Why is the enthalpy of combustion of water zero?

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IsabelJoseph54
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#1
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I don't understand why water has an enthalpy of combustion of zero. I thought energy is always released in combustion?
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Ligtning.starrr
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#2
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I THINK because water has a ph of 7 which means that water is neutral so that is why more time the enthalpy of combustion for water is zero. (It’s something my chemistry teacher told me)
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scimus63
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I dont think water burns! So enthalpy of combustion will be zero. Remember definition of enthalpy of combustion, energy released by combustion of 1 mole - so if it dont burn then no energy!
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IsabelJoseph54
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(Original post by scimus63)
I dont think water burns! So enthalpy of combustion will be zero. Remember definition of enthalpy of combustion, energy released by combustion of 1 mole - so if it dont burn then no energy!
ohhh ok thank you. does it not burn because it is the product of hydrogen burning, thus water is already burnt? does that mean any substance that has already been combusted doesnt have an enthalpy of combustion e.g Al2O3?
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scimus63
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i would say so, you cant burn it fully twice! Once it burned or been fully oxidised then thats it!- no more combustion. Definitions are here, hope it helps a bit!

https://science-revision.co.uk/A-lev...lpy_defns.html
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Pigster
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Perhaps with more formal logic: since DcH for H2 is the complete combustion of H2 in excess O2 and the product of that reaction is water, that means that water is already fully combusted and hence cannot be further combusted.

(the oxygen in) Water can be further oxidised - to H2O2, O2 or even F2O, but none of those products are achievable by a combustion reaction.
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IsabelJoseph54
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(Original post by scimus63)
i would say so, you cant burn it fully twice! Once it burned or been fully oxidised then thats it!- no more combustion. Definitions are here, hope it helps a bit!

https://science-revision.co.uk/A-lev...lpy_defns.html
thanks! and thanks for the website link
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IsabelJoseph54
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(Original post by Pigster)
Perhaps with more formal logic: since DcH for H2 is the complete combustion of H2 in excess O2 and the product of that reaction is water, that means that water is already fully combusted and hence cannot be further combusted.

(the oxygen in) Water can be further oxidised - to H2O2, O2 or even F2O, but none of those products are achievable by a combustion reaction.
thank you!
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