Chemistry - reactivity and reverse reactions

[mike_au]

Can someone explain what determines which chemicals will react in a mixture, for example:

 

I found a recipe a little while ago that involved mixing a common chemical and Al to make flash, yet other mixtures which include this chemical and Al (along with other things, C for example) burn quite slowly.

 

Since Al is more reactive (are activation energy and surface area the only factors affecting reactivity?) than the other chemicals, shouldn't it react in preference to the other chemicals?

 

i.e. Iron can be oxidized in air, but when mixed with Al it will act as an oxidizer and be reduced, why doesn't the Al reduce any C that happens to oxidize?

 

Or is it purely a question of probability, as energy is released the odds of it being absorbed by Al is lower than the odds of it being absorbed by some of the other fuels (do to there being more of them present) and therefore there is never enough Al with enough energy for it to react violently?

 

I am going to try making some fountains, is there a point where if I add too much Al then my mixture will no longer be fountain mix with Al and will become flash with C?

[justanotherpyro]

Can someone explain what determines which chemicals will react in a mixture, for example:

 

I found a recipe a little while ago that involved mixing a common chemical and Al to make flash, yet other mixtures which include this chemical and Al (along with other things, C for example) burn quite slowly.

 

Since Al is more reactive (are activation energy and surface area the only factors affecting reactivity?) than the other chemicals, shouldn't it react in preference to the other chemicals?

 

i.e. Iron can be oxidized in air, but when mixed with Al it will act as an oxidizer and be reduced, why doesn't the Al reduce any C that happens to oxidize?

 

Or is it purely a question of probability, as energy is released the odds of it being absorbed by Al is lower than the odds of it being absorbed by some of the other fuels (do to there being more of them present) and therefore there is never enough Al with enough energy for it to react violently?

 

I am going to try making some fountains, is there a point where if I add too much Al then my mixture will no longer be fountain mix with Al and will become flash with C?

There are too many factors and variables to give your question a general answer.

 

As far as the actual pyrotechnic aspect of it that involves safe production of a fountain Al does not become flash with C. Both are fuels and require an oxidizer. KNO3 is not a very strong oxidiser because the O atoms are more strongly bonded to the the N atom than the O's in say KClO4 or KClO3 where the bond holding the O atoms to the Cl is much weaker. Thus oxygen is liberated better from those two compounds. To get the sparks desired for a fountain you would use a relatively low mesh (16mesh-325 maybe)Al which even if it were mixed with a powerful oxidizer i.e. KClO4 it would still be too slow burning to act as flash although it is still dangerous.

[Mumbles]

The series of reactions are far more complex than I think you realize. Probably the easiest way to think about it, though not entirely correct, is that when burning everything reacts together at once. Proportions, particle size/shape, and overall balance of fuel and oxidizer all play significant roles. For instance, 70% perc, 30% dark flake Al will react very fast and violently making flash. If you reverse the proportions to 70% dark Al, 30% Perc, you have a flitter star.

 

As JAP said, it is very complex, and near impossible to really model accurately. Probably just best to think that the heat decomposes the oxidizer releasing the O2, and then all the other fuels grab it as it is near. There are preferential reactions, but everything can be assumed to react in good balanced formulas. There should theoretically be just enough oxygen to react everything. While the Al may react preferentially, eventually it will run out. Atomospheric Oxygen will also play a role in tailed stars, and some primes.

 

BTW, iron is never reduced from it's elemental form. There exist negative anions of it, but it still is in a positive oxidation state. I also don't think that Al is a strong enough reducing agent to reduce CO or CO2 back to elemental carbon, though Mg is.

 

You are right, in the fact that some fuels will react preferentially first over others. This could be a main contributing factor in why metals wash out colors. Preferentially here generally means the difference between micro seconds though.

[mike_au]

Sorry, I was being a bit cryptic because I don't think I am supposed to talk about how to make flash. I am aware that a redox reaction required an oxidizer.

 

What I mean is, say you have the composition Oxidizer + C mixed with Al, how is this different to Oxidizer + Al mixed with C?

 

Does the C actually change the reaction in some way, or is it purely a question of ratios that determines which one dominates the reaction.

 

Edit:

 

I wasn't very clear about the iron example either, I meant, the iron will oxidize to produce iron oxide, iron oxide can then react with a more reactive metal to produce some other oxide and iron.

 

I was wondering what prevents the same thing happening if iron, oxidizer and Al were mixed up. I guess the answer I was after is that there should be enough oxidizer that all of the fuel will react without needing to reverse the oxidation of the other fuels.

 

Quite obvious now that I think about it :/

 

Thanks

[Mumbles]

It's mostly a question of ratio.

 

70-30 perc-Al is a flash powder (obviously)

70-30 perc-benzoate is a rocket propellant

 

If you replace 5 parts of the Al with benzoate, probably no difference will be noticable. If you replace 5 parts of the benzoate with Al, you'll lose the whistle, and probably get a significantly more powerful propellant. It's the difference between adding more power to a relatively low powered mixture, versus removing a small bit of power from an already high powered mix.