Experiment 8: Rates of Chemical Reactions - Detailed Notes

Experiment 8: Rates of Chemical Reactions

Background

• The experiment investigates the effects of concentration and temperature on chemical reaction rates.
• The specific reaction studied is between sodium thiosulphate solution and hydrochloric acid.
• Reaction equation: S2O3^{2-} (aq) + 2H^+ (aq) \rightarrow S (s) + SO2 (aq) + H2O (l)
  • S2O3^{2-} is the thiosulfate ion from sodium thiosulfate (Na2S2O_3) solution.
  • H^+ is the hydrogen ion from hydrochloric acid (HCl) solution.
  • Sulfur (S) is a yellow solid; its formation causes the solution to become opaque.
• Keep thiosulfate and hydrochloric acid separate until ready to initiate the reaction and timing.
• The key to a quantitative investigation is to control all variables except the one being tested.

Pre-Lab Work

• Read the procedure and plan how to test the effects of temperature and concentration on the reaction rate.
• Review Module 7 in the Study Guide and Module 7 Lecture ppt slides.
• Watch the provided videos:
  • Video 1: Reaction Rate
  • Video 2: Temperature Dependence on Reaction Rate
  • Video 3: Concentration Dependence on Reaction Rate

Learning Outcomes

1. Laboratory safety (safe handling of chemicals, personal protective equipment).
2. Planning own experimental procedure.
3. Applying serial dilutions to prepare various concentrations.
4. Developing new skills - using automatic pipette.
5. Understanding concentration and temperature effect on rates of chemical reactions.

Skills

• Skills developed during the experiment (unspecified in the provided text).

Safety Notes

1. Inform the demonstrator of any sulphur allergy.
2. Avoid placing face/nose directly above the volumetric flask to minimise exposure to sulphur fumes.
3. If hot water comes into contact with skin, immediately rinse under a cold water tap.

Procedure

• Place 45 mL of 0.25 mol/L sodium thiosulfate (Na2S2O_3) in a small conical flask on top of a black 'X' on white paper.
• Add 5 mL of 1.0 mol/L HCl, immediately start a stopclock, and briefly shake to ensure mixing.
• Record the time taken for the 'X' to disappear when viewed from above.
• The rate of disappearance of the 'X' is an indicator of the reaction rate.

Part 1: Effect of Concentration on Reaction Rate

• Plan a series of tests where you change the concentration of one reactant at a time.
• Test a range of concentrations to identify trends.
• The standard procedure uses 45 mL of thiosulfate and 5 mL of hydrochloric acid (total volume = 50 mL).
• To change thiosulfate concentration:
  • Add deionized water and less thiosulfate, ensuring the total volume is 50 mL.
• Alternatively, keep the amount of thiosulfate constant and vary the amounts of hydrochloric acid and water, ensuring the total volume is 50 mL.
• Record all results.

Part 2: Effect of Temperature on Reaction Rate

• Utilize available laboratory equipment to change the temperature of mixtures (cooling or warming).
• Control other variables (amounts of chemicals) while changing temperature.
• Ensure thiosulfate solution and hydrochloric acid are at the desired temperature before mixing.
• Test the reaction rate at several different temperatures (e.g., 0 °C, room temperature, 40 °C, 60 °C).
• Record all results.

Post-Lab Questions

1. Present results in a table using 1/time as a measure of reaction rate.
2. Plot separate graphs:
   • Reaction rate as a function of thiosulfate ([S2O3^{2-} ])
   • Reaction rate as a function of hydrogen ion concentration ([H^+ ])
   • Reaction rate as a function of temperature.
   • X-axis: concentration (or temperature).
   • Y-axis: reaction rate.