The Run That Did Not Fit
Students study the peroxodisulfate–iodide clock reaction at 25 °C: S2O8^2− + 2I− → 2SO4^2− + I2. Each run contains 0.0050 mol L−1 thiosulfate and starch, so 0.0025 mol L−1 peroxodisulfate has been consumed when the blue colour appears. Run 1: [I−] = 0.10 M, [S2O8^2−] = 0.10 M, 42 s. Run 2: 0.20 M, 0.10 M, 21 s. Run 3: 0.10 M, 0.20 M, 20 s. Run 4 repeats Run 1, but the water bath had drifted to 32 °C; it took 28 s. A student wants to delete Run 4 as an outlier and average the rest.
1. Determine the reaction orders with respect to I− and S2O8^2− and calculate a rate constant from Runs 1–3. Show your reasoning.
Read the worked answer
Rate = Δ[S2O8^2−]/t = 0.0025/t.
- Run 1: 0.0025/42 = 5.95 × 10^−5 M s−1
- Run 2: 0.0025/21 = 1.19 × 10^−4 M s−1
- Run 3: 0.0025/20 = 1.25 × 10^−4 M s−1
Compare Runs 1 and 2: [I−] doubles while [S2O8^2−] is constant, and the rate doubles. Order in I− = log(2)/log(2) = 1.
Compare Runs 1 and 3: [S2O8^2−] doubles while [I−] is constant, and the rate increases by 1.25 × 10^−4 / 5.95 × 10^−5 = 2.10. Order in S2O8^2− = log(2.10)/log(2) = 1.07, i.e. first order within experimental uncertainty.
Rate = k[I−][S2O8^2−]. From Run 1, k = 5.95 × 10^−5/(0.10 × 0.10) = 5.95 × 10^−3 M−1 s−1. Run 2 gives the same value; Run 3 gives 6.25 × 10^−3 M−1 s−1. Mean k ≈ 6.1 × 10^−3 M−1 s−1. Run 3 is slightly high, suggesting timing or temperature variation.
2. Should Run 4 be deleted? Justify your decision using the temperature change and the purpose of the experiment.
Read the worked answer
Run 4 should not be deleted simply because it disagrees. It is not a random outlier in the statistical sense; it was deliberately or accidentally run at a different controlled variable, temperature. The rate ratio is 42/28 = 1.5 for a 7 K rise, which is a plausible temperature effect (roughly a factor of 1.5–1.7 per 10 K). Deleting it would hide a systematic variable and bias the 25 °C data.
If the aim is k at 25 °C, exclude Run 4 from that particular average, but retain and report it as a 32 °C measurement. Use it to discuss the Arrhenius relationship. The correct response is to diagnose the drift, not to erase the evidence.
3. Propose two experimental improvements and one additional run that would strengthen the investigation.
Read the worked answer
Improvements: (1) thermostat the reaction cell and pre-equilibrate all solutions to 25.0 °C before mixing; measure temperature immediately before and after each run. (2) Use the same operator and a colorimeter or photodiode to detect the blue endpoint objectively, rather than visual timing; prepare fresh thiosulfate and iodide solutions to avoid oxidation.
Additional run: repeat Run 1 at 25 °C several times to estimate precision, then run the same concentrations at 30 °C and 35 °C to obtain an Arrhenius plot and an activation energy. This would test whether the 32 °C result is consistent with a smooth temperature trend rather than being an unexplained anomaly.