Catalase or Temperature? A Reaction-Rate Claim Under Review
During a unit on reaction rates, a student compares the decomposition of 3.0% H2O2 with and without 0.50 g of MnO2. She uses a 50.0 mL gas syringe and records the volume of O2 every 10 s. The uncatalyzed trial is run first at 21.0 °C and produces 2.4 mL in 60 s. The catalyzed trial is run immediately afterward; the lab warms to 25.2 °C, and the syringe produces 19.6 mL in 60 s, though the plunger sticks near 15 mL and then jumps. She concludes that the catalyst increases the rate by a factor of about 8 and writes that the reaction is first order in H2O2. Her teacher asks her to defend the claim before the class. The class has one syringe, a thermometer, and a stopwatch; no pressure sensor is available.
1. Identify the most serious threat to the student's causal claim and describe a control procedure that could isolate the effect of the catalyst with the available equipment.
Read the worked answer
The most serious threat is confounding by temperature, not the catalyst alone. The catalyzed trial ran after the room warmed by 4.2 °C; a typical Q10 of 2–3 means that warming alone could raise the rate by roughly 30–50%, and the stuck syringe adds a mechanical artifact. A claim of an 8-fold increase is therefore not supported as a catalyst-only effect.
With one syringe, run paired trials in alternating order (control–catalyzed–control–catalyzed) after equilibrating both flasks in the same water bath at 21.0 °C. Record temperature at the start and end of each 10 s interval, use the same total liquid volume and H2O2 concentration, and discard trials in which the plunger sticks. Compare initial rates over the first 20 s, when substrate depletion is small, rather than the 60 s endpoint. If temperature still differs, report it and use the Arrhenius equation only as a sensitivity check, not as a correction.
2. The student wants to use the 60 s volumes to determine the reaction order in H2O2. Explain why this data set cannot establish the order, and describe a better experimental design using the same materials.
Read the worked answer
Order requires at least two different H2O2 concentrations with all else held constant; this data set has one concentration in each condition, and the two conditions differ in catalyst and temperature, so the effects are entangled. The 60 s volume is also not an initial rate: by 60 s the reaction may be slowing as H2O2 is consumed, and the total O2 that can be collected is limited by the amount of H2O2.
A better design: prepare 1.0%, 2.0%, and 3.0% H2O2 from the stock, add the same 0.50 g MnO2 at 21.0 °C, and record volume every 5 s. Use the slope of the steepest early linear portion (first 10–20 s) as the initial rate, convert volume to moles via the ideal gas law if pressure is known, and plot ln(initial rate) vs ln[H2O2]; the slope is the order. Include a blank without catalyst and replicate each concentration at least twice. Report syringe calibration and temperature.
3. Rewrite the student's conclusion in a form supported by the evidence and list two specific improvements for a peer-review report.
Read the worked answer
Supported conclusion: 'In this single pair of trials, adding 0.50 g MnO2 was associated with a much larger 60 s O2 volume (19.6 mL vs 2.4 mL), but because the catalyzed trial was 4.2 °C warmer and the syringe stuck, the size of the catalytic effect is uncertain and the data do not establish first-order kinetics.'
Two improvements: (1) temperature-equilibrate and randomize trial order, with at least triplicates; (2) calibrate the syringe by collecting a known volume or weighing dispensed water, and replace the sticking syringe; also use initial rates over multiple H2O2 concentrations. A peer review should note that the 8-fold factor is an observation, not a causal estimate, and that the first-order claim is unsupported.