What energies are involved in ozone production?

[Martin]

Anyone with some practical chem smarts can help me with this one.

 

I have several questions about ozone production.

 

Why are large voltages used in industrial ozone production? What determines the threshholds? If the threshholds are high, then why do small voltages---even household level or thereabouts---seem able to produce ozone in small amounts?

 

I assumed that one energy requirement would be that needed to break the O=O bond and get atomic oxygen

 

So I looked up the O=O molecule bond and found it was 498 kJ/mole

 

so I put this into google

498 kJ/Avogadro's number/eV

 

and it tells me that the bond is about 5.2 eV------around 5 electron volts.

 

I don't know if that is right, or about right. Can anybody verify? Tell me if I'm wrong.

 

what is the next thing I should consider?

 

I want to get a handle on the reactions involved when ozone is produced and I want a rough idea of the energies involved.

 

I get the impression that you want the oxygen molecule to ionize producing some oxygen ions which will then be accelerated by whatever electric field is there and crash into oxygen molecutes. Colliding with sufficient energy, they stick, and then somehow turn into neutral ozone. This is just a guess. Anybody want to clue me in?

[Mr Skeptic]

Very little energy is actually required on a per-molecule level in chemistry. Such is evident in eg batteries and electrolysis of aluminum.

 

Ozone in the stratosphere is mostly produced from ultraviolet rays reacting with oxygen:

 

O₂ + photon(radiation< 240 nm) → 2 O

 

O + O₂ → O₃

 

And (h * c) / (240 nm) = 5.16600715 electron volts.

 

But that is an inefficient production method' date=' though I don't know if ozone can actually be produced directly (3 O[sub']2[/sub] + energy → 2O₃)

 

So it seems your calculations are correct, and for a bit over 5 eV you can get two ozones.

 

---

I also looked up the minimum energy needed to form ozone, which is +142.3 kJ / mol from O₂, which turns out to be 1.47 eV per molecule.

 

I also learned that ozone can explode

[Ozone]

Wherever you have an electric discharge in air (or othe O2 bearing gas that does not contain a good amount of a quencher, like methane) you will get some ozone.

 

Old driers contain "ozonators" which were, IIRC high voltage coronal discharge tubes (which supply the UV you need to make the ozone). They gave a "fresh scent" to the dried laundry. Since this is quantized, more than 5 ev per rxn is a waste. It looks like it can be made much more easily, though:

 

Ibanez, Jorge G.; Rodrigo Mayen-Mondragon and M. T. Moran-Moran (October 2005). "Laboratory Experiments on the Electrochemical Remediation of the Environment. Part 7: Microscale Production of Ozone". Journal of Chemical Education 82: 1546. Retrieved on 2006-05-10.

 

In higher concentrations it is quite noxious (in odor and tendency) and adds very readily to double bonds to form ozonides. These ozonides can be decomposed (slowly) to yield two equivalents of carboxylic acid. This is the aspect I am most familiar with. They (ozonides) can (and sometimes do) decompose rapidly, which can be in the form of a nice explosion.

 

It appears that (I need to check this for accuracy) concentrations in excess of 10 % w/w can deflagerate if ignited:

 

http://gcmd.gsfc.nasa.gov/Resources/FAQs/ozone.html

 

I do not know if it will explode if confined. I suppose that in the absence of a reducible substrate (which would be impressive), this would depend upon the energy released heading toward dioxygen.

 

I should note that in larger discharges, the NOx from the N2 in the air is usually much more debilitating than the ozone (especially with large HV gaps and tesla coils, etc.).

 

There is surprisingly decent coverage and a reference list (avoid most of the climatic screed) here:

 

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

 

Cheers,

 

O3

[thedarkshade]

OK, fair enough but yet no answer.

 

What is the minimum voltage required to break O2 into atomic oxygen?

[John Cuthber]

Across a small enough gap, the answer might be 5V.

However there is a requirement for a large enough potential gradient to get a discharge in air. Over any practical size of ozoniser that takes a lot of volts.

[YT2095]

it`s important to rem also that even though the O=O is Delta H +496.

the Delta H of Formation O-O=O is -146.

 

leaving a DH of +350

so depending on your method and how Much you wanted to make, you Would get a little energy back although not enough to sustain the reaction, enough to keep some of it Going

[Ozone]

The static field required to breakdown (that is, jump a spark across a 1cm gap) dry air is about 30kV. A smallish practical ozonator is the Welsbach T-23 which is capable of delivering 0.5mmole O3/minute (Murray, et. al. (1972). Ozone reactions with organic compounds. ACS Adv. Chem. Ser. 112., pp. 11). I do not know, however, what the operating potential of this particular device is. One is used here (they say 120V, but I gather that this is the voltage on the primary side):

 

http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0976

 

http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p0489

 

The specs on the ozonator the testers used, a Welsbach T -series are given here:

 

http://www.welsbach.com/t-series.html

 

I do know that they usually operate between 25 and 40kV

 

For the electrochemical route:

 

From the citations given it can be found that (electrochemically):

 

3 H2O → O3 + 6 H+ + 6 e−; ΔEo = −1.53 V

 

and the competing reaction:

 

2 H2O → O2 + 4 H+ + 4 e−; ΔEo = −1.23 V

 

(6H+ + 6e-->2H2 = 0V (NHE)

 

An additional reaction is given by Bard and Faulkner:

 

O2 + H2O -->O3 + 2H+ + 2e- -2.075V (vs NHE)

 

After that it should be a matter of pushing some © of electrons around.

 

Cheers,

 

O3

[recursive]

Hmm... This is really getting complex but what I know is that if I run my 50kv tesla coil for even a few seconds I can smell the ozone! It's a great oxidiser, but don't tell that to your lungs...

[thedarkshade]

I got confused at one point! When we were discussing about ozone and it's production our teacher mentioned something that I seem not to understand!

 

He said: "When lightning strikes there is created a voltage of 380V which breaks the molecular oxygen into atomic oxygen and then the atomic oxygen reacts with molecular one and there you got ozone!"

 

I got the second part but not the first! I'm not sure if he's saying that lightning has a voltage of 380V (which is complete madness) or that 380V is enough to break molecular oxygen into atomic ones! What do you guys think?