8.1 - Simple Experiments to Illustrate Le Chatelier's Principle

Materials and Apparatus Required

Chemicals

• Cobalt (II) chloride (or cobalt (II) nitrate)
• Deionised water
• Concentrated hydrochloric acid
• Ethanol
• Sodium chloride
• Crushed ice

Apparatus

• Boiling tubes and racks
• Dropping pipettes
• 250 cm³ Pyrex beakers
• 100 cm³ measuring cylinders
• Electronic balance
• Safety glasses

Safety Precautions

• Wear safety glasses at all times.
• Concentrated hydrochloric acid is highly corrosive and produces toxic fumes. Handle it in a fume cupboard.
• Cobalt chloride is harmful if ingested or if it contacts the skin. Avoid direct contact.
• Ethanol is flammable. Keep it away from open flames.

Method

1. Dissolve 4 g of cobalt chloride in 40 cm³ of deionised water to set up the equilibrium:

$$\text{CoCl₄²⁻ (blue)} + 6\text{H₂O} \rightleftharpoons \text{Co(H₂O)₆²⁺ (pink)} + 4\text{Cl⁻}$$

2. The solution will appear predominantly pink, indicating the equilibrium lies to the right.
3. In a fume cupboard, gradually add concentrated hydrochloric acid while stirring until the solution turns violet.
4. By trial and error, produce an intermediate violet colour.
5. Divide the violet solution into six boiling tubes.
6. Perform the following tests to observe shifts in equilibrium:

• Test 1: Add water to one tube. The solution will turn pink as the equilibrium shifts right.
• Test 2: Add concentrated hydrochloric acid to another tube. The solution will turn blue as the equilibrium shifts left.
• Test 3: Place one tube in hot water. The solution will turn blue as the endothermic reverse reaction predominates.
• Test 4: Place another tube in an ice bath. The solution will turn pink as the exothermic forward reaction predominates.

Results

• Effect of water: The solution turns pink, indicating the equilibrium shifts to the right.
• Effect of $HCl$: The solution turns blue, indicating the equilibrium shifts to the left.
• Effect of heat: The solution turns blue, favouring the endothermic reverse reaction.
• Effect of cold: The solution turns pink, favouring the exothermic forward reaction.

Example Questions with Answers