Hydrochloric acid reacts with sodium thiosulfate to produce sulfur, sulfur dioxide, and sodium chloride. As sulfur forms, the solution becomes cloudy. The time taken for a marked “X” beneath the beaker to disappear is used to measure reaction rate at different concentrations of sodium thiosulfate, allowing the order of the reaction to be determined.

1. Label five 100-mL beakers 1–5 and draw a large “X” on the bottom of each. Place them on an overhead projector or light box so the marks are visible.
2. Measure the specified volumes of sodium thiosulfate solution (0.15 M) and distilled water into each beaker to prepare different concentrations:
   • Beaker 1: 50 mL Na2S2O3, 0 mL water
   • Beaker 2: 40 mL Na2S2O3, 10 mL water
   • Beaker 3: 30 mL Na2S2O3, 20 mL water
   • Beaker 4: 20 mL Na2S2O3, 30 mL water
   • Beaker 5: 10 mL Na2S2O3, 40 mL water
3. Measure 5 mL of 2 M HCl into a separate container for each beaker.
4. Add HCl all at once to the sodium thiosulfate solution in beaker 1, stir briefly, and start timing.
5. Stop timing when the black “X” is no longer visible through the cloudy solution. Record the reaction time.
6. Repeat the procedure for beakers 2–5.
7. Calculate 1/time for each trial and plot concentration versus reaction time and concentration versus 1/time to analyze the rate law.

The Disappearing Cross Experiment - Your Science Teacher:

Reaction Rate - Hydrochloric acid + Sodium Thiosulfate (Concentration) - Angles and Acid:

📄 Rate of Reaction of Sodium Thiosulfate and Hydrochloric Acid - Flinn Scientific: https://www.flinnsci.com/api/library/Download/78da6c8204aa48a294bd9a51844543ad?srsltid=AfmBOoqLBsxWmXyZ3iFfLDoScbD_tGWUaU4mgduwhXls9Z9IMRjttrNt

• Repeat the experiment varying HCl concentration to confirm the reaction order with respect to acid.
• Investigate the effect of temperature by warming or cooling the sodium thiosulfate solution before mixing.
• Downscale the experiment using a 6-well reaction plate or medicine cups for individual student trials.

• Wear safety goggles, gloves, and a chemical-resistant apron.
• Hydrochloric acid is corrosive; avoid skin or eye contact.
• Sodium thiosulfate solution is an irritant; handle with care.
• The reaction produces sulfur dioxide gas, which is an irritant; perform in a well-ventilated area and avoid inhalation.
• Clean beakers promptly after use to prevent sulfur deposits from hardening.

• Why does the “X” disappear more quickly in higher concentrations of sodium thiosulfate? (Because reaction rate increases with concentration, producing sulfur faster.)
• Why is 1/time used as a measure of reaction rate? (Because rate is inversely proportional to the time taken for the visible change to occur.)
• What does the straight-line relationship between concentration and 1/time suggest about the reaction order? (It suggests the reaction is first order with respect to sodium thiosulfate concentration.)
• Why is the reaction considered acid-catalyzed even though HCl concentration does not directly affect the measured rate? (The acid produces reactive intermediates like HS2O3⁻ that drive the decomposition.)