Question

I recently performed a lab on Chemical Equilibrium.

Here is some background on the experiment:

-We made a stock solution by combining Fe(NO3)3 and NH4SCN. This made a blood red solution. Then, we added different reagents to see the change of color and the shift of equilibrium.

I'm being asked the following question: Based on the color of solutions reported, determine the direction of the reaction's: Fe^3+(aq)+SCN^- --> [FeSCN]^2+(aq) equilibrium shift upon each addition of each reagent investigated and write the chemical reaction involving the reagent that is reponsible for the shift.

We investigated the following additions:

a. 1 M iron (III) nitrate

-Upon adding iron (III) nitrate to the blood red stock solution, it turned a somewhat lighter red.

b. 1 M ammonium thiocyanate

-Upon adding this to the stock solution, it turned from blood red to a dark red/black

c. 0.1 M tin (II) chloride

-Upon adding this to the stock solution, it turned from blood red to a cloudy orange color.

Please explain how you determined the shift and equation, I don't understand how to find either.

Thank you so much for your help!

Answer 1

Fe³⁺(aq) + SCN^-    [FeSCN]²⁺(aq)

Yellow Colorless Deep red

(a)

Additions of Iron (III) nitrate

The Iron (III) nitrate is going to dissociate in your solution into Fe³⁺ and 3Cl^-. Your elemental iron III has an oxidation number of +3 and so does the Iron atom in [FeSCN]2+. But when you add your Iron (III) nitrate and it dissociates in solution, you are effectively adding in Fe³⁺ ions. In order for the reaction to re-establish equilibrium, it will try to use up the Fe³⁺ ions you just added and the reaction will shift to the right. Therefore, you will be making more [FeSCN]²⁺ and the reaction will become a darker red.

(b)

Additions of Ammonium thiocyanate

The NH4SCN is going to dissociate in your solution into NH4⁺ and SCN^-. When you add your NH4SCN and it dissociates in solution, you are effectively adding in SCN^- ions. In order for the reaction to re-establish equilibrium, it will try to use up the SCN^- ions you just added and the reaction will shift to the right. Therefore, you will be making more [FeSCN]²⁺ and the reaction will become a darker red.

(c)

Additions of Tin (II) chloride

SnCl₂ is tin(II), a reducing agent. The only thing that could be reduced is Fe(III) on the reactant side. Reducing Fe(III) to Fe(II) will lower the concentration of Fe(III), shifting the equilibrium to the left.