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I am aware that since 2005, IUPAC guidelines established that hydrogen is more electronegative than all elements from group V in the periodic table (you can check official documentation). Thus, $\ce{NH3}$ is now named nitrogen trihydride and $\ce{PH3}$ is phosphorous trihydride. Nevertheless, now I am revisiting some material regarding chemical equations balancing and I found this reaction as an example:

$$\ce{4 ClO3- + 3 N2H4 -> 6 NO + 4 Cl- + 6 H2O}$$

In this reaction, $\ce{Cl^5+}$ reduces to $\ce{Cl^1-}$ and, supposedly, nitrogen (-2) from $\ce{N2H4}$ (hydrazine) oxidises to nitrogen +2 in $\ce{NO}$. Nevertheless, according to the new rules, nitrogen in hydrazine should have a positive oxidation number (+2) and hydrogen a negative one (-1). In such a case, there wouldn't be a redox reaction since nitrogen's oxidation number is not changing. So, what is the final answer? Are IUPAC ruled not applicable? or, based on the new guidelines, is this reaction not a redox type anymore?

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  • $\begingroup$ Chlorine's oxidation state is changing, so it must be a redox. Unless... were the oxidation state guidelines for any elements besides hydrogen changed? $\endgroup$
    – unstable
    Apr 7 at 3:30
  • $\begingroup$ If Cl goes from +$5$ to $-1$, Hydrogen must go from $-1$ to $+1$, and nitrogen stays at +$2$, as strange as it may look. . $\endgroup$
    – Maurice
    Apr 7 at 12:21

1 Answer 1

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The guidelines for naming compounds do not coincide with the guidelines for oxidation state, because:

(1) the formal electronegativity order for compound naming does not match the actual electronegativities of the elements as determined by bond polarities or electronic properties of the atoms;

(2) since 2016, oxidation state is defined in terms of molecular-orbital structure, which correlates with electronegativity differentials most but not all of the time (see here).

In the case of ammonia, the molecular-orbital structure is rendered as it has always been: the occupied (bonding) orbitals are polarized towards nitrogen, so that element is given the negative oxidation state no matter how we name the compound.

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