Reaction Kinetics — KMnO₄ + H₂O₂ Prac Protocol

Year 10 science: collision theory in action

Published

May 12, 2026

Year 10 Chemistry · Reactivity Rocket Project · Phase 1 Lab 1 · Simulation companion

Learning intentions. By the end of this prac, students can: (1) describe how concentration and temperature affect reaction rate using collision theory, (2) measure gas evolution and link it to stoichiometry, (3) explain why a reaction may proceed safely in one configuration and dangerously in another, and (4) reason about the engineering safety factors that separate the two.

1 The reaction

2\text{KMnO}_4 + 5\text{H}_2\text{O}_2 + 3\text{H}_2\text{SO}_4 \rightarrow 2\text{MnSO}_4 + \text{K}_2\text{SO}_4 + 8\text{H}_2\text{O} + 5\text{O}_2

In acidic solution, permanganate oxidises hydrogen peroxide. The reaction is highly exothermic (ΔH ≈ −200 kJ per mole of permanganate) and produces oxygen gas, water, and the brown manganese dioxide / pale pink Mn²⁺ products depending on pH.

2 Materials

Item Quantity Notes
Hydrogen peroxide solution, 3% w/w 50 mL per group Pharmacy-grade. Do not substitute stronger solutions.
Potassium permanganate solution, 0.005 M 20 mL per group Pre-mixed; deep purple
Dilute sulfuric acid, 1 M 10 mL per group Provides acidic medium
Conical flask, 100 mL 1 per group Open-mouthed; gas must vent freely
Gas collection tube, ≥250 mL with water trough 1 per group Sized for full stoichiometric yield
Digital thermometer, ±0.1 °C 1 per group Probe placed in solution, not headspace
Stopwatch 1 per group
Water bath, 25 °C ± 2 °C 1 per pair of groups Provides thermal mass for heat sink
Magnetic stirrer with stir bar 1 per group Continuous stirring required
Safety glasses, lab coat, gloves per student Standard PPE

3 Method

  1. Set up the gas collection apparatus. Verify the tube has capacity of at least 250 mL — the full stoichiometric yield from 50 mL of 3% H₂O₂ at room temperature is approximately 250 mL of O₂.
  2. Place 50 mL of 3% H₂O₂ in the conical flask. Add 10 mL of dilute H₂SO₄. Place the flask in the water bath and begin stirring at medium speed.
  3. Insert the thermometer. Record starting temperature.
  4. Add the KMnO₄ solution in one pour. Start the stopwatch.
  5. Record gas volume at 15-second intervals. Record solution temperature at the same intervals. Continue until gas evolution slows to less than 5 mL per 15-second interval, then for two further intervals.
  6. Repeat at two other water-bath temperatures (15 °C, 35 °C) — temperature must not exceed 40 °C.

4 Observations table

Time (s) Volume O₂ (mL) Solution T (°C) Notes
0 0 Pour completed
15
30
…

5 Analysis

  1. Plot volume of O₂ versus time for each temperature.
  2. Determine the initial rate (the gradient of the first linear portion) for each run, in mL/s.
  3. Calculate the moles of O₂ produced at completion and compare to the theoretical maximum from stoichiometry.
  4. Plot ln(initial rate) versus 1/T (in kelvin). The gradient is −Eₐ/R. Estimate the activation energy and compare to the literature value of 58 kJ/mol.

6 Collision theory questions

  1. The collision frequency in a solution at 25 °C is approximately 10²⁹ collisions per litre per second. The fraction with energy above Eₐ at 25 °C is about 5 × 10⁻¹¹. How many successful collisions occur per litre per second?
  2. If the temperature rises by 10 K, what happens to (a) the collision frequency and (b) the fraction above Eₐ? Which effect dominates?
  3. Doubling the concentration of H₂O₂ does what to the collision frequency between H₂O₂ and MnO₄⁻ ions? Why does this matter for the reaction rate?

7 Engineering questions — for high-agency students

  1. The autocatalytic decomposition threshold for H₂O₂ is around 70 °C. In an unstirred test tube, a hot spot near the catalyst can be 15–20 K above the bulk temperature. If a thermometer in the bulk reads 50 °C, what might the hot spot temperature be? What is the safety margin to autocatalytic onset?
  2. The decomposition rate roughly doubles for every 10 K rise. Above 70 °C the reaction also releases enough heat to keep itself decomposing without external input. Why do these two effects together mean that “stop heating it” is not a safe shutdown action above 70 °C?
  3. A 50 mL beaker holding 30 mL of reactants is producing O₂ at 5 mL/s and the temperature is rising at 0.5 K/s. The bulk thermometer reads 45 °C. (a) How long until the bulk temperature reaches the 70 °C threshold? (b) If the hot spot is 15 K above the bulk, how long until the hot spot reaches the threshold? (c) Which time is the relevant one for safety?
  4. Commercial H₂O₂ is sold at 3% (pharmacy), 6% (hair bleach), 35% (pool clarifier), 50% (industrial), 70% (research), and 90% (rocket propellant). It is never sold at 99%. Suggest why, using ideas about decomposition kinetics, thermal mass, and storage stability.

8 Why this reaction can become dangerous

The prac you have just performed is the same chemistry used as a rocket propellant in the German V-2 missile, the British Black Arrow rocket, and the Russian Soyuz launch escape system. The difference between the prac on your bench and a propellant system is not the chemistry — it is the engineering controls:

  • The school version uses dilute reagents (3% peroxide), small volume (50 mL), open vessel (gas vents freely), and active heat removal (water bath plus stirring).
  • The propellant version uses concentrated reagents (80%+ peroxide), larger volume, closed combustion chamber (gas is the point), and no heat removal (heat is the point).

Removing one safeguard at a time, the reaction transitions from a calm bubbling demonstration to a self-sustaining decomposition that cannot be stopped once started. The simulations that accompany this prac let you explore those transitions without doing them on a bench.

9 Safety reasoning

The temperature safety factor for this prac is built from four independent contributions:

T_\text{bulk, max} \le T_\text{critical} - \Delta T_\text{spread} - \Delta T_\text{measurement} - \Delta T_\text{uncertainty}

Where T_critical = 70 °C, ΔT_spread (hot spot above bulk in stirred test tube) ≈ 5 K, ΔT_measurement ≈ 3 K, and ΔT_uncertainty (reagent batch variation) ≈ 7 K. This gives a working bulk temperature ceiling of approximately 55 °C, and a standard prac ceiling of 40 °C is well below that.

If the thermometer reads above 45 °C during the prac: stop adding reagent, remove from the water bath, place in a cool water bath, do not cap or stopper the vessel, notify the teacher.

10 What is happening when the reaction “looks fun”

The videos online of this reaction being run at higher concentrations show chemistry that has already crossed three of the four engineering thresholds before the camera started recording. The lessons from those videos are about what failure looks like, not what success looks like. The success looks like a slow bubbling beaker on your bench with a thermometer reading 32 °C and an O₂ tube filling smoothly.

11 References for student further reading

  • Schumb, W. C., Satterfield, C. N., Wentworth, R. L. Hydrogen Peroxide. ACS Monograph 128, Reinhold (1955). Still the standard reference on industrial H₂O₂.
  • Sutton, G. P. Rocket Propulsion Elements, 9th ed. Wiley (2017). Chapter 7 covers monopropellant systems including H₂O₂.
  • Australian Standard AS 2714 — Hydrogen Peroxide. Storage and handling requirements.