Equilibrium of Iron (III) Thiocyanate

The equilibrium between iron(III) ions, thiocyanate ions, and the red-brown ferric thiocyanate complex can be shifted by adding or removing reactants. Color changes from yellow to red-brown or vice versa demonstrate Le Chatelier’s principle in action.

1. Prepare the ferric thiocyanate solution by mixing equal volumes of dilute ferric chloride solution and potassium thiocyanate solution.
2. Pour a thin layer of this reddish-brown solution into a glass petri dish.
3. Add a small amount of iron(III) chloride solid to one area of the dish and observe the color darkening to red-brown.
4. Add potassium thiocyanate solid to another area and observe a similar red-brown darkening.
5. Add potassium phosphate solid to a different area, which removes Fe³⁺ ions by precipitation, shifting the equilibrium back and causing a more yellow color.
6. Add silver nitrate solid to another area, which removes SCN⁻ ions by precipitation, also shifting the equilibrium left and producing more yellow.
7. Compare all four effects side by side in the same dish for a striking visual display.

Le Chatelier's Principle - Iron (III) Thiocyanate - Chemistry and Biochemistry Demo lab at OSU:

Iron(III) Thiocyanate Equilibrium - Science Ready:

📄 Equilibrium of Iron (III) Thiocyanate - Harvard Natural Sciences Lecture Demonstrations: https://sciencedemonstrations.fas.harvard.edu/presentations/equilibrium-iron-iii-thiocyanate

• Warm the solution to observe temperature effects on the equilibrium position.
• Use a spectrophotometer to measure absorbance changes and determine the equilibrium constant.
• Demonstrate the reaction in a cuvette under a document camera for larger audiences.

• Wear safety glasses, gloves, and a lab coat when handling chemicals.
• Handle silver nitrate with care; it stains skin and clothing.
• Work with small amounts of solid chemicals to minimize waste and exposure.
• Dispose of all solutions and solids in an inorganic chemical waste container.

• Why does adding FeCl₃ or KSCN make the solution more red-brown? (Adding reactants shifts equilibrium to the right, forming more FeSCN²⁺ complex.)
• Why does adding potassium phosphate or silver nitrate make the solution more yellow? (They remove Fe³⁺ or SCN⁻, shifting equilibrium left toward reactants.)
• How does this experiment illustrate Le Chatelier’s principle? (The system responds to stresses such as added or removed species by shifting equilibrium to counteract the change.)
• How could you determine the equilibrium constant experimentally? (By measuring absorbance of FeSCN²⁺ at a known wavelength and applying Beer’s Law.)