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

HAZARDS

Concentrated solution of HC1 will burn skin or damage clothing; avoid skin contact with acid. Reactions should be carried out in a hood or well-ventilated room; some poisonous SO2(g) is produced. Students should be cautioned against leaning over reaction vessel too closely as they observe the "x" under the beaker. Goggles should be worn throughout the experiment.

MODIFICATIONS/SUBSTITUTIONS

. Sodium thiosulphate is available from photographic supply stores as "hypo." "Fixer", also available from photographic supply stores, contains thiosulfate (usually in the form of Na2S2O3) and other chemicals used as hardening agents which do not seem to interfere with the reaction. Satisfactory results were obtained by using 47.4 g "fixer"/liter of solution.

2. HC1 solution is available from a hardware store as muriatic acid, 28% HCl; dilute this solution with 1 part distilled or deionized water to 1 part of muriatic acid to make a solution that is about 4.5 M and can be used in this experiment.

3. Plastic cups may be used in place of beakers.

PROCEDURE

(It is suggested that students work in pairs to facilitate the timing.)

. 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)

25

0

2

20

5

3

5

0

4

0

5

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." Add 5 mL HCl solution and begin timing the reaction as soon as the acid touches the sodium thiosulfate solution. Stir the reaction mixture at a constant rate throughout the reaction.

5. Stop timing when the "x" under the beaker is no longer visible through the solution. Record this time in your data table.

6. Repeat steps 2-4 for the remaining samples.

7. In step 1, why was it necessary to keep the total volume constant at 25 mL? As the volume of sodium thiosulfate solution used was decreased, how did the concentration of Na2S2O3 change?

8. Make a graph of your data by plotting the time (in sec) for each reaction on the y-axis against the volume (in mL) of sodium thiosulfate stock solution on the x-axis.

9. What relationship exists between the volume of sodium thiosulfate solution used and the time it takes for the reaction? What relationship exists between concentration and time?

0. Calculate the relative rate of each reaction by taking the reciprocal of each time measurement. Record these results in your data table.

1. Make a second graph by plotting the relative rate of each reaction on the y-axis against the volume of sodium thiosulfate solution on the x-axis.

2. 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

. 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.

REFERENCES

Dreyfus Curriculum Module, Chemical Kinetics and Catalysis, 1982. (No longer in print.) The use of the disappearing "x" to time the reaction is described.

Masterton, W., Slowinski, E., and Walford, E., Chemistry in the Lab, Rinehart and Winston, New York, 1980, p. 139. The same reaction is used to investigate the effects of concentration and temperature by timing until the first hint of cloudiness is observed.