Ferric-Thiocyanate Equilibrium

Interactive Simulation of Le Chatelier's Principle

Reaction: $Fe^{3+}_{(aq)} + SCN^-_{(aq)} \rightleftharpoons [Fe(SCN)]^{2+}_{(aq)}$
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📖 Lab Manual: Aim, Principle & Procedure
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1. Aim

To study the shift in equilibrium between Ferric ions ($Fe^{3+}$) and Thiocyanate ions ($SCN^-$) by increasing or decreasing the concentration of either of the ions.

2. Principle

When Ferric Chloride ($FeCl_3$) reacts with Potassium Thiocyanate ($KSCN$), a reversible reaction occurs:

$Fe^{3+}_{(aq)} + SCN^-_{(aq)} \rightleftharpoons [Fe(SCN)]^{2+}_{(aq)}$
(Yellow) + (Colorless) $\rightleftharpoons$ (Deep Red)

Le Chatelier's Principle: if a system at equilibrium is disturbed by changing a concentration, it shifts in the direction that partially opposes that change. Work out for yourself which way each addition drives this equilibrium before you test it.

3. Materials

  • $0.1M$ Ferric Chloride ($FeCl_3$)
  • $0.1M$ Potassium Thiocyanate ($KSCN$)
  • Solid Oxalic Acid ($H_2C_2O_4$)
  • Reference Color Control (Tube A)

4. Procedure

  1. Tube A: Control tube (Do not alter).
  2. Tube B: Add $FeCl_3$ to increase $[Fe^{3+}]$.
  3. Tube C: Add $KSCN$ to increase $[SCN^-]$.
  4. Tube D: Add Oxalic Acid to decrease $[Fe^{3+}]$. (Oxalic acid forms a stable complex $[Fe(C_2O_4)_3]^{3-}$, removing $Fe^{3+}$ from the equilibrium).

🔮 Predict before you experiment

The red colour deepens as more [Fe(SCN)]²⁺ forms and fades as it breaks down. For each addition, predict which way the equilibrium shifts, then lock it to unlock the tubes.

⚠️ Safety

  • Potassium thiocyanate (KSCN) is toxic — never mix it with acids in a way that could release toxic HCN/thiocyanate fumes, and do not taste or ingest it.
  • Oxalic acid is toxic and corrosive — harmful if swallowed and irritating to skin/eyes; wear goggles and wash hands.
  • Ferric chloride solution is acidic and staining — avoid skin/clothing contact.
  • Use small volumes in the test tubes; do not point a tube mouth at anyone.
  • Collect the iron/thiocyanate waste in the labelled container — do not pour down the sink.

🧪 Reagents Bench

$FeCl_3$
$KSCN$
Oxalic Acid
Select a reagent to add to a test tube
A
B
C
D

📝 Observations Log

Tube Color Change Shift
A Deep Red Reference
B - -
C - -
D - -

Equilibrium Inference

Lock your prediction, then add a reagent to a tube to observe Le Chatelier's Principle in action.
Reagent added!