Background+of+Experiment

= Chemical Cocktail: An In-depth Analysis of Clock Reactions = We have been introduced to the wonderful world of Chemistry the last two years by our teacher Mrs. Barnes. Through-out those years we have

seen and worked through many different reactions but the reactions that seem to stand out to many are the colour-changing reactions. Unfortunately I

could not get an exact break-down of the reaction studied for my ISU, so I will be focusing on other reactions that include colour changes which are

mainly referred to as clock reactions. An ever famous colour changing reaction is the Briggs-Rauscher Oscillating Colour Change Reaction. This

reaction is one of the most common demonstrations of a chemical oscillator reaction. Another famous clock reaction is the Old Nassau reaction. Both of

these reactions, and my ISU all include iodine as a material. Iodine is an essential trace element, it is a nonmetallic halogen element occurring at

ordinary temperatures as a grayish-black crystalline that sublimes to a dense violet vapour when heated. These visually exciting reactions will be

explained through-out this paper.

Clock reactions represent a unique class of chemical reactions, clock reaction mechanisms can be explained by considering a hypothetic system

which consists of four reactants: A, B, C, and D. In this system reactant A reacts with reactant B to produce a product P:  A + B → P (Reaction 1) The formed product P than reacts with reactant C and is therefore consumed – yielding product R: P + C → R (Reaction 2) If the concentrations of A and B are high, the overall effect of the Reaction 1 and Reaction 2 is consumption of reactant C. As soon as the reactant C is

completely removed from the reaction system, the product of Reaction 1 (P) cannot further react. Therefore the concentration of product P raises

rapidly, making its amount of molecules high enough to react with the fourth reaction D. In this reaction a coloured produced is finally formed and this is

the moment when we observe the sudden change in solution colour: P + D → coloured product (Reaction 3) In most clock reactions these three reactions repeat at least ten times.

The Briggs-Rauscher oscillating reaction is one of a very small number of known oscillating chemical reactions. This oscillating reaction was

developed by Thomas S. Briggs and Warren C. Rauscher of Galileo High School in San Francisco. The first oscillating reaction was reported by W. C.

Bray in 1921. The Briggs-Rauscher reaction is a great demonstration of a clock reaction because of its dramatic and visually striking colour changes.

Three colourless solutions are mixed and stirred on a magnetic stirrer. (Solution A: hydrogen peroxide and distilled water. Solution B: potassium iodate,

distilled water and sulfuric acid. Solution C: malonic acid, manganese (II) sulfate mono hydrate, distilled water and starch.) The three colourless

solutions when mixed will slowly turn an amber colour, suddenly changing to a very deep blue. This slowly fades again to a colourless solution then the

process repeats about ten times before ending as a deep blue liquid which smells strongly of iodine.

The Old Nassau reaction is also known as the Halloween reaction. In this reaction there are three different solutions. The first solution contains

sodium hydrogensulfite and starch solution. The second solution contains mercury (II) chloride solution. And the third solution contains a potassium

iodate solution. Contents of the third beaker were added to the first beaker, then the contents of the second beaker added to the second third mixture.

Several seconds later the colourless mixture turns an orange/red colour and after seconds the mixture suddenly turns black. The reaction in this

experiment takes place in several steps that are responsible for this clock mechanism. Initially, iodate ions are reduced by hydrosulfite ions to produce

iodide ions. Once the concentration of iodide ions is high enough to exceed the solubility product of mercury (II), orange HgI2 precipitate is formed.

When all mercury (II) ions are precipitated in the reaction with iodide ions, the excess of iodides reacts with iodate ions and elemental iodine is formed.

Iodine formed in this reaction reacts with starch indicator to give a blue coloured complex. As the concentration of potassium iodate decreases from the

first to the last beaker, so does the rate of the reaction.

Chemistry is everything. Chemistry is the science of matter and the changes it undergoes. The science of matter may also be addressed by

physics, but while physics takes a more general fundamental approach, chemistry is more specialized, being concerned with the composition, behaviour,

structure, and properties of matter, as well as the changes it undergoes during chemical reactions. There are very many chemical reactions in the world

of chemistry, but the one I chose to study my ISU on was clock reactions. Their visually exciting and interesting and would definitely keep an audiences

attention. Seeing the reaction takes place, it almost seems very simple. Little do a lot of people know is in a matter of seconds three or more reactions

are taking place to undergo the wonderful colour changes. From Briggs-Rauscher oscillating reaction to the iodine clock reaction lab to the halloween

reaction, we've seen many colour changing reactions that grasp our attention. Hopefully these reactions grasped your attention as they did mine.