Leaving Cert Chemistry Reaction Rates: To investigate the effect of con

Question

To investigate the effect of concentration on a reaction rate, a student measured 100 cm³ of a 0.10 M solution of sodium thiosulfate into a conical flask, added 10 cm³ of 1.0 M hydrochloric acid, and then placed the flask on top of a cross on a sheet of white paper as shown in the diagram. The student noted the time (in minutes) taken for the cross on the sheet of paper to become obscured by the pale yellow precipitate formed in the solution. The reciprocal of the time (1/time) was used as a measure of the initial rate of the reaction.

Samples of the 0.10 M solution of sodium thiosulfate were diluted to make 100 cm³ portions of 0.08, 0.06, 0.04 and 0.02 M sodium thiosulfate. Each was then, in turn, reacted with 10 cm³ of 1.0 M hydrochloric acid as described above. The results obtained are shown in the following table.

| Concentration of sodium thiosulfate (M) | Time taken for the cross to become visible (min) | 1/time (min$^{-1}$) | Rate (M min$^{-1}$) |
|------------------------------------------|------------------------------------------------|---------------------|---------------------|
| 0.08                                     | 2.00                                           | 0.50                | 0.04                |
| 0.06                                     | 2.30                                           | 0.43                | 0.0258              |
| 0.04                                     | 3.08                                           | 0.32                | 0.0158              |
| 0.02                                     | 6.25                                           | 0.16                | 0.0080              |

a) Identify the pale yellow precipitate that obscured the cross on the sheet of paper.

b) Describe the procedure for preparing the 0.08 M solution of sodium thiosulfate from the 0.10 M solution.

c) Plot a graph to show the relationship between the initial rate of this reaction (1/time) and the concentration of sodium thiosulfate. The axes should be labelled.

d) Use the graph to determine how long it would take for the cross on the sheet of paper to become obscured in the reaction of the 0.05 M sodium thiosulfate solution.

e) Explain why the reciprocal of the time (1/time) may be used as a measure of the initial rate of the reaction.

a) Identify the pale yellow precipitate that obscured the cross on the sheet of paper.

b) Describe the procedure for preparing the 0.08 M solution of sodium thiosulfate from the 0.10 M solution.

c) Plot a graph to show the relationship between the initial rate of this reaction (1/time) and the concentration of sodium thiosulfate.

d) Use the graph to determine how long it would take for the cross on the sheet of paper to become obscured in the reaction of the 0.05 M sodium thiosulfate solution.

e) Explain why the reciprocal of the time (1/time) may be used as a measure of the initial rate of the reaction.

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