how are ions atomised in a flame test??? (1 Viewer)

[Eagle Mum]

“However, sodium metal oxidises rapidly in air and sodium oxide (Na2O) melts at 1132 degC and sublimes around 1275 degC. Even if sodium metal is present, placing it into a flame with an ample supply of air guarantees it will become sodium ions as part of an oxide rapidly.”

Yes, I agree, some of the electrons in the inner orbits which reach the 3p orbit would also decay into the 3s orbit. I’m not entirely sure about the comment that there’s an ample supply of air. Inside the flame, most of the oxygen would be consumed by combustion of the natural gas fueling the flame. I’m not convinced that both species (atoms & ions) aren’t in the mixture.

 

[Eagle Mum]

“that does not mean that excitations can't be 2p to 3s to 3p and reverse, to give the flame colour, with an alternative being a 2p to 3d excitation with a 3d to 3p to 3s to 2p relaxation pathway.”

Do you have the specifications on the energy required to excite a 2p electron to the 3d orbit level?

 

[Eagle Mum]

“For a flame test with sodium chloride, the question that @dumNerd asks is, for me, critical... if sodium chloride were somehow producing uncharged sodium atoms in a flame test, where is the electron coming from to reduce the sodium cation?”

I understand that spectroscopy has shown C3H3+, H3O+, CHO+ species downstream of unseeded flames suggesting that the hydrocarbons and their oxidised products are species that have given up electrons.

 

[CM_Tutor]

“However, sodium metal oxidises rapidly in air and sodium oxide (Na2O) melts at 1132 degC and sublimes around 1275 degC. Even if sodium metal is present, placing it into a flame with an ample supply of air guarantees it will become sodium ions as part of an oxide rapidly.”

Yes, I agree, some of the electrons in the inner orbits which reach the 3p orbit would also decay into the 3s orbit. I’m not entirely sure about the comment that there’s an ample supply of air. Inside the flame, most of the oxygen would be consumed by combustion of the natural gas fueling the flame. I’m not convinced that both species (atoms & ions) aren’t in the mixture.

Flame tests should be done with the bunsen burner's air hole open so you do not have an orange flame in which incomplete combustion is occurring. Further, no one would do a flame test using actual metallic sodium, it would be with a salt and hence with sodium cations.

If sodium cations are introduced into a flame, what process are you suggesting by which they are reduced to sodium metal? Where is the electron coming from for

Na⁺(g) + e^- -----> Na(g)

 

[Eagle Mum]

Flame tests should be done with the bunsen burner's air hole open so you do not have an orange flame in which incomplete combustion is occurring. Further, no one would do a flame test using actual metallic sodium, it would be with a salt and hence with sodium cations.

If sodium cations are introduced into a flame, what process are you suggesting by which they are reduced to sodium metal? Where is the electron coming from for

Na⁺(g) + e^- -----> Na(g)

Crossed posts.
Spectroscopy has shown C3H3+, H3O+, CHO+ species downstream of unseeded flames suggesting that the hydrocarbons from the fuel source and their oxidised products are species that have given up electrons.

 

[CM_Tutor]

“For a flame test with sodium chloride, the question that @dumNerd asks is, for me, critical... if sodium chloride were somehow producing uncharged sodium atoms in a flame test, where is the electron coming from to reduce the sodium cation?”

I understand that spectroscopy has shown C3H3+, H3O+, CHO+ species downstream of unseeded flames suggesting that the hydrocarbons and their oxidised products are species that have given up electrons.

Indeed, flames contain all sorts of unusual species. The combustion pathway for methane suggests radicals including OH, CH₃, CH₃OO, CH₃O, CHO, HC(O)OO, and HCO₂, not to mention NO and NO₂, plus molecules of CH₄, H₂CO, CO₂, CO, and H₂O.

Most electron transfers are not with a free electron, of course; they are usually accompanying atom transfers as species collide. In the above, a typical example might be the transfer of an O atom from a perxoide or hydroperoxide, such as:

CH₃OO + NO -----> CH₃O + NO₂

OOH + NO -----> OH + NO₂

However, I am struggling to think of any species E that might collide with a sodium cation and

Na⁺ + E -----> Na + E⁺

then occurs.

 

[Eagle Mum]

Well, this has turned out to be an interesting thread in the area of chemistry physics which I haven’t thought about for several decades. Thanks.

I think it is noteworthy that many of the well accepted textbooks for secondary school chemistry teach that the colour from the flame tests are from the excitation and decay of the valence electrons in the outer orbit. If it is otherwise, than the textbooks might need to be rewritten, so it’s great to have a rigorous discussion about what actually is producing the flame colour.

 

[CM_Tutor]

I was having a look around and found this reference:

http://www.chemguide.co.uk/inorganic/group1/flametests.html

which supports your view of there being uncharged sodium atoms formed in a flame. Though it gives no clear explanation, it does attribute the information as coming from a Professor of Physical Chemistry in 2016, which is well after I learned this chemistry.

The author, Jim Clark, is author / joint author of the Edexcel Chemistry for International GCSE books, fyi.

So, you may be right. Certainly Na atoms would give rise to the colour. I just remain puzzled as how they could come to be present (and what happens after, as gas containing Na atoms will not be good for anyone nearby when those atoms reach the mucous membranes in the nose and mouth.