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I was under the impression that when talking about oxidation/reduction, we are referring to specific elements, and the changes in their oxidation numbers.

However, I saw the following reaction referred to as the oxidation of ammonia:

$$\ce{4 NH3 + 3 O2 -> 2 N2 + 6 H2O}\tag1$$

$\ce{N}$ changes its oxidation state from -3 to 0, whilst $\ce{H}$ stays in +1. It makes sense that we refer to this as the oxidation of nitrogen. Can we extend this understanding and also say that the compound $\ce{NH3}$ is being oxidized?

  1. Is it correct to talk about compounds being oxidized/reduced in this way?

  2. If so, how do we deal with more ambiguous cases where there is both oxidation and reduction of a compound happening? For example:

$$\ce{2 ClO3- -> 2 Cl- + 3 O2}\tag2$$

In this case $\ce{Cl}$ goes from +5 to -1 and is reduced, whereas oxygen goes from -2 to 0 and is oxidized. Is there a rule for which part of the compound you consider when talking about whether its oxidized or reduced? Or do you say the compound is being both oxidized and reduced?

Or do we consider the net movement of electrons/net change in oxidation state?

  1. Oxidation and reduction are also defined in terms of oxygen gain/loss. In a hypothetical reaction like the one below

$$\ce{CS2 + 2 H2O -> CH4 + SO2}\tag3$$

which element do you consider to be the 'main' part of the compound when talking about oxygen gain/loss? Here you have the carbon gaining hydrogen (reduction) whilst the sulfur is gaining oxygen (oxidation). Do you consider carbon to be the 'main' element and talk about the compound as being reduced due to carbon gaining hydrogen, or do you consider sulfur to be the main element and talk about the compound being oxidized due to it gaining oxygen?

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Your first impression was correct; it is elements that get oxidized/reduced. Talking of compounds as being oxidized/reduced is an abuse of language. This can be allowed when it creates no ambiguity, and can't otherwise.

To your examples:

  1. $\ce N$ get oxidized, $\ce H$ stays as it was. We talk about the compound as a whole getting oxidized, and that's just fine.
  2. $\ce{Cl}$ gets reduced, $\ce O$ gets oxidized. We simply don't talk about the compound as a whole getting either oxidized or reduced. These words are just not applicable here, they would make about as little sense as words like "vandalized" or "glorified". There is no rule to determine which way it would be, because there is nothing to determine. Neither do we say that a compound is being both oxidized and reduced.
  3. This is no different from the previous example. You may assign the oxidation states all right, and see that the two definitions of oxidation/reduction are in fact equivalent. $\ce C$ gets reduced, $\ce S$ gets oxidized, end of story. There is no main element.

So it goes.

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There is a historical change about the word oxidation. Before the middle of the $19$th century, oxidation was a reaction of a substance with oxygen. It was similar to "burning". For example glucose $\ce{C6H12O6}$ was oxidized when "burning" with oxygen $\ce{O2}$ and producing $\ce{CO2 + H2O}$. Today we say that it is not glucose, but the carbon atom which is oxidized in the same reaction. In a similar manner, $\ce{NH3}$ was oxidized when burning with $\ce{O2}$. Today we consider that only the nitrogen atom of $\ce{NH3}$ is oxidized in this reaction.

I agree this definition is somewhat ridiculous today. But you should go back to the times when the only experimental parameter available to the first chemists was the balance.

Same thing for reduction. Before 19th century, a reduction was a loss of oxygen atom and a "reduction" of solid mass. For example, $\ce{CuO}$ was reduced to copper atom when reacting with $\ce{H2}$, $\ce{CO}$ or $\ce{CH4}$, because the mass of the solid stuff ($\ce{CuO}$ becoming $\ce{Cu}$) decreases (= is reduced) between the beginning $\ce{CuO}$ and the final $\ce{Cu}$ of the reaction.

Same thing for potassium chlorate. Before the middle of the $19$th century, potassium chlorate $\ce{KClO3}$ was "reduced" to $\ce{KCl}$ when loosing oxygen $\ce{O2}$, because the mass of the solid stuff was "reduced" or decreased during the transformation.

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    $\begingroup$ Oxidation / reduction terminology for compounds is still current. Not in the sense of combustion, but in the sense of other, less extreme kinds of chemical conversions, such as oxidizing ethanol to acetic acid. $\endgroup$ Feb 24 at 20:26
  • $\begingroup$ @Jojn Bollinger. Thank you for the remark. You are right. I have not thought of this terminology, alcohols being oxidized to aldehydes and then to acids. Thank you. $\endgroup$
    – Maurice
    Feb 25 at 19:42

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