# Question 0322c

Jul 13, 2017

Here's what I got.

#### Explanation:

The trick here is that the iron(III) cations will combine with the thiocyanate anions to form the thiocyanatoiron(III) complex ion, "Fe"("SCN")^(2+).

Now, you could say that the balanced chemical equation that describes this reaction looks like this

${\text{Fe"("NO"_ 3)_ (3(aq)) + 3"KSCN"_ ((aq)) -> "Fe"("SCN")_ (3(aq)) + 3"KNO}}_{3 \left(a q\right)}$

However, that would not be an accurate depiction of what happens in solution because "Fe"("SCN")_3 is actually soluble, so it will not exist as "Fe"("SCN")_3 in solution.

If you keep in mind the fact that the reaction produces the thiocyanatoiron(III) complex ion, you can say that the complete ionic equation that describes this reaction looks like this

"Fe"_ ((aq))^(3+) + color(red)(cancel(color(black)(3"NO"_ (3(aq))^(-)))) + color(red)(cancel(color(black)(3"K"_ ((aq))^(+)))) + overbrace("SCN"_ ((aq))^(-) + color(red)(cancel(color(black)(2"SNC"_ ((aq))^(-)))))^(color(blue)("= 3SCN"_ ((aq))^(-)))-> "Fe"("SCN")_ ((aq))^(2+) + color(red)(cancel(color(black)(2"SNC"_ ((aq))^(-)))) + color(red)(cancel(color(black)(3"K"_ ((aq))^(+)))) + color(red)(cancel(color(black)(3"NO"_ (3(aq))^(-))))

The nitrate anions, the potassium cations, and $2$ of the $3$ thiocyanate anions are spectator ions because they are present on both sides of the chemical equation, so you can say that the net ionic equation that describes this reaction will be

"Fe"_ ((aq))^(3+) + "SCN"_ ((aq))^(-) -> "Fe"("SCN")_ ((aq))^(2+)

Now, you can write an even more accurate depiction of what happens in solution by taking into account the fact that iron(III) cations exist as hexaaquairon(III) complex ions in aqueous solution.

"Fe"_ ((aq))^(3+) + 6"H"_ 2"O"_ ((l)) rightleftharpoons "Fe"("H"_ 2"O")_ (6(aq))^(3+)

The reaction between the hexaaquairon(III) complex ion, "Fe"("H"_2"O")_6^(3+), and the thiocyanate anion to form the pentaaquathiocyanatoiron(III) complex ion, ["Fe"("H"_2"O")_5("SCN")]^(2+).

So the net ionic equation that describes this reaction can be written like this

"Fe"("H"_ 2"O")_ (6(aq))^(3+) + "SCN"_ ((aq))^(-) rightleftharpoons ["Fe"("H"_ 2"O")_ 5("SCN")]_ ((aq))^(2+)#

This reaction is used a test for iron(III) cations because the addition of the clear solution of thiocyanate anions to a yellowish solution that contains the iron(III) cations as the hexaaquairon(III) complex ions will change the color of the solution to blood red.