Core Practical 14: Determine Activation Energy (Arrhenius Experiment)

Reaction Summary

Overall reaction:
C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

The bromine is generated in situ from bromate(V) and bromide ions in acidic solution.

Indicator reaction: Once phenol is used up, excess Br₂ reacts with methyl red, causing a colour change (disappearance of colour), which marks the endpoint.

Safety Notes

• Phenol is corrosive and toxic – wear gloves and use in a fume cupboard.
• Sulfuric acid is an irritant.
• Potassium bromate(V) is an oxidising agent.
• Wear eye protection, lab coat, and conduct a risk assessment.

Apparatus and Chemicals

• 0.01 mol dm⁻³ phenol solution
• Mixed bromide/bromate solution:
  • 0.1 mol dm⁻³ KBr
  • 0.02 mol dm⁻³ KBrO₃
• 0.5 mol dm⁻³ sulfuric acid
• Methyl red indicator
• 2 boiling tubes, pipettes, beakers
• Water bath or kettle, thermometer (0–110 °C), stop clock

Procedure

1. Prepare hot water bath (start at 75 °C).
2. Pipette 10 cm³ of phenol solution and 10 cm³ of bromide/bromate solution into a boiling tube.
3. Add 4 drops of methyl red.
4. In a second boiling tube, pipette 5 cm³ sulfuric acid.
5. Place both tubes into the water bath until they reach temperature.
6. Mix both solutions by transferring them quickly between the tubes. Start timing.
7. Leave the tube in the bath. Stop timing when the methyl red colour disappears.
8. Repeat for other temperatures: 65, 55, 45, 35, 25, and 15 °C. Use ice for the lowest temperature if required.

Table to Record Results

Calculations and Graphing

1. Convert °C to K by adding 273.
2. Calculate 1/T and ln(t).
3. Plot a graph of ln t (y-axis) against 1/T (x-axis).
4. The gradient (m) of this graph = Ea / R.
5. Rearrange: Ea = gradient × R (R = 8.314 J mol⁻¹ K⁻¹).