• In $\ce{CH4}$, carbon has 8 electrons due to 4 covalent bonds.
  • In $\ce{CO2}$, carbon has 8 electrons due to 2 double bonds.

In both cases, carbon has 8 electrons. So why is carbon oxidised? Doesn't oxidation occur when electrons are lost?

The crux of my question is how to treat covalent bonds in redox reactions.

I know we can say carbon has gained oxygen so is oxidised, but I am after an answer in terms of the electron definition of reduction and oxidation.

Redox scheme of the reaction of methane and oxygen (Image Source)

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    $\begingroup$ Oxygen is more electronegative than hydrogen and carbon , so it attracts the electron cloud more. $\endgroup$ – user14857 Oct 30 '16 at 20:06
  • $\begingroup$ So then is every single reaction a redox reaction because one of the elements in a molecule (unless its a diatomic molecule with the same atoms) will be more electronegative than the other and will have the electrons slightly towards it? $\endgroup$ – K-Feldspar Oct 30 '16 at 20:09
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    $\begingroup$ Actually you need to check the oxidation numbers for identifying redox reactions. In methane carbon has -4 oxidation number but in carbon dioxide it is +4. Increase in oxidation number means oxidation. $\endgroup$ – user14857 Oct 30 '16 at 20:14

To determine oxidation states and thus the outcome of redox reactions, polar covalent (even if they are only very slightly polar) are treated as fully separated ionic bonds.

In methane, carbon is more electronegative than hydrogen ($2.5 > 2.1$), so when setting up oxidation states, we treat methane as if it were composed of $\ce{C^4-}$ and $\ce{4 H+}$ — leading to the oxidation states of $\mathrm{-IV}$ on carbon and $\mathrm{+I}$ on hydrogen.

Likewise, oxygen is more electronegative than carbon ($2.5 < 3.5$), so in carbon dioxide we assume the electrons to be completely transferred over to oxygen; as if it were made up of $\ce{C^4+}$ and $\ce{2 O^2-}$ leading to the oxidation states of $\mathrm{+IV}$ and $\mathrm{-II}$, respectively.

This is an entirely formal treatment. However, there is some experimental basis behind it, namely that in methane carbon has a slightly higher electron density and a slightly negative partial charge while in carbon dioxide it is the oxygens that have a partial negative charge. The on-paper treatment is, however, an extremely gross exaggeration.

For ionic compounds, of course you observe complete electron transfer from one species to another signified in the ion charge, which may seem as if the concept is much more ‘true’ to ionic compounds.


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