were getting plated in a very thin layer of copper metal. The mill was dry, but there could have been a little moisture in the mixture.

Does contact between two perfectly dry solids usually allow any appreciable reaction to occur?

I am thinking that prolonged milling of two dry solids would allow a reaction between them to proceed to completion. What do you guys think? I would try a few reactions but I just ruined my ball bearings somehow when grinding zinc oxide.

UnintentionalChaos - 21-3-2008 at 21:54

Reactions when milling can occur...usually this is only an issue with highly reactive materials...Ive seen a preperation of Magnesium diboride involving ball milling magnesium metal power together with boron trioxide for a few days under argon.

12AX7 - 21-3-2008 at 22:38

Such reactions have been known since antiquity -- for instance, grinding cinnibar in a copper mortar coats it with a silver layer of mercury.

I seem to recall there's a special name for this reaction.

Tim

microcosmicus - 21-3-2008 at 23:03

Also, remember that when the balls collide, there is considerable
pressure at the point where the ball impacts the oxide and, for the
brief duration of the impact, a relatively large amount of energy is
being introduced into a tiny volume, which could activate
chemical reaction. Under such extreme conditions, I could see Cr
reducing Cu++. As for reactivity, remember that the standard cell
potential of 2 Cr + 3 Cu++ --> 2 Cr+++ + 3 Cu is 1.08 V, so this
reaction is definitely energetically favored. I suppose that what
is going on here is something like a miniature thermite
reaction initiated by the impact.

[Edited on 22-3-2008 by microcosmicus]

Nicodem - 22-3-2008 at 03:02

Solid/solid reactions under thermal conditions are very well known (and the basis of polycrystalline materials science). They are generally called calcinations or sometimes sintering (this is not strictly correct since sintering is a process which is not necessarily accompanied by a chemical transformations). The reactions are based on the diffusion of ions from one crystal to the other until a polycrystalline material of new chemical composition is obtained.
As was already said, on the impact points during ball milling the temperature and pressure can get quite extreme. So it is not surprising that solid/solid reactions can be conducted using planetary ball mills and similar equipment even at low average temperature. In a sense this is similar to sonochemistry where the reactions occur in cavitation points with extreme temperature and pressure, regardless of much lower average.

ScienceGeek - 22-3-2008 at 03:59

Also, don' forget the classicall reaction between Ammonium Nitrate and a Barium salt!

CyrusGrey - 22-3-2008 at 09:46

Interesting. I did not consider that reactions in a ballmill would be high temperature/pressure reactions. I'll try and see if I can make a more chemically resilliant ballmill using bismuth shot.

ScienceGeek: doesn't that reaction proceed because it produces water (From the hydrated salt) which dissolves a small amount of the reagents so they can react?

[Edited on 22-3-2008 by CyrusGrey]

YT2095 - 22-3-2008 at 10:49

Quote:
Originally posted by ScienceGeek
Also, don' forget the classicall reaction between Ammonium Nitrate and a Barium salt!


forget... I`v never heard of it in the 1`st place?

what do you mean?

Mr. Wizard - 22-3-2008 at 11:36

There is another possibility; the chrome plating has worn through, and the base coating of copper has shown through. Any good chrome plate has a base plate of copper then nickel. Maybe somebody can add to that.

microcosmicus - 22-3-2008 at 14:03

Quote:

nteresting. I did not consider that reactions in a ballmill
would be high temperature/pressure reactions.


Think about it for a moment --- if it weren't for high pressures,
how would your mill grind? To crush your CuO, you are
going to have to apply quite a bit of pressure. Basically,
the balls act like miniature hammers and anvils crushing
the bits of CuO into powder. As for temperature, when you
apply pressure to something, some of the energy comes
out as heat.
Quote:

I'll try and see if I can make a more chemically resilliant
ballmill using bismuth shot.


The problem with bismuth is that it is too soft and will bend
from the impact as opposed to crushing your CuO. What
you want instead is something that is extremely hard but
not likely to react with what you are trying to grind. A
better choice might be another oxide like SiO2 or Al203.
Basically, get yourself some pebbles or marbles and use
them instead of metal balls.

UnintentionalChaos - 22-3-2008 at 14:22

For non-reactive grinding media, I think the best choices are likely unglazed fired porcelain pellets (You could make yourself If you have a capable kiln...or you could buy alumina pellets) or possibly something like flint or chert pebbles (cryptocrystalline SiO2). I would think that marbles would be a poor choice since glass is brittle.

microcosmicus - 22-3-2008 at 14:43

Just to clarify, when I said "marble", I didn't necessarily mean glass!
Rather, I was using the term to mean artificially produced spheres
as opposed to naturally occurring pebbles. In particular, there are
industrial marbles made from ceramics which could be used
for grinding in a ball mill.

[Edited on 22-3-2008 by microcosmicus]

12AX7 - 22-3-2008 at 20:19

Lead (hardened with antimony) is a popular medium for producing gunpowder. Hard materials are unsuitable because they may spark. Simple elastic impact between balls is a good model for high-energy ball mills, where the balls (or rods, as the case may be) are lifted to a high level and fall onto balls below, but leaves much to be desired for a slower rolling ball mill, where the balls cascade over each other, producing more of a shearing and mashing action rather than impact.

Tim

blogfast25 - 25-3-2008 at 10:25

Another less known solid state reaction is the thermite reaction between a rusty steel ball and a clean aluminium ball: when smacked together the sound and spark of a small amount of the iron (III) oxide being reduced by the Al can clearly be heard and seen.

There's a nice classroom demo of it somewhere on the Tinkerwebs...

a_bab - 26-3-2008 at 06:18

My theory is that the chemical reaction are taking place all the time, with almost any participants. The key is the speed; conditions like temperature, pressure, concentration would change the speed to visible "amount".

For instance, the old paper (newspapers) are yellow due to the slow oxidation that takes place in tens of years. As the time passes by, the paper tends to turn darker and darker, it gets crumbly and it may even become black (I saw some 3000+ years old fabric in a museum and it was completly black and crubly, as being carbonized).

If you put a piece of paper under heat, you can see the same reactions speeded up by magnitudes of millions. Such slow "burning" reactions are taking place all the time, and they can accelerate if the generated heat is not eliminated. It is a well known fact that lots of flammable stuff are not to be held together in huge packs, as they can catch fire for no apparent reason. Examples are cotton, paper, hay, and even foreworks, where the slow reactions between the components may even render the producs useless.

That's why we have an "expiry date" for almost every product, due to these slow reactions.


On a side note: the infamous Armstrong mixture type is a good example of "touching solids reaction"; if the pressure between the reactants is not enough, the reaction takes place too slow; when pressure is applied the crystal structure breakes, and the atoms of the P and the oxidant get the activation energy and react at a faster rate that will accelerate.

Same with the spontaneous explosions; only an extra tiny little bit of energy may be needed to get the reaction going in an "avalanche".

The atoms from a substance are moving all the time; they must be like a the bubbles of a foam, but moving instead of popping.
Also, when you rub with the finger even the hardest matter some of the atoms from that matter will remain on your finger.

microcosmicus - 26-3-2008 at 06:38

Quote:

My theory is that the chemical reaction are taking place
all the time, with almost any participants.


Sure, it's just a question of overcoming the activation
energy. Under ordinary circumstances, this will only
happen when a molecule happens to have above average
energy due to a themal fluctuation or through quantum
tunneling. The probabilities of both tunneling and
fluctuations decrease exponentially with the size of
the energy barrier to be overcome, which is why the
reaction is as sluggish as a sleepy snail at ordinary
temperatures. If a reaction is energetically favored,
it will proceed to equilibrium, even if it takes centuries
to get there.

[Edited on 26-3-2008 by microcosmicus]

CyrusGrey - 26-3-2008 at 13:25

Quote:

Another less known solid state reaction is the thermite reaction between a rusty steel ball and a clean aluminium ball: when smacked together the sound and spark of a small amount of the iron (III) oxide being reduced by the Al can clearly be heard and seen.


Cool. Can you post more details, like the amount of force required or if the aluminum needs to be stripped of it's oxide layer first. This sounds like it would be a neat experiment to record for the Home Chemistry Society website.

Also what is Tinkerwebs?

I heard it mentioned somewhere on the forums that bismuth shot makes a grinding media comparable to antimony-lead media. I can't find the thread now though. Another think I have heard while trying to find bismuth shot is that bismuth shatters rather than deforms like lead does, sounds like its harder than regular lead at least.

[Edited on 26-3-2008 by CyrusGrey]