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Non Life Sciences => Chemistry => Topic started by: Bill.D.Katt. on 08/08/2010 18:14:55

Title: Equation balancing for Al redox?
Post by: Bill.D.Katt. on 08/08/2010 18:14:55
The exothermic reaction between Al and KNO3 (a slow flash powder) oxidizes Al. If Al2O3 is one of the products, what are the others? A thick white smoke was released, I'm guessing it was some form of oxidized N. What is the balanced equation? Since there was no H involved, it seems that K2O must be included.
Title: Equation balancing for Al redox?
Post by: lightarrow on 08/08/2010 20:57:57
Quote from: Bill.D.Katt. on 08/08/2010 18:14:55
The exothermic reaction between Al and KNO3 (a slow flash powder) oxidizes Al. If Al2O3 is one of the products, what are the others? A thick white smoke was released, I'm guessing it was some form of oxidized N. What is the balanced equation? Since there was no H involved, it seems that K2O must be included.
It depends on the relative amounts. You could have:

2Al + KNO3 → Al2O3 + K + ½N2↑

or:

10Al + 6KNO3 → 5Al2O3 + 3K2O + 3N2↑

or:

2Al + 2KNO3 → Al2O3 + K2O + 2NO↑

or:

...
...
...
Title: Equation balancing for Al redox?
Post by: Bill.D.Katt. on 09/08/2010 01:36:50
Could it really create elemental K? I didn't think that it would get hot enough to do so. And if it did, there would be a secondary reaction between the K and the Al2O3 right?
Title: Equation balancing for Al redox?
Post by: Bored chemist on 09/08/2010 06:53:03
At least one of the products will be potassium aluminate.
Very litle elemental K will be made; any that is wil react with the air.
Title: Equation balancing for Al redox?
Post by: lightarrow on 10/08/2010 12:45:04
Quote from: Bill.D.Katt. on 09/08/2010 01:36:50
Could it really create elemental K? I didn't think that it would get hot enough to do so.

Looking for standard enthalpy of formations of K2O and Al2O3:

K2O
http://webbook.nist.gov/cgi/cbook.cgi?ID=C12136457&Units=SI&Mask=2#Thermo-Condensed

Al2O3
http://webbook.nist.gov/cgi/cbook.cgi?ID=C1344281&Units=SI&Mask=2#Thermo-Condensed

we can infer that the reaction:

3K2O + 2Al → Al2O3 +2K

has an enthalpy of -586.19 kJ/mol, that is, it's very exothermic.

Actually, you would be amazed of how strongly aluminium remove oxigen from chemicals; it removes oxygen from almost every oxide of elements; there are just a few exceptions, for example BeO, MgO, CaO, probably also Li2O, BaO and no much more than these...

Quote
And if it did, there would be a secondary reaction between the K and the Al2O3 right?
Of course not, if the reaction A → B is very favoured, the opposite reaction B → A is very disfavoured (not sure of the english terms).