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The purpose of this experiment is to determine the effect of changing the concentration of a reactant upon the rate of a reaction.

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sciencee PURPOSE The purpose of this experiment is to determine the effect of changing the concentration of a reactant upon the rate of a reaction. DESCRIPTION This investigation is appropriate for a general or first-year college-prep course. The effect of changing the concentration of thiosulfate ion is studied by observing the time required for a fixed amount of product to form. As the reaction proceeds, the solution becomes cloudy due to the formation of a colloidally dispersed precipitate of sulfur. In order to determine the point at which a specific amount of product has formed, an "x" is observed through the solution. The reaction is timed until the "x"" is no longer visible. TIME REQUIRED 40 to 50 minutes. MATERIALS Chemicals: 0.15 M sodium thiosulfate solution (23.7 g Na2S2O3 dissolved in deionized or distilled water to make one liter of solution)* 6 M HCl solution (dilute 500 mL concentrated HCl solution to one liter with distilled or deionized water)* distilled or deionized water Equipment: 250-mL beakers stirring rods 25-mL graduated cylinder clock which can measure seconds white paper *See Modifications / Substitutions HAZARDS Concentrated solution of HC1 will burn skin or damage clothing; avoid skin contact with acid. ...read more.


3. Plastic cups may be used in place of beakers. PROCEDURE (It is suggested that students work in pairs to facilitate the timing.) 1. Obtain five 250-mL beakers, about 30 mL of hydrochloric acid solution, and about 80 mL of sodium thiosulfate solution. Label the beakers from 1 to 5. Add the amounts of sodium thiosulfate solution and distilled or deionized water to each cup indicated in the following table: Beaker Number Volume of Sodium Thiosulfate (mL) Volume of distilled or deionized water (mL) 1 25 0 2 20 5 3 15 10 4 10 15 5 5 20 2. Note that the total volume in each beaker is 25 mL. 3. Make a table that shows the information in the table above and also includes a column for time (sec) and relative rate (sec-1). 4. Make a small "x" on a sheet of white paper with a pencil. Place a beaker containing the sodium thiosulfate solution over this "x." ...read more.


12. How does the relative rate of the reaction vary with concentration of sodium thiosulfate solution? If the concentration of the sodium thiosulfate solution doubled, what would happen to the relative rate of the reaction? DISPOSAL Small amounts of colloidal suspensions of sulfur can be washed down the drain. TIPS 1. Because "fixer" contains additional hardening agents exact molar concentrations of sodium thiosulfate cannot be prepared. For this reason, students are asked to plot time vs. volume of sodium thiosulfate solution rather than molar concentration. If "hypo" or pure sodium thiosulfate are used and students are able, they could calculate exact molar concentrations and use this information in making their graphs. In either case, they will reach the same conclusion about the relationship between concentration and rate of the reaction. 2. This experiment could be done as a demonstration on the overhead projector by placing a transparency on which an "x" has been made with a marker under the beaker or petri dish in which the reaction is carried out. 3. Students could extend this experiment by determining the effect of changes in HCl concentration on the rate of reaction. ...read more.

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