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In the formation of NaCl, Na gets oxidised because it loses an electron, while Cl gets reduced because it gains an electron.

But how does this concept work in the case of covalent compounds, especially those compounds that have non-polar bonds?

For example, according to my textbook, in the formation of methane Hydrogen is getting oxidised from state 0 to 1, while Carbon is getting reduced from state 0 to -4.

But how is hydrogen losing an electron here? Doesn't it gain an electron due to sharing? C-H bonds are non polar so we cannot say that carbon pulls hydrogen's electrons towards it either right?

Pumpkin_Star
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  • In these cases of nonpolar covalent bonding, oxidation and reduction is formal, similarly as oxidation states/numbers. If a bond is easily broken, like N-Cl, ON can be formally determined from ON of products – Poutnik Nov 03 '22 at 09:15
  • @Poutnik What do we mean by oxidation and reduction is formal? What does formal here? – Pumpkin_Star Nov 03 '22 at 10:12
  • There is no gaining nor loosing electrons in $\ce{C(s) + 2 H2(g) -> CH4(g}$). There is speculative gaining and loosing electrons in thought heterolytic $\ce{H3C-H -> H3C- + p}$ bond breaking, on which the formal oxidation number is based on. – Poutnik Nov 03 '22 at 11:50
  • @Poutnik Oh, Ok. Thank you so much.. So in throry, if we take an isolated ch4 atom then on breaking one of its bonds we will end up with a methyl anion and a hydrogen cation? has this been experimentally checked too? And pardon me for asking a slightly silly question but umm, is it possible to just break one bond while leaving rest of the three untouched? – Pumpkin_Star Nov 03 '22 at 13:03
  • @Poutnik another doubt that I have is, that according to this speculation, if we keep on breaking the bonds, then we keep on creating more H cations or in other words protons. But I have been taught that CH4 disassociates into H2O and CO2 but here how will protons form H20? – Pumpkin_Star Nov 03 '22 at 13:11
  • Have you invented transmutation available in chem labs? How can CH4 dissociate to CO2 and H2O ? – Poutnik Nov 03 '22 at 13:17
  • @Poutnik I am very very sorry. The reaction I gave u was that of ch4 burning in air. Ignore that comment. Just read the one above it. And no, I dont even know whay transmutation means sorry – Pumpkin_Star Nov 03 '22 at 13:28
  • Redox is just changing oxidation numbers https://chemistry.stackexchange.com/questions/69798/an-introduction-to-oxidation-state-oxidation-number – Mithoron Nov 03 '22 at 13:47
  • @Mithoron Then why do we define oxidation and reduction this way.. Also the answer did not mention how oxidation and reduction makes things easier for chemists. – Pumpkin_Star Nov 03 '22 at 13:52
  • Transmutation is supposed possibility of chemical way of turning one element to another, mostly common or less precious metals to gold. It was widely believed as possible in medieval era in Europe. – Poutnik Nov 03 '22 at 15:51
  • @Poutnik So just to confirm we are speculating that CH4 will break into CH3- and H+ and that is why the oxidation state of carbon is negative here right? – Pumpkin_Star Nov 03 '22 at 16:40
  • @Poutnik Also why can't we perform this reaction in the lab? Like maybe in vacuum? – Pumpkin_Star Nov 03 '22 at 16:42
  • It is thought formal operation, based solely on electronegativity difference. In reality, bond C-H usually breaks homolytically, forming radicals. – Poutnik Nov 03 '22 at 16:44
  • @Poutnik So basically, CH4 breaks homolyticaly, but since we think that if it were to break heterolyticaly it would form CH3- and H+, we write the oxidation states of H as 1 and C as -4. Have I understood this correctly? – Pumpkin_Star Nov 04 '22 at 16:06
  • Yes, that is correct. – Poutnik Nov 06 '22 at 17:06
  • @Poutnik Thanks! This helped me a lot!! :) – Pumpkin_Star Nov 07 '22 at 03:13

1 Answers1

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Carbon is slightly more "electronegative" than hydrogen, so by convention C in CH4 is -4 and each hydrogen is +1, even though the bonds are considered covalent.

Differences in electronegativity are important in understanding polar molecules. More reading here.

With CO2, the carbon oxidation state is +4, because oxygen has a stronger affinity for electrons than carbon.

Robert DiGiovanni
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  • So the definition that oxidation is the loss of electrons is wrong in the case of CH4 right? The only reason H gets oxidised is because its oxidation state increases from 0 to1. And the reason we write its oxidation state as 1 is that carbon is more electronegative than hydrogen. Nothing about electrons right? – Pumpkin_Star Nov 04 '22 at 05:53
  • @Obinna we can observe that the net charge of elemental H, C, and molecular CH4 remain 0, and the bonding is "covalent", or (more or less) sharing the electrons to fill vacant orbitals. – Robert DiGiovanni Nov 04 '22 at 07:14