The procedure described below outlines this experiment as a demonstration, best done on a large scale for the greatest visual impact. The demonstration itself takes less than one minute.
For a student investigation, the quantities required would be smaller and they would have to measure the quanities quite accurately with, for example, disposable plastic syringes.
A white background will help so that the impact of the sudden and spectacular colour change is not lost. Scaling up the volumes of solution that are mixed may help in a large room. There is no warning of when the blue colour is about to appear. It may help understanding if the students are already familiar with the reactions of starch and iodine, and iodine and sodium thiosulfate, so it may be worth demonstrating these beforehand.
For more advanced discussions or investigations — this reaction is the rate determining step and is first order with respect to both H 2 O 2 and I —. As soon as the iodine is formed, it reacts with the thiosulfate to form tetrathionate ions and recycles the iodide ions by the fast reaction:.
As soon as all the thiosulfate is used up, free iodine or, strictly, I 3 - ions remains in solution and reacts with the starch to form the familiar blue-black complex. This collection of over practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures.
A video and teacher notes providing an introduction to reaction rates and kinetics, in which a colourless solution changes to dark blue. Video: Iodine Clock demonstration. Sometimes performance does not reflect understanding. Use this practical to investigate how solutions of the halogens inhibit the growth of bacteria and which is most effective.
Site powered by Webvision Cloud. Skip to main content Skip to navigation. Four out of five No comments. Solution A note 1 : Soluble starch, 0. The solutions will keep overnight, but best results are obtained if the solutions are made up on the day.
Sodium thiosulfate will react with acids to give sulfur dioxide and a precipitate of sulfur, hence the sodium thiosulfate and ethanoic acid are separated in solutions A and B respectively. If you have access to 1 M dilute ethanoic acid, use cm 3 of this to make solution B. Mix cm 3 of 20 volume hydrogen peroxide with cm 3 of 1M ethanoic acid.
Health, safety and technical notes Read our standard health and safety guidance Wear eye protection. Sodium thiosulfate—5—water, Na 2 S 2 O 3. Handling glacial ethanoic acid requires care and should be done in a fume cupboard using gloves and eye protection. Procedure Solution A is made up as follows Make a paste of 0. Pour onto this approximately cm 3 of boiling water and stir. Pour the resulting solution into a 1 dm 3 beaker and dilute to around cm 3.
Add 4. Stir until all the solids have dissolved and allow to cool to room temperature. Pour the mixture into a 1 dm 3 volumetric flask and make up to 1 dm 3 with water. Solution B is made up as follows In a 1 dm 3 volumetric flask mix cm 3 of 20 volume hydrogen peroxide with 30 cm 3 of glacial ethanoic acid and dilute to 1 dm 3 with water.
The demonstration Measure cm 3 of solution A and cm 3 of solution B in separate cm 3 measuring cylinders. Both solutions are colourless although solution A will be slightly cloudy. Pour both solutions simultaneously into a cm 3 beaker to mix. Ensure thorough mixing with a stirring rod or magnetic stirrer.
After about 20 seconds at room temperature the mixture will suddenly turn dark blue. The appearance of the blue colour may be timed — an assistant or a student can start and stop the timer.
Sodium thiosulfate solution reacts with dilute hydrochloric acid:. The sulfur forms a cloudy yellow-white precipitate during the reaction. The time taken for this to achieve a given cloudiness provides a way to measure the reaction time. Record the results in a table. This table gives some example results. Describe the effect of increasing the temperature of the reaction mixture on the rate of reaction.
Use the graph to help. The rate of reaction increases as the temperature increases. The rate increases by a greater amount at higher temperatures. Suggest a reason why the same person should look at the black cross each time. Different people may decide that they cannot see the cross at different amounts of cloudiness, leading to errors in deciding when to take the reaction time. Evaluate the hazards and the precautions needed to reduce the risk of harm.
For example:. Fran Scott demonstrates how to measure the rate of reaction and how to increase it. Core practical - observing colour changes Investigate the rate of a reaction by observing a colour change There are a number of ways to investigate the rate of a reaction in Chemistry. Aims To investigate the effect of changing the temperature on the rate of a reaction. Method Using a measuring cylinder, add 50 cm 3 of dilute sodium thiosulfate solution to a conical flask. Place the conical flask on a piece of paper with a black cross drawn on it.
0コメント