The Melting Point of Diamonds: Burning or No Burning?

[Weissritter]

I heard something that I adopted as a rule some years ago: "Every combustion produces H₂O and CO₂". I used it blindy until hydrogen burning came to mind.
Clearly, H₂ does not produce CO₂.
Now, what happens when a diamond reaches its melting point, and, does it burn afterwards? If so, what kind of reaction happens?
And yes, this is not homework.

 

[Borek]

In the air diamond will burn long before reaching its melting point.

 

[M Quack]

Things like Cs and Mg will burn in air, too. I guess the real question is how do you define combustion...

 

[Weissritter]

Diamond's internal bonding is highly stable, strong and pressure-resisting. All of these mean the diamond would have a damned high fusion point. But it is organic. With proper conditions, any organic structure can quickly and aggressively break their bondings and create CO2 and H2O.
So what could happen if, instead of just fusing the diamond, and gradually getting it into gas state, the proper combustion could be given, no matter time nor energy wasted, would it burn? If so, what compounds would we have as products.
My own way of describing combustion is an aggressive oxidation of the carbon in an organic compound, making big structures little ones.

 

[chemisttree]

Diamond is pure carbon. You repeatedly reference the burning of hydrogen and the production of H2O. Where does the hydrogen come from when you burn something that is pure carbon?

 

[AGNuke]

Diamond simply burns to give CO₂ at 973 K, way lower than its melting point. One doesn't simply burns a substance by making it in gaseous form.

 

[snorkack]

Look at ordinary charcoal or coke.

It is pure carbon - all hydrogen was driven away in charring it. What was left behind was small crystals of graphite and some pores between them.

On heating in air, charcoal will neither melt nor evaporate. Instead, the oxygen in air reacts directly with the surface of graphite releasing heat and carbon oxides.

If any flames issue from coals of fire, they are caused by carbon monoxide reacting with more oxygen.

If charcoal is heated in inert environment like argon, then even below melting point it is annealed - the small graphite crystals grow and distort, closing the pores. But at 1 bar, graphite will not melt at all - sharing that property with substances like carbon dioxide (dry ice) and arsenic. The sublimation temperature of graphite at 1 bar is estimated as about 3700 Celsius.

Diamonds in air burn much like pieces of charcoal or graphite - though note they are not porous as charcoal.

Diamond does not melt or sublime at 1 bar either. On heating in inert atmosphere between 1500 and 2500 Celsius, diamond converts into graphite.

 

[russ_watters]

I heard something that I adopted as a rule some years ago: "Every combustion produces H₂O and CO₂".

Not sure where you heard it, but it is incorrect. As others stated, the products depend on the reactants. What is always the same in a combustion process is the oxygen. So that's the definition: chemically combining a substance with oxygen in a rapid reaction, creating heat and light.

The definition is somewhat loose as it pertains to the speed of the reaction though, since the word "fast" is qualitative and the reactions can otherwise be identical whether happening "fast" or "slow".

 

[Weissritter]

Look at ordinary charcoal or coke.

It is pure carbon - all hydrogen was driven away in charring it. What was left behind was small crystals of graphite and some pores between them.

On heating in air, charcoal will neither melt nor evaporate. Instead, the oxygen in air reacts directly with the surface of graphite releasing heat and carbon oxides.

If any flames issue from coals of fire, they are caused by carbon monoxide reacting with more oxygen.

If charcoal is heated in inert environment like argon, then even below melting point it is annealed - the small graphite crystals grow and distort, closing the pores. But at 1 bar, graphite will not melt at all - sharing that property with substances like carbon dioxide (dry ice) and arsenic. The sublimation temperature of graphite at 1 bar is estimated as about 3700 Celsius.

Diamonds in air burn much like pieces of charcoal or graphite - though note they are not porous as charcoal.

Diamond does not melt or sublime at 1 bar either. On heating in inert atmosphere between 1500 and 2500 Celsius, diamond converts into graphite.

Diamond simply burns to give CO₂ at 973 K, way lower than its melting point. One doesn't simply burns a substance by making it in gaseous form.

These two are the [STRIKE]droids[/STRIKE] answers I was looking for. Thanks

Diamond is pure carbon. You repeatedly reference the burning of hydrogen and the production of H2O. Where does the hydrogen come from when you burn something that is pure carbon?

Yes, I am aware this reaction would not produce H2O. I just didn't make it clear enough, it seems. Most of the answers also said something I already knew, so...I'll make things more clear in a near future. Thanks all.