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In this experiment we measure the activation energy for the following reaction:
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S2O32-(aq) + 2H+(aq) ⇒ SO2(aq) + S(s) + H2O(l) |
In order to measure an activation energy, we draw a graph of natural log of the rate of reaction against the reciprocal of the absolute temperature. This involves measuring the rate of reaction at various temperatures. We use a water bath to obtain the different temperatures, and the production of the sulfur enables us to measure the reaction rate. As the solid sulfur is produced, the solution becomes cloudy. We place a dot or cross behind a boiling tube containing the reaction mixture, and measure the time it takes for the cross to become obscured. The longer it takes, the smaller is the reaction rate. We obtain a measure of the reaction rate from the reciprocal of the time taken to obscure the cross (that is 1/t).
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Some sodium thiosulfate solution and hydrochloric acid was measured out and placed in a water bath to get up to temperature. It will take around 5 minutes for the temperature to be reached. We need to measure the temperature of the reacting solution itself, rather than that of the water bath. The temperature was measured before mixing and just after reaction. Averaging these results gives the temperatures you can see on the video. Care has to be taken to avoid cross contamination, otherwise the reaction will start before the time is taken. The start time is a bit more difficult to assess as it takes time for the two solutions to mix. I suggest you take the start time half way through the addition of the solutions. You must use the same boiling tube for the reaction throughout. Students often assume that all the tubes will be the same, but they do vary in thickness and this will affect the time taken to obscure the cross. However, the tube becomes coated in a layer of sulfur during the reaction, so you will have to clean and dry it between experiments. |
The following video shows five experiments. Make a note of the temperature and the time taken between the solutions being mixed and the cross becoming obscured in each case. You will need to use a watch or stop clock for the timing. Use these results to complete a table like the one on the student instruction sheet. Then plot a graph of ln(1 over time) against 1 over temperature. Remember that temperature must be in Kelvin. Try to work out the activation energy for this reaction, and then compare it with the results below.
Video - measuring activation energy
