What is happening in Thermite that allows Aluminum and Iron Oxide to burn so hot? Also, how is aluminum and iron able to burn like that considering they are metals?


What is happening in Thermite that allows Aluminum and Iron Oxide to burn so hot? Also, how is aluminum and iron able to burn like that considering they are metals?

In: 2

Like all chemical reactions it’s based on the energy in the bonds. The Aluminum-Oxygen bond is just at a much lower energy than the Iron-Oxygen bond so a lot of energy is released by this reaction.

Thermite reaction is also know as a reduction oxidation reaction or redox for short. In order to get a thermite reaction you need a starter of high heat. Typical this is done with a simple magnesium fuse. Once that happens the bond between the iron and oxygen atoms (ie rust) is given enough energy to break. So the oxygen is now free floating but it is not stable on its own. So it reacts with the next most stable thing, aluminum. As the oxygen bonds with the aluminum it releases energy to form that new bond. That energy is enough to break free other oxygen atoms from the rust and cause them to bond with the aluminum. So you have a chain reaction until all the aluminum and iron oxide are reduced.

To answer the second part of your question:

> Also, how is aluminum and iron able to burn like that considering they are metals?

Most metals will burn VERY well if exposed to oxygen and heat. Burning is just rapid oxidation, after all, and oxygen is voracious and will not hesitate to bond with anything even remotely accepting. Metal fires are a major concern in some settings and are both incredibly hot and very difficult to extinguish – they are even their own “class” of fire (class D).

There are three things that keep metal from being on fire basically 100% of the time:

– a high heat requirement to ignite, as opposed to corrosion which happens at lower temperatures (but takes longer).
– the fact that for large chunks of metal, the outside has either already reacted with oxygen and blocks access for oxygen to get to the unoxidized metal inside.
– in some cases the metal has either entirely or partially been made into an alloy that is more resistant to oxidation – think stainless steel, which is iron mixed with gallium in order to strengthen metallic bonds and keep oxygen from joining the party and causing rust, or

If you want to burn a metal, simply give it lots of heat and free oxygen, and increase its surface area (typically done by grinding it to a powder). And that’s how thermite works. You heat aluminum powder and iron oxide enough with a starter, and the iron oxide will separate and the desperate oxygen will grab whatever it can bond with – and at the temperatures of thermite, aluminum is something it can bond with. This reaction generates heat (much like it takes heat to separate the iron from the oxygen), and that leads to a chain reaction where the aluminum oxide formation generates enough heat to free more iron and oxygen and cause more reactions with aluminum. Eventually you’ll run out of aluminum or iron oxide, but in the meantime you’ll generate enormous amounts of heat.

‘Burning’ can be more precisely called ‘rapid oxidation’. Or, it’s when lots of oxygen binds to something quickly. It also usually involves breaking bonds that were there previously, which is usually started by there being enough heat to start the bonds breaking, and then once that starts the energy released from the oxygen bonding creates enough heat to keep the reaction going. How much heat is dependent on how reactive the thing oxidizing is, but most metals burn as long as there’s enough oxygen and heat to keep the reaction going. In fact, one of the ways to identify unknown metals is to burn them and observe what color the flame is!

Despite how neutral we view it, Aluminum (Al) is actually really reactive with oxygen. Wikipedia lists this as a “high [chemical affinity](https://en.wikipedia.org/wiki/Chemical_affinity) to oxygen”, but you can think of it as Al will combine with oxygen faster and more aggressively than most other chemicals when given the chance. If you exposed pure Al to liquid oxygen, it would explode in fire! It’s just that Al usually reacts specially, quickly creating a film of Al oxide that protects the Al beneath it when it’s just atmospheric exposure. In ideal conditions, all Al would burn as hot (or hotter) as thermite because that’s the temperature Al burns at.

So the way thermite works is by taking a stable and concentrated source of non-air oxygen (iron oxide), finely mixing it with Al in a way to maximize the contact of their surface area, and then heating it enough that the oxygen starts to break away from the iron and instead reacts with the Al. This allows the Al and oxygen to react as fast as it can, which generates a huge amount of heat.

As a bonus, a byproduct is molten iron, which will have enough mass and heat to it to spread the heat created from the burning Al onto whatever is underneath the thermite (and what gets splashed by the bubbling mixture will get some scorching too).

It’s not really hotter than other fuel-oxidizer fires, *but* it releases almost no gas, so all of the heat stays right there instead of floating away.

As for why metal can burn, pretty much all metals can burn. Metals are not naturally stable and want to react with oxygen, which our atmosphere is full of. Usually the process is pretty slow or even grinds to a halt, but having a ton of oxygen available (in the form of iron oxide) and making the metal a fine powder to expose it more really speeds things up.