Required practical · A-level chemistry
RP 3: Rate vs temperature: the disappearing cross
Hazards
⚠SO₂ evolved from thiosulfate + acid — toxic; ventilate, dispose promptly
⚠Dilute acids — irritant
Method
- Set up the cross tubesDisappearing cross (AQA reduced-scale exemplar): Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S. Put 10.0 cm³ of 0.05 mol dm⁻³ sodium thiosulfate in a tube over a marked cross; have ~10 cm³ of 1.0 mol dm⁻³ HCl ready in a second tube.
- Mix, time, and record both temperaturesRecord the start temperature, add 1 cm³ of the acid, start timing, and stop when the cross viewed from above is no longer visible. Record the final temperature and use the mean T for that run.
- Straight into the stop-bathEmpty the cloudy mixture straight into an alkaline stop-bath (sodium carbonate solution with phenolphthalein — replace it when the colour fades) rather than leaving it to stand.
- Five temperatures — never above 55 °CRepeat at ≥5 temperatures — room T, ~25, ~35, ~45, ~55 °C — warming the tubes in a water bath before mixing. AQA's limit: never above 55 °C (CLEAPSS advises staying below ~50 °C, because hot acidified thiosulfate gives off more SO₂).
- Plot rate = 1/t, then ArrheniusPlot rate = 1/t against mean temperature; for the Arrhenius extension plot ln(1/t) against 1/T — the gradient is −Ea/R.
Mistakes examiners see every year
| The mistake | Why it costs marks |
|---|---|
| Plotting time against temperature and never mentioning rate | AQA's 2019 examiner report calls this the most common reason for losing marks. Convert to rate = 1/t before plotting. |
| Assuming water-bath T equals mixture T | Record the mixture's own temperature at the start and end; use the mean. |
3 more examiner traps for this practical
Plus the step-by-step Lab Mode wizard with apparatus diagrams that highlight as you work.
Open it in ChemLabWhat to expect
Time falls sharply as temperature rises — rate roughly doubles per ~10 K (AQA spec 3.1.5.3). The ln(1/t) vs 1/T plot should be a straight line; a typical classroom Ea is ~45–50 kJ mol⁻¹ (indicative ballpark). Safety: SO₂ is toxic and asthmatics must be closely monitored — well-ventilated room, reduced-scale quantities (CLEAPSS SSS 35 / PP141).