[shafaifer]

Hello,
 
I have these reactions (they should be correct):
 
CaCO3(s) + 2HNO3(aq) --> H2O(l) + Ca2+(aq) + CO2(g) + 2NO3-(aq)                Eq. 1
 
Calcium cations will now form a complex with water according the following reaction:
 
Ca2+(aq) + 6H2O(l) --> [Ca(H2O)6]2+(aq) + 6H2O(l)                       Eq. 2
 
Now comes the combustion: This is a project of AAS (atomic absorption spectroscopy), and I want to measure the absorbance for calcium when irradiated with a hollow cathode lamp. I am only interested in calcium so I ignore nitrate and CO2(g). As a combustion agent / oxidising agent, C2H2(g)/O2(g) is used:
 
[Ca(H2O)6]2+(aq) + C2H2(g)/O2(g) -->                                  Eq. 3
 
What is the product of this equation 3? I am sure solid calcium is formed (thus reduced). But how can it be reduced when the compounds used for the combustion are oxidizing agents? I am considering C2H2(g)/O2(g).

[kriggy]

The acetylen+O₂ is used for atomization of your sample it gets decomposed to single atoms which then absorb the radiation from cathode lamp.

[shafaifer]

Thank you very much, you said single atoms, so is this correct:

[Ca2+(H2O)6]2+(aq) + acetylene/O2 --> 12H(g) + Ca(s) + 6O(g)

I assume acetylene/O2 mixture does not influence the reaction in addition to its role as catalyst, am I right?

 

[kriggy]

No its not a catalyst.You need the atoms to be separated from each other and to do that, you need high temperatures. Its just a way how to do it, there are other possibilities like electrothermal atomizers.

[shafaifer]

What is the correct appreciation of this combustion reaction?

Can you (som others) agree with me that we have:

 [Ca(H2O)6]2+(aq) + C2H2(g)/O2(g)  

What are the products?

If you know, telling me this will help me very much in learning as well as in time saving. I need this equation for my project that will be delivered 03-06-2014: 23:59.

 

[Arkcon]

Its kinda hard to understand your question.  I know what you're asking, I just don't know why.  Briefly, eventually, the calcium will exist as a finely powdered ash, of some refectory compound, in your case CaO.  This has no bearing on how a AAS instrument works.  At least some of the calcium salt sample, when dispersed in a high energy flame, exists as free Ca²⁺. Its the ionic Ca²⁺ that absorbs the wavelengths of light from the hollow cathode lamp.  If you have to balance the equation for the overall reaction, then you have to use CaO as a product, but this has no bearing on how AAS works.

[Borek]

What is the correct appreciation of this combustion reaction?

[Ca(H2O)6]2+(aq) + C2H2(g)/O2(g)

As you were told several times already, this is not what is happening, and the combustion part is just a trivial

C₂H₂ + O₂  ...

[shafaifer]

Thank you very much for this helpful aswer, Arkcon.

[Ca(H₂O)₆]²⁺(aq) + C₂H₂(g)/O₂(g) → CaO(s) + 6H₂O(g) + C₂H₂(g)/O₂(g)         Eq. 3
Remark: When heating at this temperature the mixture will become an aerosol.
To balance the equation 3, I am not sure how. An oxygen-atom is missing on the left side of the equation. The acetylene/O2 mixture has its own reaction, it is like the reaction between CH4(g) and O2(g):

CH₄(g) + 2O₂(g) 2H₂O(l) + CO₂(g).

As you said, Arkcon, not all of the [Ca(H2O)6]2+(aq) is converted into CaO(s) – what is left will absorb wavelengths from the cathode lamp.

I am very interested in this and I want to give the best answer possible.

[Borek]

To balance the equation 3, I am not sure how.

You are trying to force a multi stage process, combining several different reactions and processes, into a single reaction. This will never work, and you will be never able to balance the reaction, period.

The longer you disagree with that, the more time you waste - but you are not going any closer to understanding.