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I am trying to find the net-ionic equation for the reaction of potassium thiocyanate and iron(II) acetate.

It seems reasonable to me that the molecular equation is

$$\ce{2KSCN + Fe(CH3COO)2 -> 2CH3COOK + Fe(SCN)2}$$

which would give the net-ionic form

$$\ce{SCN- + Fe^2+ -> Fe(SCN)2}$$

My chemistry teacher says the correct form is

$$\ce{4SCN- + Fe^2+ -> Fe(SCN)4^2-}$$

First I don't know what molecular equation would give rise to this. Second, why in the world is my guess, which is simpler, wrong and this one correct?

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  • $\begingroup$ In the molecular equation, you have written cyanide (CN-) on the left and thiocyanate on the right. The next equation you have written, has one SCN- on the left and two on the right so it is not balanced. The last equation is balanced, but it does not represent the molecular equation because the product is not the same. $\endgroup$ – DavePhD Jan 13 '15 at 12:53
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If you add an excess of cyanide ions to a iron(II) solution you could conceivably get precipitate of $\ce{Fe(SCN)2}$ which would then dissolve to form a complex with four thiocyanate ions. However, I can't find any references to such a complex on the internet.

The equations would be: $$\ce{Fe^{2+}_{(aq)} + 2SCN-_{(aq)} -> [Fe(SCN)2]_{(s)}}$$ $$\ce{[Fe(SCN)2]_{(s)} + 2SCN^{-}_{(aq)} -> [Fe(SCN)4]^{2-}_{(aq)}}$$

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As best I can tell, your answer is correct. I can't find any reference for iron(II) thiocyanate in the form your teacher says is correct, and several references that say $\ce{Fe(SCN)2}$ is the correct form.

For precipitation reactions, the charge will never change. You should always assume that the charges in the product will balance. There might be some cases where the product has an unexpected formula that you can't predict based on charges alone, but I can't think of any off the top of my head, and you certainly shouldn't be expected to know them at this stage.

edit:

This is the best reference I could find, it's a google books link to the CRC handbook

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  • $\begingroup$ I guess there is a difference here whether you talk about the complex (which can be a negative anion with more than two thiocyante coordinating it), and the stoichiometry of a solid, which will be di-thyociano per definition for the charge equilibrium (but it is not necessarily the number of ligands coordinating the metal). $\endgroup$ – Greg Jan 13 '15 at 1:13
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    $\begingroup$ @Greg - could be, but this type of question is usually asked in general chem I, so I don't know how the student would be expected to know that, let alone predict it from the reactants. $\endgroup$ – thomij Jan 13 '15 at 3:33
  • $\begingroup$ The textbook may include reference for given complexes, and teacher would like to test that knowledge. So the short answer is that you don't predict it, you learn it. While I don't necessarily agree with this practice, I have seen it in several occasion (including high-school level texts..). $\endgroup$ – Greg Jan 13 '15 at 9:11
  • $\begingroup$ @Greg - while we are making conjectures, consider also the possibility that the teacher is just wrong. Since I can't find any references to that complex, I think that possibility is more likely. $\endgroup$ – thomij Jan 13 '15 at 14:02
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    $\begingroup$ Well, another trouble is that this is NOT a precipitation reaction. Fe(SCN)$_2$ is water-soluble, so a genchem student who didn't know about complex formation would have to conclude that all of the ions were spectators and that there was no reaction. $\endgroup$ – Fred Senese Jan 13 '15 at 15:37

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