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Experiment 24: Iron(III) Thiocyanate Equilibrium

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Topic:
Chemical Equilibrium
Level:
Ordinary & Higher Level
Exam Focus:
Le Chatelier’s Principle

1. Experiment Snapshot

FeatureDetails
ApparatusTest tubes, droppers, test tube rack, measuring cylinders, white tile.
Chemicals/ReagentsIron(III) chloride solution, potassium thiocyanate solution, potassium chloride or iron(III) nitrate if used, distilled water.
Main observationA blood-red complex forms; colour intensity changes when concentration changes.
Main measurementColour intensity comparison.

2. Aim

To investigate the equilibrium between iron(III) ions and thiocyanate ions and observe how changes in concentration affect equilibrium position.

3. Quick Theory

Iron(III) ions react with thiocyanate ions to form a red complex ion. Changing concentration shifts the equilibrium according to Le Chatelier’s principle.

Fe³⁺ + SCN⁻ ⇌ [FeSCN]²⁺
Examiner Tip: The red colour becomes darker when equilibrium shifts to the product side.

4. Apparatus, Chemicals and Safety

CategoryDetails
ApparatusTest tubes, droppers, test tube rack, measuring cylinders, white tile.
ChemicalsIron(III) chloride solution, potassium thiocyanate solution, potassium chloride or iron(III) nitrate if used, distilled water.
Safety: Wear goggles. Iron(III) salts and thiocyanate solutions can irritate skin/eyes. Avoid ingestion and wash hands.

5. Labelled Experiment Diagram

Iron(III) Thiocyanate Equilibrium — Premium Practical Setup
Iron(III) Thiocyanate Equilibrium Reactants / SampleFe³⁺ + SCN⁻ Main Processequilibrium mixture Observation / Productred complex Colour intensity shows equilibrium shift.

6. Procedure

  1. Mix small volumes of iron(III) chloride and potassium thiocyanate solution.
  2. Observe the blood-red colour formed.
  3. Divide the mixture into several test tubes.
  4. Add extra iron(III) solution to one tube and observe colour change.
  5. Add extra thiocyanate solution to another tube and observe colour change.
  6. Dilute another tube with water and observe the colour becoming paler.
  7. Compare all tubes against a white background.
  8. Explain observations using Le Chatelier’s principle.
Examiner Tip: Use a white tile/background so colour changes are easier to see.

7. Observations and Results

Stage / TestObservationConclusion
Fe³⁺ addedRed colour darkensEquilibrium shifts right
SCN⁻ addedRed colour darkensEquilibrium shifts right
Water addedRed colour becomes palerMixture diluted/equilibrium affected
Reactant removed if applicableRed fadesEquilibrium shifts left
Conclusion: Changing concentration shifts the equilibrium to oppose the change, causing visible changes in red colour intensity.

8. Calculations / Key Chemical Reasoning

No complex calculation is normally needed; focus on direction of equilibrium shift.

Fe³⁺ + SCN⁻ ⇌ [FeSCN]²⁺
Worked Example:
Adding Fe³⁺ increases a reactant, so equilibrium shifts right to use up Fe³⁺ and forms more red complex.

9. Sources of Error and Improvements

Source of ErrorEffectImprovement
Too concentrated solutionsColour too dark to compareUse dilute solutions
Dirty test tubesContamination affects colourUse clean tubes
Different volumesUnfair comparisonUse equal volumes
No white backgroundHard to see colourUse white tile
Poor mixingUneven colourMix gently

10. Student Common Mistakes

  • Saying equilibrium stops.
  • Forgetting the reaction is reversible.
  • Saying darker red means less product.
  • Not applying Le Chatelier.
  • Using different volumes in comparison tubes.

11. Examiner Tips

Tip: Red complex is product.
Tip: Darker red usually means shift right.
Tip: Equal volumes make comparison fair.
Tip: Use Le Chatelier language: oppose the change.

12. Examiner Traps

Trap: Dilution can make colour paler without meaning reaction vanished.
Trap: Equilibrium is dynamic.
Trap: Adding reactant shifts equilibrium to product side.

13. 🎮 Interactive Simulator

Equilibrium Colour Shift

Choose concentration change and observe colour.



Observation Window

Pale red mix
Choose a condition and run the simulation.

Lab Score: 0/0

14. Past Paper Style Questions

  1. State the aim of this experiment. [2 marks]
  2. Name the main apparatus or reagent used. [2 marks]
  3. Describe the key observation expected. [2 marks]
  4. Give one safety precaution. [1 mark]
  5. Give one source of error and one improvement. [2 marks]
  6. Explain the chemistry behind the result. [3 marks]

15. Mark Scheme Answers

Q1. To investigate concentration effect on FeSCN equilibrium [2].

Q2. Test tubes, Fe³⁺ solution, SCN⁻ solution, droppers [2].

Q3. Blood-red colour forms/changes intensity [2].

Q4. Wear goggles and avoid skin contact [1].

Q5. Different volumes; use equal volumes [2].

Q6. Adding reactant shifts equilibrium right forming more red complex [3].

16. Quick Revision Card

Key PointMemory Hook
Red species[FeSCN]²⁺
Add Fe³⁺darker red
Add SCN⁻darker red
PrincipleLe Chatelier
Key ideadynamic equilibrium

17. Mastery Checklist

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