Le Chatelier's principle with color – FeSCN²⁺ and cobalt chloride

What it shows

Le Chatelier's principle says that a system at equilibrium responds to a change by shifting so as to partly oppose it. Here two classic color reactions make the shift visible: blood-red FeSCN²⁺ formed from Fe³⁺ and SCN⁻, and the pink-to-blue change of cobalt(II) as [Co(H₂O)₆]²⁺ turns into [CoCl₄]²⁻. Forward and reverse rates depend on concentration, so the mixture relaxes until Qc equals Kc. Concentration changes and dilution only move Qc, while temperature changes Kc itself. Time is slowed down, and the cobalt Kc and ΔH are illustrative values.

How to use

In the iron tab, let the mixture settle, then press Add Fe³⁺ or Add SCN⁻ and watch the red deepen. Press Add AgNO₃ or Add NaOH to precipitate an ion and see the color fade. Try Add water and compare with simple dilution. In the cobalt tab, move Water-bath temperature to heat or cool the tube, press Add conc. HCl, and read the Qc vs Kc gauge after each change.

Parameters you can change

• Equilibrium system Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺ (blood red), [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O (pink ⇌ blue)
• Initial Fe³⁺ concentration 0.5–4 mM
• Initial SCN⁻ concentration 0.5–4 mM
• Initial Cl⁻ concentration (cobalt system) 2–10 M
• Initial water-bath temperature (cobalt system) 0–90 °C

Questions to explore

1. Why does adding water make the FeSCN²⁺ solution paler than dilution alone would?
2. From the colors on heating and cooling, is the forward cobalt reaction exothermic or endothermic?
3. After adding Fe³⁺ and waiting for equilibrium, has Kc changed, and what happened to [SCN⁻]?