Question

Iron (III) ion and thiocyanate ion exists in equilibrium with iron thiocyanate ion. The reaction, as written, is exothermic. Given that the FeNCS²⁺
product is a deep red color and the reactants are nearly colorless, which of the following will cause the solution to bedarker in color at equilibrium?
Fe³⁺(aq) + NCS^–(aq) ↔ FeNCS²⁺(aq)

		Increasing the solution temperature
		Adding FeNO3
		Removing Fe3+ ion by adding NaOH which reacts with Fe3+ ion.
		Removing NCS– ion by adding AgNO3 with reactacts with NCS– ion.
		None of these

Answer 1

1) The reaction is exothermic in nature ,so on increasing the temperature the equilibrium will shift in reverse direction(according to Le Chatlier's principle).In exothermic reaction, the reactants are higher in energy than the products ,so raising temperature yields more reactants.

The reactants are colorless,so color intensity decreases.

Answer: 2)Adding Fe(NO3)3 will increase the amount of reactant Fe3+ ,so the equilibrium will shift in forward direction to counteract the change ,thereby yielding more product.As the product is red in color ,so an increase in product concentration will intensify the red color.

3) 3NaOH(aq) +Fe³⁺(aq) ---->Fe(OH)3(s) + 3Na+ (aq)

As Fe3+ or reactant concentration decreases, the equilibrium will shift in reverse direction to yield more Fe3+ ions (thereby counteracting the change as per Le Chatlier's principle).Thus , product concentration decreases.

As the colored product FeSCN2+ concentration decreases, the color of the solution becomer lighter.

4)AgNO3 +SCN- ---->AgSCN (aq) +NO3-(aq)

The reaction of AgNO3 with SCN- ion produces soluble Ag-complex thereby decreasing the concentration of reactant SCN-.So, to counteract the decrease in SCN- concentration ,the equilibrium will shift in reverse direction.The concentration of FeSCN2+(red colored product) decreases and so the color intensity of the solution will decrease.