15.3.6 CIA Demonstration: Shifting the Equilibrium of FeSCN2+

Question 1

CIA Demonstration: Shifting the Equilibrium of FeSCN2+

Answer 1

• The color of thiocyanoiron(III) ion (FeSCN 2+ ) can be used to demonstrate Le Châtelier’s principle.
• Increasing a reactant shifts the equilibrium to produce more product making the solution darker.
• Decreasing a reactant shifts the equilibrium to produce more reactants making the solution lighter.

Question 2

note

Answer 2

• The color of the FeSCN 2+ ion can be used to demonstrate Le Châtelier’s principle. The FeSCN 2+ ion is a dark orange-red color, allowing visualization of the relative concentration of reactants and products.
• Increasing a reactant shifts the equilibrium to produce more product, making the solution darker.
• In the example, the concentration of iron(III) ion (Fe 3+ ) is increased by the addition of iron(III) nitrate (Fe(NO 3 ) 3 ). The ICE diagram shows the changes in the reactants and products.
• The same result can be obtained by adding thiocyanate ion (SCN – ).
• Decreasing a reactant shifts the equilibrium to produce more reactants, making the solution lighter.
• In the example, the concentration of iron(III) ion is decreased by the addition of hydroxide ion (OH – ). The hydroxide ion binds with the iron(III) ion and precipitates as iron(III) hydroxide (Fe(OH) 3 ). This causes the equilibrium to shift in favor of the products, producing more iron(III) ion to replace the loss. The shift is visualized as a lightening in the color of the solution. The ICE diagram shows the changes in the reactants and products.
• The same effect can be obtained through the addition of silver ion (Ag + ). Silver ion forms a precipitate with the thiocyanate ion, thus decreasing its concentration.