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Effect of Temperature

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Summary

The effect of temperature on the rate of the iodine clock reaction is shown. Click on any image to enlarge it.

Video

These images are associated with the video Effect of Temperature

Video Narration

Narration

This experiment tests the effect of temperature on the rate of the iodine clock reaction. The reaction rate is greater at higher temperatures.

Temperatures (20 ^o C, 8 ^o C)

Discussion

In the experiment testing the influence of volume on reaction rate, the color changes in the four vials do not occur at the same instant, because not all four solutions were mixed at exactly the same time. The important point is that the color changes were nearly simultaneous, especially compared to the overall time for the reaction. As long as the concentrations are the same, solution volume should not affect the rate of reaction.

The first step in this reaction is the formation of triiodide ion:

H₂O₂ + 3 I^- + 2 H⁺ --> I₃^- + 2 H₂O

In the absence of thiosulfate ion the triiodide ion would form the characteristic blue complex with starch. However, the triiodide ion is rapidly reduced back to iodide ion by thiosulfate:

I₃^- + 2 S₂O₃^2- --> 3 I^- + S₄O₆^2-

In this clock reaction thiosulfate ion is the limiting reactant. The blue starch-triiodide complex forms only when all the thiosulfate ion has been consumed.

The initial concentration of thiosulfate ion is the same in every reaction in this demonstration. Since the initial rate of every reaction is approximately

[S₂O₃^2-] /t

and [S₂O₃^2-] is the same for every reaction, relative rates will be proportional to 1 /t.The time between the start of the reaction and the color change can be determined from the timer readings. It should be possible to show that the reaction is first order in[H₂O₂],[I^-], and [H⁺] by showing that a plot of the initial concentration of the reactant whose concentration varies against 1/t is linear, or by plotting log concentration vs log (1/t) and observing that the slope is approximately 1.

Video Discussion

In the experiment testing the influence of volume on reaction rate, the color changes in the four vials do not occur at the same instant, because not all four solutions were mixed at exactly the same time. The important point is that the color changes were nearly simultaneous, especially compared to the overall time for the reaction. As long as the concentrations are the same, solution volume should not affect the rate of reaction.

The first step in this reaction is the formation of triiodide ion:

H₂O₂ + 3 I^- + 2 H⁺ --> I₃^- + 2 H₂O

In the absence of thiosulfate ion the triiodide ion would form the characteristic blue complex with starch.  However, the triiodide ion is rapidly reduced back to iodide ion by thiosulfate:

I₃^- + 2 S₂O₃^2- --> 3 I^- + S₄O₆^2-

In this clock reaction thiosulfate ion is the limiting reactant. The blue starch-triiodide complex forms only when all the thiosulfate ion has been consumed.

The initial concentration of thiosulfate ion is the same in every reaction in this demonstration. Since the initial rate of every reaction is approximately

[S₂O₃^2-] /t

and [S₂O₃^2-] is the same for every reaction, relative rates will be proportional to 1 /t.The time between the start of the reaction and the color change can be determined from the timer readings. It should be possible to show that the reaction is first order in[H₂O₂],[I^-], and [H⁺] by showing that a plot of the initial concentration of the reactant whose concentration varies against 1/t is linear, or by plotting log concentration vs log (1/t) and observing that the slope is approximately 1.

Keywords

Kinetics | Oxidation / Reduction | Reactions | Stoichiometry

Included in These ChemEd DL Resource Groups

Iodine Clock Reaction
Oxidation-Reduction Aqueous Solutions

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