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When $\ce{FeCl3}$ is added in water it gives a pale yellow color, but on the addition of $\ce{KSCN}$ it changes to intense red color, so how would we explain the color change?

What I thought that the intensity would for sure change as when $\ce{FeCl3}$ is added with $\ce{KSCN}$ it would form a tetrahedral complex because $\ce{SCN^-}$ acts as a very weak field ligand, and as tetrahedral does not have center of symmetry La Porte Rule cannot be applied while in water it would form octahedral complex and on drawing the electronic configurations La Porte Rule will fail there, thus I came to the conclusion that color of the $\ce{KSCN}$ on would be more intense.

But how would we tell about the color change?

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    $\begingroup$ If someone is downvoting my post, would they at least tell me why are they doing so, I don't know all my previous posts were also heavily downvoted, and it only happens on chemistry stack exchange, on physics and math people do reply properly. $\endgroup$ Sep 13, 2023 at 12:52
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    $\begingroup$ For future reference: for the body of questions, answers, and comments, chemistry.se offers to use mhchem as a comfortable method to add chemical equations. $\endgroup$
    – Buttonwood
    Sep 13, 2023 at 13:12
  • $\begingroup$ @KutubkhanBhatiya "all my previous posts" -> there is only one other post and that was closed as HW. Perhaps, you can edit the question and clarify so it can get opened. Downvoters are not obligated to give an explanation. (I am not the downvoter btw) $\endgroup$ Sep 13, 2023 at 17:16
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    $\begingroup$ @NilayGhosh I am not trying to offend someone, but I deleted my previous posts as they were all closed due to downvotes $\endgroup$ Sep 13, 2023 at 17:38
  • $\begingroup$ @NilayGhosh And I also changed the questions by following the guidelines they attached about the homework question and I followed it as well but then too it didn't get open, so that's why $\endgroup$ Sep 13, 2023 at 17:51

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The choice of «intensity» is not a good one here, because the change from (pale) yellow to red relates to different positions of the absorption band(s) in terms of an energy split between orbitals / eventually recorded absorption wavelengths.

Second, in aqueous solutions $\ce{Fe^{3+}}$ typically is said to be an hexaaqua cation. The overall reaction in aqueous solution hence is better described as

$$\ce{[Fe(H2O)6]^{3+} + 3 SCN^- <=> [Fe(SCN)3(H2O)3] + 3 H2O}$$

However, the situation is a bit more complex (literally) because exchange of water by $\ce{SCN^-}$ is stepwise. This is to say that upon addition of the aqueous solution of $\ce{KSCN}$ you simultaneously have the equilibria of

$$\ce{[Fe(H2O)6]^{3+} + SCN^- <=> [Fe(SCN)(H2O)5]^{2+} + H2O}$$

and

$$\ce{[Fe(SCN)(H2O)5]^{2+} + SCN^- <=> [Fe(SCN)2(H2O)4]^+ + H2O}$$

as well as

$$\ce{[Fe(SCN)2(H2O)4]^+ + SCN^- <=> [Fe(SCN)3(H2O)3] + H2O}$$

which each can be described by an individual thermodynamic constant $K$.

Third, tetrahedral complex refers to a cation in the centre with four bonds extending to ligands. Neither the simplified description of $\ce{[Fe(SCN)3]}$, nor the better one of $\ce{[Fe(SCN)3(H2O)3]}$ fits this well. (Here, both water and $\ce{SCN^-}$ are monodentate ligands. So the number of ligands equates to the number of «bonds» between the ligands and the central metal cation.) A typical shape of complexes of formula $\ce{MA3B3}$ (with both $\ce{A}$ and $\ce{B}$ monodentates) is the octahedral one.

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  • $\begingroup$ Oh got it, but just to make it clear I am a first year college student and this is my first chemistry course, and this was one of the previous year question of this course, and we barely even covered tetrahedral geometry, so like about the color change is it that that as ligands are changing the value of $\Delta$ changes, that is why there is a change in color? $\endgroup$ Sep 13, 2023 at 16:53
  • $\begingroup$ @KutubkhanBhatiya How the MOs split (which are the orbitals lower, or higher in energy, relative to each other; the split of the ligand field to be square quadratic, tetrahedral, cubic, octahedral; how large is the energy gap between MOs) depends on the ligands, their number of bonds (monodentate/chelating ligand; hapticity of the central cation); initial electron configuration of the cation. For $\ce{ML6}$ complexes, depending if additional electrons «enter» orbitals antiparallel in spin to already present electrons, there are low-spin/high spin complexes. It certainly takes time to learn it. $\endgroup$
    – Buttonwood
    Sep 14, 2023 at 11:33
  • $\begingroup$ Hey @Buttonwood you seem pretty good, maybe you can attempt my question here chemistry.stackexchange.com/questions/172258/… $\endgroup$ Sep 15, 2023 at 0:58
  • $\begingroup$ @NealConroy I speculate someone deeply invested in quantum chemistry could provide an estimate which electronic transitions should be considered for $\ce{SCl2}$ and $\ce{S2Cl2}$, but it isn't me. $\endgroup$
    – Buttonwood
    Sep 15, 2023 at 15:35

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