The Egyptian Nights Experiment Lizzie Stewart

The Egyptian Nights ExperimentLizzie StewartO654546March 31 2015PurposeThe purpose of this demonstration is to provide an eye­catching illustration of theproperties and behavior of reaction rates and kinetics in chemistry.Note: This experiment is commonly referred to as both the Egyptian Nights Experimentand the Iodine Clock Reaction. I prefer the former and will use it exclusively movingforward.Idea SourceThe idea for this demonstration comes from the British Science Association websitearticle titled “Get set… demonstrate” posted in 2013. The article explains the materialsand equipments required for the successful execution of the experiment includingdetailed instructions for the procedure. The URL for this source is included in thesources section of this report. Please see the attached print­out for the original procedure.BackgroundPresent a background on the principle the demonstration illustrates. Explain thechemistry behind the principle demonstrated including equations where appropriate.This particular reaction has become a classical demonstration used to display kineticchemistry in action. The experiment was originally developed by Swiss chemist HansHeinrich Landoldt in 1886. This type of reaction can be referred to as a chemical clock; amixture of reacting compounds in which the concentration of one compound displays anabrupt change accompanied by a visible color appearing. In self­indicating reactions ofthis type in which nothing appears to happen for a period of time and then a changesuddenly become visible the onset of the color change may be used to time the reaction.Equations:H2O2(aq) 2I­(aq) 2H (aq) ? 2(aq) 2H2O(l)I2S2O32­(aq) I2(aq) ? 4O62­(aq) 2I­(aq)SProcedureFor the Egyptian Nights reaction the following materials are required:0.2g soluble starch1 4.1g sodium acetate50g potassium iodide9.4g sodium thiosulfate30ml acetic acid100ml hydrogen peroxide (30% concentration)Distilled water approximately 1.5LGlass stirring rod3 x 1L beakers2 x 1L volumetric flasks50ml and 100ml graduated cylinderstop­loading balancesafety glassesrubber gloveslab coatThroughout this demonstration it is important that protective glasses and gloves be wornat all times. Sodium acetate potassium iodide sodium thiosulfate hydrogen peroxide and acetic acid can cause skin and eye damage.The soluble starch sodium acetate potassium iodide and sodium thiosulfate must beweighed carefully using plastic weighing boats on the top­loading balance. The boatscontaining the chemicals must be labeled as the reactants are extremely similar inappearance.In a clean dry 1L beaker the soluble starch is mixed with the glass stirring rod into a thinpaste using approximately 5ml of distilled water. When a homogenous consistency hasbeen achieved the solution is diluted to 800ml.Next the sodium acetate potassium iodide and sodium thiosulfate are stirred into thesolution. It is important to stir the solution thoroughly as the potassium iodide can bechunky and slow to dissolve. Careful stirring will help achieve a transparent colorlesssolution which is key in achieving the shocking affect of this reaction.This solution is now complete and can be carefully poured into a clean dry 1Lvolumetric flask labeled “solution A.”2 The next solution requires 500ml of 6% hydrogen peroxide. As 30% hydrogen peroxideis all that is available to the lab a dilution must be made. In order to form 500ml of 6%concentration 100ml of 30% H2O2 is measured using a 100ml graduated cylinder andthen diluted to 500ml with distilled water in a 1L beaker.30ml of acetic acid is carefully measured using a 50ml graduated cylinder. The aceticacid is added to the hydrogen peroxide solution and is next diluted to 1L with distilledwater. This solution is now complete. It must be poured into the remaining volumetricflask and labeled “solution B.”At this stage the experimenter has 2 identical­looking solutions labeled A and B. Thesolutions are clear and colorless – they look as though they could be water. It is time toperform the demonstration now.300ml of each solution is poured into the remaining 1L beaker together. This newsolution must be stirred vigorously for a few seconds with the stirring rod. After about 20seconds a dramatic color change will occur; the solution will change from transparentand colorless to blue­black in an instant.There is enough of each solution to perform the experiment several times. Additionally the timing of the color change can be manipulated by adding more hydrogen peroxide tosolution B if desired.When the experiment is complete the resulting solutions can be disposed of down thesink drain. The glassware is washed dried and put away.Results/ObservationsIt is interesting to observe that while the reactants used in this experiment are eitherwhite or colorless the final result produces a spectacular color change.Soluble starch appears as a fine white powder it is dull and does not reflect light similarin appearance to flour used in baking. Sodium acetate and sodium thiosulfate are nearlyidentical in appearance. They are both small white crystals similar in appearance to tablesalt. The only visible difference is that the sodium thiosulfate is a brighter more vibrantwhite color. The potassium iodide is also a white crystal however it appears in largecoarse chunks and must be chipped apart with an ice pick in order to isolate a smallportion for measuring.The 30% hydrogen peroxide is translucent odorless and colorless. It looks exactly likewater. The acetic acid is identical in appearance to the hydrogen peroxide however itemits an eye­burning vinegary odor which is very intense and lingering.In preparing solution A the mixture required vigorous stirring for approximately 7minutes in order to dissolve the potassium iodide completely. Initially the soluble starchcontent led to a slightly milky appearance but with vigorous stirring this dissipatedalmost entirely leaving just a trace of cloudiness detectable.3 When mixing solution B the addition of the acetic acid was similar in appearance to inkbeing dropped into water. While both the acetic acid and the hydrogen peroxide solutionare colorless a swirling effect can initially be detected.Upon mixing solutions A and B in equal parts and stirring them together a 20 secondperiod passes before the reaction occurs. This waiting time allows doubt and suspense tobuild making the change even more dramatic.In videos of the experiment I’d viewed online the reaction occurs within a fraction of asecond changing from transparent and clear to completely black. During my practice runin the lab however the reaction occurred over a period of several seconds. This wasdisappointing to me; I decided it was an error in the dilution of the H2O2 which must bedone very carefully the day of the demonstration to achieve aswift color change.DiscussionDiscuss any changes made to the procedure or chemicals as a result of your practiceruns. How do the changes make your demonstration more effective? Are there anydifferences between your practice runs and your demonstration. Indicate how thedemonstration reinforced the principle it was meant to illustrate. Comment on thesuccess of the demonstration – whether it was visually effective interesting to theaudience appropriate to illustrate the principle it was meant to illustrate. Indicate anyproblems with the demonstration.Include any suggestions which might improve the demonstration.ReferencesBritish Science Association. Get Set Demonstrate. 2013. [Internet article.] Cited March2 2015. Available from:­chemistry/resource/res00001384/iodine­clock­demonstration