Quick answer: Redox reactions transfer electrons: oxidation is loss of electrons and an increase in oxidation number, reduction is gain of electrons and a decrease. This Chemistry 11-12 lab on redox reactions gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How strong is the hydrogen peroxide sold for bleaching hair or cleaning wounds? You titrate a diluted sample against purple potassium manganate(VII) in acid, find the end point from the first permanent pink, and then convert the titre into a concentration and a percentage strength.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.2
Aim
To find the concentration of a hydrogen peroxide solution by titration with acidified potassium manganate(VII).
Equipment
- 0.0200 mol/dm³ potassium manganate(VII) in a burette
- Hydrogen peroxide solution diluted by the teacher (5.00 cm³ made up to 250 cm³)
- 1.0 mol/dm³ sulfuric acid, 25.0 cm³ pipette, conical flasks, white tile, funnel
Safety
Wear eye protection and gloves. Hydrogen peroxide of the original strength bleaches skin. Sulfuric acid of 1 mol/dm³ is an irritant. Manganate(VII) stains clothes and skin brown. Wipe spills at once.
Method
- Rinse the burette with the manganate(VII) solution, fill it and record the initial reading.
- Pipette 25.0 cm³ of the diluted peroxide into a conical flask and add about 20 cm³ of sulfuric acid.
- Run in the manganate(VII) while swirling. The purple fades slowly at first, then faster.
- Stop when one drop gives a pale pink that stays for 30 seconds. Record the final reading.
- Do one rough titration, then repeat until you have three results within 0.10 cm³.
Results
Sample burette readings:
| Titration | Initial (cm³) | Final (cm³) | Titre (cm³) |
|---|---|---|---|
| Rough | 0.00 | 18.10 | 18.10 |
| 1 | 0.50 | 18.15 | 17.65 |
| 2 | 18.15 | 35.85 | 17.70 |
| 3 | 0.20 | 17.80 | 17.60 |
Analysis questions
Question 1 Mean titre
Identify the concordant results and calculate the mean titre.Question 2 Why no indicator
Explain why no indicator is added and how the end point is recognised.Question 3 Choice of acid
Explain why sulfuric acid is used and not hydrochloric acid or nitric acid.Question 4 Moles of manganate(VII)
The equation is 2MnO₄⁻ + 5H₂O₂ + 6H⁺ → 2Mn²⁺ + 5O₂ + 8H₂O. Calculate the moles of MnO₄⁻ in the mean titre.Question 5 Hydrogen peroxide concentration
Calculate the moles of H₂O₂ in 25.0 cm³ of diluted solution and the concentration of the original solution in mol/dm³ and as a percentage (g per 100 cm³, Mr H₂O₂ = 34.0).Question 6 Uncertainty
Each of the two readings that make up a titre carries ±0.05 cm³. Work out the percentage uncertainty of a titre of 17.65 cm³.Question 7 Improving the method
Describe one way to improve accuracy and one observation that suggests a catalyst is at work at the start.
