Quick answer: A reaction mechanism is a sequence of elementary steps whose sum is the overall equation. This AP Chemistry lab on reaction mechanism 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.
What does a catalyst do to the rate of a reaction, and how can its effect be measured? Manganese dioxide speeds up the decomposition of hydrogen peroxide, giving oxygen gas that you collect in a syringe. By reading the volume against time you find the rate, check the order and show that the catalyst is not used up.
Level: AP Chemistry · Curriculum: AP Chem Unit 5
Aim
Measure the volume of oxygen released from 3 % hydrogen peroxide with manganese dioxide as catalyst, determine the rate constant, and test whether the catalyst is restored.
Equipment
- 3.0 % (w/v) hydrogen peroxide solution, manganese dioxide powder (0.20 g per run)
- 100 cm³ conical flask with side-arm bung, 100 cm³ gas syringe, stopwatch
- 10 cm³ measuring cylinder, filter paper and funnel, balance (±0.01 g)
Safety
Wear eye protection. 3 % hydrogen peroxide may irritate eyes and skin; rinse splashes with water. Do not seal the flask without the syringe, because the gas must escape. Keep flames away: oxygen supports burning. Do not inhale manganese dioxide dust.
Method
- Weigh 0.20 g of manganese dioxide on a small paper.
- Measure 10.0 cm³ of hydrogen peroxide into the flask and connect the gas syringe.
- Tip in the catalyst, close the bung at once and start the stopwatch.
- Read the syringe every 20 s until the plunger stops moving.
- Filter the mixture, dry the residue and weigh it.
- Repeat the run with no catalyst for 5 minutes as a control.
Results
| Time (s) | 0 | 20 | 40 | 60 | 80 | 100 | 120 |
|---|---|---|---|---|---|---|---|
| Volume of O₂ (cm³) | 0 | 42 | 67 | 82 | 92 | 97 | 101 |
Final volume when reaction stops: 106 cm³. Control without catalyst: no visible gas in 5 minutes. Mass of dried residue: 0.20 g.
Analysis questions
Question 1 Theory yield
Write the equation and calculate the expected volume of O₂ at room temperature (1 mol of gas = 24 dm³) from 10.0 cm³ of 3.0 g per 100 cm³ H₂O₂ (M = 34.0 g/mol).Question 2 Shape of the curve
Describe how the volume curve changes and explain why the slope decreases.Question 3 Average rates
Calculate the average rate between 0 and 20 s and between 100 and 120 s.Question 4 Order and rate constant
Calculate (106 − V) at each time from 20 s to 120 s, plot its natural logarithm against time and find k.Question 5 Is the catalyst used up?
State two pieces of evidence from the results that the catalyst is restored.Question 6 Control variables
List three variables to keep constant in a comparison of two catalysts.Question 7 Improving the lab
Suggest two improvements and predict the effect of doubling the mass of powder.
