[DannyBoy]

Hi there:

I'm trying to balance a reaction in which methane is oxidized by citric acid in solution. Both methane and citric acid will be transformed into carbon dioxide. To begin, I checked that the reaction is indeed a redox reaction.

C₆H₈O₇→CO₂

So, I calculated the oxidation number for carbon goes from +6 to +4. A reduction.

CH₄→CO₂

I calculated the oxidation number for carbon goes from -4 to +4. An oxidation. This seems reasonable so far. Then, I balanced my half-reactions.

C₆H₈O₇+5H₂O→6CO₂+18H⁺+18e^-

 CH₄+2H₂O→CO₂+8H⁺+8e^-

Now comes the part that has me baffled. How do I combine these half-reactions and not end up with loads of electrons since they are on the right hand side of both half-reactions?! Clearly, I've done something very wrong. Any help would be appreciated.

[AWK]

This reaction without additional oxygen is impossible. Moreover you oxidation number +6 is wrong.

[DannyBoy]

This reaction without additional oxygen is impossible. Moreover you oxidation number +6 is wrong.

Could you explain how you came to that conclusion? Simply because the half-reactions can't balance?

Also, can you tell me what the oxidation number for carbon in citric acid should be? The way I calculated it was as follows:

Oxygen: 7× -2 = -14
Hydrogen: 8 × +1 = 8
Therefore, since citric acid has no charge C must be +6. Is this reasoning flawed?

[AWK]

But you have 6 carbon atoms in molecule.
You should calculate oxidation number for 1 atom.

[DannyBoy]

But you have 6 carbon atoms in molecule.
You should calculate oxidation number for 1 atom.

Ah, right. Thanks for that. And therefore carbon is going from +1 to +4, so both half-reactions are oxidations, so no redox reaction. Right?

[AWK]

Exactly.
But you can try obtaining CH4 + other hydrocarbon and CO2 from citric acid without additional oxygen. Such a "paper chemistry" will work.

[DannyBoy]

Exactly.
But you can try obtaining CH4 + other hydrocarbon and CO2 from citric acid without additional oxygen. Such a "paper chemistry" will work.

Okay. Thanks for the help – it's much appreciated!

The problem I'm actually tackling is the apparent oxidation of methane in the presence of citric acid. I mistakenly thought that the citric acid would be reduced to carbon dioxide, but clearly that makes no sense. I realise I'm digressing from the original problem, but do you see a way (on paper) that methane and citric acid have the potential of reacting in this way?

[AWK]

Methane is rather unreactive compound. This is a bacterial product in anaerobic conditions (marsh gas). Almost all organic compounds may be a reactants in these processes.