The method and the results are on the rate equations lab. Do the analysis yourself first, then check the answers below.
Quick answer: A rate equation, rate = k[A]^m[B]^n, shows how concentration controls reaction speed; orders come from experiments, not from the balanced equation. These answers go with the Chemistry 11-12 lab on rate equations: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The model answers use the sample times in the tables. Check how concentration and 1/t are combined to deduce the order, how the Arrhenius gradient gives the activation energy, and how the sources of error are linked to improvements.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R2.2
Dividing 1000/t by concentration gives 379, 372, 375, 379 and 379, a constant value within experimental error. Rate is proportional to concentration (a straight line through the origin), so the reaction is first order in thiosulfate.
Question 3 Why use 1/t
The same amount of sulfur precipitate must form to hide the cross in every run, so the average rate is proportional to the amount divided by the time, that is to 1/t.
Question 4 Data for the Arrhenius plot
T = 293, 303, 313 and 323 K. 1/T = 3.413 × 10⁻³, 3.300 × 10⁻³, 3.195 × 10⁻³, 3.096 × 10⁻³ K⁻¹. ln(1/t) = −3.61, −2.94, −2.30, −1.70.
Judging when the cross vanishes varies between people (random error, giving scatter), and the temperature drifts during the run (systematic error that makes the high temperature rate appear too low). The thermometer also reads only to 1 °C.
Question 7 Improvements
Use a light sensor and a data logger to detect when the transmitted light falls to a set value, and run the mixture in a thermostatic water bath so the temperature stays constant. The same person should judge the end point if a sensor is not available.
Question 8 Plan to find the order in acid
Keep the thiosulfate volume, total volume and temperature fixed and change only the volume of acid (making up the volume with water). Measure t each time, calculate 1000/t and compare how it changes with acid concentration: a straight line through the origin means first order.
