$\ce{NCl3}$ on hydrolysis produces $\ce{NH4OH}$ and $\ce{HOCl}$.

$\ce{PCl_3}$ when hydrolyzed produces $\ce{P(OH)3}$ i.e. $\ce{H3PO3}$.

$\ce{AsCl_3}$ when hydrolyzed produces $\ce{As(OH)3}$.

However when $\ce{SbCl3}$ or $\ce{BiCl3}$ are hydrolyzed they produce $\ce{SbOCl}$ and $\ce{BiOCl}$ respectively.

Why are the products so widely varying ? How can we explain the reactions theoretically ?

  • 1
    $\begingroup$ The first one is because nitrogen is more electronegative than chlorine while all the others are electropositive with respect to chlorine. The antimony and bismuth reactions are probably due to some stability of those compounds that I, unfortunately, don’t know about. $\endgroup$
    – Jan
    Dec 16, 2016 at 22:59

1 Answer 1

  1. Nitrogen trichloride

Nitrogen trichloride is quite unstable and hydrolyse easily:

$$\ce{NCl3 + 3 H2O → NH3 + 3 HOCl~~~~~~~~(1)}$$

$$\ce{NH3 + H2O -> NH4OH~~~~~~~(2)}$$

$$\ce{NCl3 + 4H2O -> NH4OH + HOCl~~~~~~~(1) +(2)}$$

  1. Phosphorus trichloride

$$\ce{PCl3 + 3H2O → H3PO3 + 3HCl}$$

  1. Arsenic trichloride

$$\ce{AsCl3 + 3 H2O → As(OH)3 + 3 HCl~~~~~~~~(1)}$$

This is the first and main reaction of hydrolysis of arsenic chloride to form arsenous acid. But other species (arsenite ions) like $\ce{[AsO(OH)2]−}$ and $\ce{[AsO2(OH)]^2−}$ also exist in solution and are the conjugated bases.

$$\ce{AsCl3 + 4H2O ⇄ H[As(OH)4] + 3HCl~~~~~~~(2)}$$

$$\ce{AsCl3 + 2H2O ⇄ HAsO2 + 3HCl~~~~~~~~~(3)}$$

And at last aresnic trichloride fully hydrolyse to form arsenic(III) oxide

$$\ce{2AsCl3 + 3H2O → As2O3 + 6HCl~~~~~~~~~~(4)}$$

  1. Antimony trichloride

$$\ce{SbCl3 + H2O → SbOCl + 2HCl~~~~~~~~(1)}$$

With more water it forms $\ce{Sb4O5Cl2}$ which on heating to 460° forms $\ce{Sb8O11Cl2}$.

$$\ce{SbCl3 + H2O ⇄ Sb(OH)Cl2 + HCl~~~~~~~(2)}$$

$$\ce{4SbCl3 + 5H2O ->[50 C] Sb4O5Cl2 + 10HCl~~~~~~~(3)}$$

Final reaction to form antimony trioxide.

$$\ce{2SbCl3 + 3H2O ->[\Delta] Sb2O3 + 6HCl~~~~~~~~~~(4)}$$

Note that speculation on the composition of antimony oxychloride has been raised. Some say it is a mixture of antimony trioxide and antimony trichloride.

  1. Bismuth tricloride

$$\ce{BiCl3 + H2O -> BiOCl + 2HCl~~~~~~~~(1)}$$

Main reaction to form bismuth oxychloride. Further hydrolysis yield and intermediate monohydrate $\ce{BiCl3.H2O}$.

$$\ce{BiCl3 + H2O(vap) ->[50 C] BiCl3.H2O~~~~~~~~(2)}$$

$$\ce{$$BiCl3 + 2H2O → Bi(OH)2Cl + 2HCl~~~~~~~~~(3)}$$


Now, you may been be wondering that hydrolysis of nitrogen and phosphorus yield one product but that of arsenic, antimony and bismuth yield 3 or 4 products. This is because of presence of vacant d or f orbitals where electrons can reside easily. For this reason, $\ce{Sb^3+, Bi^3+}$ solvate in aqueous solution to form antimonyl or bismuthyl ($\ce{SbO+, BiO+}$) and other species like $\ce{[Bi6O4(OH)4]^6+, [Bi6(OH)12]^6+ , [Bi(H2O)9]^3+}$ and similar antimony ions. Other factors also play role like relativistic effects, lanthanoid contraction, inert pair effect etc.


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