Quick answer: A rate law, rate = k[A]^m[B]^n, shows how reaction rate depends on reactant concentrations, with orders found by experiment. This AP Chemistry lab on rate law 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.
How does the concentration of sodium thiosulfate change the rate of its reaction with acid? In this lab you time the formation of a fixed amount of sulfur at five concentrations, turn the times into relative rates and use a graph to find the order in thiosulfate. The method suits a school lab at AP level.
Level: AP Chemistry · Curriculum: AP Chem Unit 5
Aim
Determine the order of reaction with respect to thiosulfate ions in the reaction S₂O₃²⁻(aq) + 2 H⁺(aq) → S(s) + SO₂(g) + H₂O(l), using 1/t as a measure of initial rate.
Equipment
- 0.100 M sodium thiosulfate solution, 1.0 M hydrochloric acid, distilled water
- Two 50 cm³ measuring cylinders, one 10 cm³ cylinder, 100 cm³ conical flask
- Stopwatch, white paper with a black cross, stirring rod
Safety
Wear eye protection. The reaction releases sulfur dioxide, which irritates the lungs: use small volumes, work in a ventilated room and rinse the flask in a fume cupboard or open area. Asthmatic students should not handle the mixtures.
Method
- Place the flask on the paper cross.
- Measure 50 cm³ of thiosulfate solution (no water) into the flask.
- Add 5.0 cm³ of 1.0 M acid, start the stopwatch and swirl once.
- Look down from above and stop the stopwatch when the cross disappears. Record t.
- Repeat with 40, 30, 20 and 10 cm³ of thiosulfate, adding water to make 50 cm³ each time, so the acid volume and total volume stay fixed.
- Rinse and dry the flask between runs and use the same observer each time.
Results
| Volume of thiosulfate (cm³) | Volume of water (cm³) | Time t (s) |
|---|---|---|
| 50 | 0 | 24 |
| 40 | 10 | 30 |
| 30 | 20 | 41 |
| 20 | 30 | 62 |
| 10 | 40 | 125 |
Analysis questions
Question 1 Fair test
Explain why the total volume and the acid volume are kept constant in all runs.Question 2 Mixed concentration
Calculate [S₂O₃²⁻] in the 55 cm³ final mixture for each run (0.100 M stock).Question 3 Relative rate
Calculate 1/t for each run and explain why it can stand in for the initial rate.Question 4 Graph
Describe a graph of 1/t against [S₂O₃²⁻] and state what its shape would show for first order.Question 5 Order from two points
Use the 20 cm³ and 40 cm³ runs to find the order in thiosulfate.Question 6 Uncertainty
The timing error is about ±1 s. Calculate the percentage uncertainty in t for the 24 s and 125 s runs.Question 7 Improvement
Suggest two improvements to the method and predict the effect of doing the same experiment at 10 °C higher.
