Chemistry Equilibrium and Ligand Effects: Test Tube Reactions and Color Changes

Part 1

What happens in test tube B ? (Fe(NO3)3 added)

Fe(NO3)3 added, shifts equilibrium right (becomes darker red) as reactant concentration is increased. More FeSCN2+ forms

What happens in test tube C?:KSCN added and red colour becomes darker as SCN- increases reactants, Q decreases and Q < K, the equilibrium shifts right to form more FeSCN²⁺, causing the solution to become darker red.

What happens when NaOH is added?

Red fades significantly; solution may turn cloudy/light yellow, OH⁻ reacts with Fe³⁺ → Fe(OH)₃(s) forms, Fe³⁺ removed from solution, Q increases → equilibrium shifts left

What happens in test tube E (KCl added)?

Little to no visible change (may become very slightly lighter)Cl⁻ does not form a strong complex with Fe³⁺, Does not significantly change Q, No meaningful equilibrium shift

Part 2 What happens when water is added after HCl?

Pink solution, More water decreases the "concentration" effect of Cl⁻ → shift LEFT → becomes pink

What happens when you add HCl?

Solution turns blue, High Cl⁻ concentration added, Q decreases → equilibrium shifts right, Chloride ligands replace water ligands

What happens when you heat the system?

Pink to blue (shift right), Forward reaction is endothermic

What happens when you cool the system?

Blue → pink, Removing heat favors exothermic direction, Equilibrium shifts left

Part 3 What happens when you add water to CuSO₄ to test tube A?

Light blue solution, Cu(H2O)6]2+ complex

What happens when you add water to HCl to test tube B?

Blue-green → shifts toward CuCl complex → greenish-blue, Cl⁻ ligands bind copper, Equilibrium shifts right (CuCl₄²⁻)

Test Tube C Add NH₃?

Light blue → deep royal blue, Explanation: NH₃ is a stronger ligand than water, Forms Cu(NH3)4]2+

What happens when you add NH₃ to Test Tube B?

Yellow-green → cloudy blue green→ deep blue, NH₃ displaces Cl⁻ ligands, Equilibrium shifts toward ammonia complex, Color not uniform at first, NH₃ begins competing with Cl⁻ for Cu²

Add HCl to Test Tube C

Deep blue → fades → green/yellow-green, HCl protonates NH₃ (forms NH₄⁺), NH₃ removed → equilibrium shifts left, Chloride complex reforms