Sodium Thiosulfate Reaction

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2009

Publication No. 91860

Rate of Reaction of Sodium Thiosulfate and Hydrochloric Acid Rate Laws
Introduction
The purpose of this demonstration is to investigate the effect of sodium thiosulfate concentration on the rate of reaction of sodium thiosulfate with hydrochloric acid. The reaction, which produces solid sulfur, will be followed by measuring the time needed for the reaction mixture to become opaque. The results will be analyzed graphically to determine the order of reaction— the mathematical relationship between the reactant concentration and the rate.

Concepts
• Kinetics • Order of reaction • Rate law • Concentration

Materials
Hydrochloric acid solution, HCl, 2 M, 25 mL Sodium thiosulfate solution, Na2S2O3, 0.15 M, 150 mL Distilled or deionized water Beakers, 100-mL, 5 Graduated cylinders, 50- or 100-mL, 2 Graduated cylinders, 10-mL, 5 Overhead projector or light box Permanent marker Stirring rods Stopwatch or timer

Safety Precautions
Hydrochloric acid solution is corrosive to eyes and skin. It is moderately toxic by ingestion and inhalation. Sodium thiosulfate solution is a body tissue irritant. The reaction of sodium thiosulfate and hydrochloric acid generates sulfur dioxide gas, which is a skin and eye irritant. Perform this demonstration in a well-ventilated lab only. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, temperature-resistant gloves, and chemical-resistant apron. Please review current Material Safety Data Sheets for additional safety, handling, and disposal information.

Procedure
1. Label five 100-mL beakers 1–5 and clean the bottom of each beaker. 2. Draw a large “X” across the bottom on the outside of each beaker. Place the beakers on an overhead projector stage or a light box so that students can view the “X.” 3. Using separate graduated cylinders for the solution and water, measure and add the required amounts of 0.15 M sodium thiosulfate and distilled water to each beaker. Be as precise as possible. Beaker 0.15 M Na2S2O3 Distilled Water 1 50.0 mL 0 mL 2 40.0 mL 10.0 mL 3 30.0 mL 20.0 mL 4 20.0 mL 30.0 mL 5 10.0 mL 40.0 mL

4. Have students calculate the final concentration of sodium thiosulfate in each beaker 1–5. 91860 011509

Flinn Scientific—Teaching Chemistry eLearning Video Series

5. Record the following information in a data table: Beaker, volume of Na2S2O3 solution, volume of distilled water, concentration of Na2S2O3, reaction time (sec), and 1/reaction time (reaction rate). See the Sample Data and Results table in the Discussion section. 6. Measure 5.0 mL of 2 M hydrochloric acid into each of five 10-mL graduated cylinders. 7. Starting with beaker #1, carefully add the HCl all in one pour to the sodium thiosulfate solution. Stir the solution once with a stirring rod and immediately start timing. 8. Stop timing when the black “X” is no longer visible. Record the reaction time in seconds in the data table. 9. Repeat steps 7 and 8 with beakers 2–5. 10. Calculate 1/reaction time for each trial. Plot concentration vs. time and concentration vs. 1/time on separate graphs.

Disposal
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. Collect the leftover reaction mixtures and filter to separate the solid sulfur product. The sulfur may be disposed of in a landfill according to Flinn Suggested Disposal Method #26a. The filtrate may be neutralized and disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Tips
• This activity may be performed as a chemical demonstration with classroom participation or as a student activity. The contents of the beakers project well on an overhead projector and the time to the disappearance of the black “X” is easily seen and measured. • The reaction may be downsized for a student lab activity. Carry out individual trials in separate...
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