Equilibrium of Iron (III) Thiocyanate
Publication information:
Abstract
What it Shows
Le Chatelier’s principle is demonstrated by color changes that result from the addition of solid chemicals to the iron (III) thiocyanate equilibrium system.
Link to Equilibrium of Iron (III) Thiocyanate video can be found here https://harvard.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=d12cdb20-d654-4a48-a96c-ae370148a6c2
How it Works
The ferric thiocyanate complex (FeSCN2+) is produced by mixing aqueous solutions of ferric chloride (FeCl3) and potassium thiocyanate (KSCN). Chloride and potassium ions are spectators, so the following equilibrium system is established:
Fe3+(aq) (yellow) + SCN-(aq) (colorless) ⇆ FeSCN2+ (reddish-brown)
According to Le Chatelier’s principle, when a reactant is added, the equilibrium moves to the right to produce more FeSCN2+ (more red-brown color). When a reactant is removed, the equilibrium moves to the left, producing more reactants to restore equilibrium (more yellow color).
Setting it Up
Safety: Wear safety glasses, gloves, and lab coat.
Make the ferric thiocyanate solution by mixing equal volumes of 0.0025 M FeCl3 solution with 0.005 M KSCN solution (concentrations are approximate). The demo is set up on a tray. A 100 mm glass petri dish has a thin layer of ferric thiocyanate solution, and four small, covered glass petri dishes have a small amount of solid chemical in each: iron (III) chloride, potassium thiocyanate, potassium phosphate monobasic, and silver nitrate. Each of the small dishes has a dedicated spatula with which to add small amounts of the solid chemical to the larger dish.
Starting with the original ferric thiocyanate solution under a document camera, add each of the following chemicals in small amounts without mixing. Iron (III) chloride makes the solution around the addition darker red. Potassium thiocyanate also makes a darker red ring around the addition. Each of these compounds adds to the reactant side of the equilibrium equation, resulting in more of the product ferric thiocyanate.
Adding potassium phosphate precipitates iron phosphate, removing Fe3+. Adding silver nitrate precipitates silver thiocyanate. These additions remove the reactants, which results in less of the product ferric thiocyanate, so more yellow results.
All four additions to the large petri dish can be seen at once if small amounts are used. Swirling the dish gently will clear the red brown color completely.
Disposal: Dispose of the solutions and dry chemicals in the inorganic chemical waste bucket.
Comments
This is a good small scale demonstration of equilibrium and Le Chatelier's principle, less toxic than the cobalt chloride complex equilibrium. Changing the temperature of the solution also shifts the equilibrium, since the equilibrium constant varies with temperature. Finding the equiibrium constant of this reaction using UV-Vis spectroscopy is a common lab experiment in gen chem. According to Reference 1, the equilibrium constant is approximately 113 at 20°C.
References
1. Colorimetric Determination of the Iron(III)−Thiocyanate Reaction Equilibrium Constant with Calibration and Equilibrium Solutions Prepared in a Cuvette by Sequential Additions of One Reagent to the Other. Frazier Nyasulu and Rebecca Barlag. Journal of Chemical Education 2011 88 (3), 313-314
Full text
What it Shows
Le Chatelier’s principle is demonstrated by color changes that result from the addition of solid chemicals to the iron (III) thiocyanate equilibrium system.
Link to Equilibrium of Iron (III) Thiocyanate video can be found here https://harvard.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=d12cdb20-d654-4a48-a96c-ae370148a6c2
How it Works
The ferric thiocyanate complex (FeSCN2+) is produced by mixing aqueous solutions of ferric chloride (FeCl3) and potassium thiocyanate (KSCN). Chloride and potassium ions are spectators, so the following equilibrium system is established:
Fe3+(aq) (yellow) + SCN-(aq) (colorless) ⇆ FeSCN2+ (reddish-brown)
According to Le Chatelier’s principle, when a reactant is added, the equilibrium moves to the right to produce more FeSCN2+ (more red-brown color). When a reactant is removed, the equilibrium moves to the left, producing more reactants to restore equilibrium (more yellow color).
Setting it Up
Safety: Wear safety glasses, gloves, and lab coat.
Make the ferric thiocyanate solution by mixing equal volumes of 0.0025 M FeCl3 solution with 0.005 M KSCN solution (concentrations are approximate). The demo is set up on a tray. A 100 mm glass petri dish has a thin layer of ferric thiocyanate solution, and four small, covered glass petri dishes have a small amount of solid chemical in each: iron (III) chloride, potassium thiocyanate, potassium phosphate monobasic, and silver nitrate. Each of the small dishes has a dedicated spatula with which to add small amounts of the solid chemical to the larger dish.
Starting with the original ferric thiocyanate solution under a document camera, add each of the following chemicals in small amounts without mixing. Iron (III) chloride makes the solution around the addition darker red. Potassium thiocyanate also makes a darker red ring around the addition. Each of these compounds adds to the reactant side of the equilibrium equation, resulting in more of the product ferric thiocyanate.
Adding potassium phosphate precipitates iron phosphate, removing Fe3+. Adding silver nitrate precipitates silver thiocyanate. These additions remove the reactants, which results in less of the product ferric thiocyanate, so more yellow results.
All four additions to the large petri dish can be seen at once if small amounts are used. Swirling the dish gently will clear the red brown color completely.
Disposal: Dispose of the solutions and dry chemicals in the inorganic chemical waste bucket.
Comments
This is a good small scale demonstration of equilibrium and Le Chatelier's principle, less toxic than the cobalt chloride complex equilibrium. Changing the temperature of the solution also shifts the equilibrium, since the equilibrium constant varies with temperature. Finding the equiibrium constant of this reaction using UV-Vis spectroscopy is a common lab experiment in gen chem. According to Reference 1, the equilibrium constant is approximately 113 at 20°C.
References
1. Colorimetric Determination of the Iron(III)−Thiocyanate Reaction Equilibrium Constant with Calibration and Equilibrium Solutions Prepared in a Cuvette by Sequential Additions of One Reagent to the Other. Frazier Nyasulu and Rebecca Barlag. Journal of Chemical Education 2011 88 (3), 313-314