Something that we haven’t discussed in this course is the speed of a chemical reaction. It turns out that not all reactions happen instantaneously; in fact some reactions can be quite slow. Take a while to think about how reactions happen on a molecular scale and what might make a reaction happen faster or slower. If hydrogen peroxide (H₂O₂), sodium thiosulfate (Na₂S₂O₃), potassium iodide (KI), acetic acid (CH₃CO₂H), and sodium acetate (CH₃CO₂Na) are mixed in the appropriate quantities the mixture will stay colorless for several seconds before turning a dark blue/black color. In this project your mission is to make this mixture change color at a preset time (designated by your instructor), in effect making a ‘chemical clock’. Of course in order to do this you will need to understand how the various chemicals interact and determine the best way to control the reaction. We suggest that you split the project into four parts.

The first part of the project might be to investigate the effects of the different chemicals on the reaction. We suggest that you do this on a small scale first by adding equal amounts of different combinations of chemicals into a test tube. For example what happens when you mix just potassium iodide and starch together? What about potassium iodide, starch and sodium thiosulfate? Try as many combinations as you can. Which chemicals are necessary to produce the blue/black color? Which chemical affects the speed of the reaction?

Once you have determined which chemical is best used to control the speed of the reaction, you will need to investigate the relationship between the amount of chemical and the reaction speed. Any good experiment involves changing one variable at a time and measuring the effect. We suggest that using graduated dropping pipets and a test tube you change the amount of one chemical and keep other concentrations constant. Since we are working with concentrations, the total volume has to be constant. This is probably best achieved by diluting to a constant volume with water. What is the relationship between the chemical concentration and the reaction speed? If you double the concentration does the reaction time double?

We know from experience that heating often makes reactions happen faster. Perhaps controlling the temperature is the best way to control the reaction rate. You should design an experiment to determine the effect of temperature on reaction speed.

Now that you have fully investigated this reaction, it is time to use your knowledge to make your chemical clock. Your instructor will give you a time to aim for. You will have to decide on the amounts of chemicals and temperature that you will need to use in order to make 100mL of solution change color in the specified time. Although this part of the project should not be included in your journal article, your ability to successfully complete this task will form part of your grade for the course. GOOD LUCK!

We are treating this as a research project so the assignment will be a full journal article incorporating all of the journal sections that you have written for previous experiments including abstract, introduction, materials and methods, results, discussion and references. You should provide details of your results as appropriate in tables and figures. Your article should discuss parts 1, 2 and 3 of the project. Although some data may be shared between groups your data treatment (e.g. any calculations, graphs etc) and write up should be prepared independently. Your introduction should include a couple of references so a literature search will be necessary (you may also come up with something that will be helpful for your project work). Your journal article must be submitted before the end of classes (Friday December 13^th) in order to receive a grade.