lab_iodine_clock_reaction

Lab: The Iodine Clock Reaction The rate of a chemical reaction depends on many physical and chemical factors, including temperature, the chemical nature of the reacting species, and the initial concentration of the reactants. In the following experiment, all factors except one are held constant. The iodine clock reaction is the classic experiment used to investigate rates of reaction. It is called a clock reaction because at first, when two colourless solutions are mixed, no reaction appears to occur. Then at a specific time, the mixture suddenly changes colour. The reaction combines aqueous iodate (IO 3 - (aq) ) and bisulfite (HSO 3 - (aq) ) ions and involves three reactions in sequence. In the first reaction, aqueous iodate ions are reduced by the bisulfite ions to form iodide ions. IO 3 - (aq) + 2 HSO 3 - (aq)  3SO 4 2- (aq) + I - (aq) + 3H + (aq) In the second reaction, the iodide ion is changed to molecular iodine. 6H + (aq) + IO 3 - (aq) + 5I - (aq)  3I 2(aq) + 3H 2 O (l) In the final step, the iodine reacts with starch suspended in solution to form a blue-black complex. I 2(aq) + starch  blue-black complex However bisulfite can also reduce iodine according to the following equation I 2(aq) + HSO 3 - (aq) + H 2 O (l)  2I - (aq) + HSO 4 - + 2H + (aq) The concentration of iodine can’t build up until while bisulfite is present, so the blue black complex won’t appear until the bisulfite is used up. The first reaction is slow in comparison to the rest, so the appearance of the blue black complex can be used to indicate the time of reaction of the first reaction. The shorter the time required, the greater the rate of reaction. Because the times of reaction are related to the rates of reaction, we will be able to make rate-concentration comparisons. Graphical analysis is an efficient way to determine order of reaction for iodate, m, where rate=k[IO 3 - (aq) ] m . Purpose: The purpose of this investigation is to gather and analyze experimental observations to determine the rate dependence of a reactant in a chemical system. Question: What is the order of reaction with respect to the initial concentration of iodate ions in the iodine clock reaction? Pre-lab: What will happen to the reaction time as the initial concentration of iodate ions is increased? What will happen to the rate? Materials:  0.020 mol/L potassium iodate solution (Solution A)  0.0010 mol/L sodium bisufite/hydrochloric acid/starch solution (Solution B)  micropipettes  goggles  stopwatch  spot plates  3 100 mL beakers