Why Does Fe(H2O)₆³⁺ Appear Brown Instead of Yellow in Water?

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    Inorganic chemistry
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Discussion Overview

The discussion revolves around the color appearance of the Fe(H2O)₆³⁺ complex in water, specifically addressing why it appears brown instead of yellow. Participants explore the implications of oxidation states, complex formation, and the subjective nature of color descriptions in chemistry.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant proposes a reaction equation involving the oxidation of Fe²⁺ to Fe³⁺ in the presence of Cl₂, questioning the correctness of their formulation.
  • Another participant agrees with the general idea of the reaction but raises the possibility that not all H₂O molecules are replaced by Cl⁻, suggesting a different complex may form.
  • Concerns are expressed about the discrepancy between textbook claims that Fe³⁺ is yellow and the observation that it appears brown in this context.
  • A participant notes that the difference between yellow and brown is largely a matter of intensity and suggests that discussing colors without reference to spectra lacks rigor.

Areas of Agreement / Disagreement

Participants express differing views on the color of the Fe(H2O)₆³⁺ complex, with some asserting it is yellow while others observe it as brown. The discussion remains unresolved regarding the reasons behind this color discrepancy.

Contextual Notes

There are limitations regarding the definitions of color and the subjective nature of color perception in chemistry, as well as unresolved questions about the formation of different complexes and the role of pH.

sdfsfasdfasf
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I've been tasked with writing an equation to show the reaction of the Fe2+ complex with Cl2. I believe that the oxidation state of the Iron has changed, so I would write 2Fe(H2O)(6)^2+ + Cl2 -> 2Cl- + 2Fe(H20)(6)^3+, but I can't be sure this is correct.

Is the 2Fe(H20)(6)^3 ion not yellow in water due to the formation of its conjugate base? If so, is there any hint in the question as to why it should appear brown? THANK YOU!!
 
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sdfsfasdfasf said:
2Fe(H2O)(6)^2+ + Cl2 -> 2Cl- + 2Fe(H20)(6)^3+

General idea is OK, whether some H2O molecules are not replaced with Cl-, producing a bit different complex, is another question.

sdfsfasdfasf said:
Is the 2Fe(H20)(6)^3 ion not yellow in water due to the formation of its conjugate base?

It is yellowish/brown even in low pH solutions, where the conjugate base is not present.
 
It's just strange because my textbook (from which this exam question is based off) claims that hydrated Fe3+ is yellow, and all other exam questions on this topic use that piece of information, its just this one that says its brown.
 
Difference between yellow and brown is mostly just intensity, these are not completely different colors. Especially when you take into account that these names of colors are very handwavy, there is no rigor behind. The only way to treat these things seriously is to talk not about "colors" but about spectra.
 
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The joys of chemistry.
 
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