When you mix aqueous iron(III) nitrate and potassium thiocyanate together you get a red solution. Then if you add sodium fluoride, the red colour disappears.

This is basically because the $\ce{Fe(SCN)^2+}$ ions are red, but $\ce{FeF^2+}$ ions are colourless.

I get Le Chatelier's principle in that, the more iron ions that are stolen by the fluoride, the less $\ce{Fe(SCN)^2+}$ ions there will be because they keep dissociating etc.

The thing I wonder about is why the fluoride atoms get all the iron. I can think of a couple of potential reasons for this, but I don't know which, if any of them, are true, even after googling a fair bit.

Could it be the case that iron is a lot more attracted to fluoride than thiocyanate? Or that the fluoride reaction isn't reversible? And if either of those is true, can this experiment be seen as evidence for that?

  • $\begingroup$ Well, I guess Fe(III) (aq) is "hard" enough to prefer F- from SCN- $\endgroup$ – Mithoron Apr 7 '17 at 15:22

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