Measuring the Rate of Chemical Reactions

Overview

• Focus: Measuring reaction rate between Sodium Thiosulfate and Hydrochloric Acid.
• Observations: Formation of sulfur precipitate turning solution from clear to cloudy yellow.
• Method: Timed observation of a cross disappearing under the reaction vessel.

Experiment Setup

• Solutions: 50 cm³ of Sodium Thiosulfate (starting concentration: 40 g/dm³), 10 cm³ of Hydrochloric Acid.
• Equipment: Measuring cylinder, beaker, timer, pencil-drawn cross, distilled water for dilution.

Procedure

1. Measure 50 cm³ of Sodium Thiosulfate.
   • Use a measuring cylinder at eye level for accuracy.
   • Pour into a beaker.
2. Measure 10 cm³ of Hydrochloric Acid.
   • Use a smaller measuring cylinder or pipette for accuracy.
3. Start the timer as soon as acid is mixed with Sodium Thiosulfate.
   • Mix with a swirl and observe the cross from above.
   • Note the time until the cross disappears.

Initial Observation

• Most concentrated Sodium Thiosulfate: 27.44 seconds for cross to disappear.

Varying Concentrations

1. Measure 40 cm³ of Sodium Thiosulfate.
   • Add 10 cm³ distilled water for dilution.
2. Repeat mixing with 10 cm³ Hydrochloric Acid.
   • Observe the reaction time (32 seconds observed).

Theory

• Reaction rate dependent on particle collision frequency and energy.
  • More diluted solution: fewer Sodium Thiosulfate particles -> lower collision frequency -> slower reaction rate.

Consistency & Safety

• Constant variables: total solution volume (50 cm³), observer, observing conditions.
• Safety: Avoid inhaling sulfur dioxide fumes.

Data Collection

• Collect times for multiple concentrations (five values recommended).
• Repeat experiments for reliability and calculate mean averages.

Data Analysis

• Plot average reaction time against concentration of Sodium Thiosulfate.
• Expect a curved graph (line of best fit: a smooth curve).
  • Increased concentration -> decreased reaction time.
• Optional: Plot 1/time for reaction rate analysis.