E4-4

Many reactions, however, proceed in a number of small steps. Let us consider the hypothetical composite reaction 2A + B D. Of the many possible ways this reaction might occur, two are given below:

mechanism (1)A + A C, followed rapidly by C + B D, ormechanism (2)A + B C, followed rapidly by C + A D

To determine which of these two possible mechanisms is operating, one could carry out kinetic experiments on the dependence of the rate of formation of D on the concentrations of the reactants A and B. Because in each mechanism the first step is much slower than the second step, it will be a bottleneck and totally determine the rate of formation of D. Under these conditions the rate of formation of D, v, for the two mechanisms would be given by:

rate of formation of D via mechanism (1):v = k[A]2, orrate of formation of D via mechanism (2):v = k[A][B].

Therefore, measurements of the rate of the reaction at different concentrations of A and B would allow one to distinguish easily between the two mechanisms. If the rate depended on the square of the concentration of A and not at all on the concentration of B, this would support mechanism (1). If the rate of the reaction were directly proportional to the concentrations of both A and B, this would support mechanism (2). If the reaction rate does not conform to either of these relationships, then some other mechanism must apply. This is a simple example of how kinetics research can be used to determine how a reaction occurs.Iodine Clock ReactionThe iodine clock reaction was discovered by the Swiss chemist Hans Heinrich Landolt in 1886. There are a number of variations of it, but all of them involve the mixing of two colourless solutions. Initially there is no visible reaction, but after a certain period of time the mixed solution suddenly turns dark blue. In this experiment you will study the iodate variation of the iodine clock reaction, which involves the reaction between iodate ions, IO3, hydrogensulfite ions, HSO3, and hydronium ions, H+. The stoichiometry of the overall reaction is as follows:2IO3(aq) + 5HSO3(aq) + 2H+(aq) I2(aq) + 5HSO4(aq) + H2O(l)(1)If the solution contains starch, it will react with I2, the elemental iodine produced by the reaction, and form a blue starch-iodine complex. But why doesnt the blue colour appear immediately? The trick is that as soon as any I2 is formed, it immediately reacts with any HSO3 still present and is converted into colourless I:I2(aq) + HSO3(aq) + H2O(l) 2I(aq) + HSO4(aq) + 2H+(aq)(2)Thus, reaction (2) efficiently removes any I2 produced by reaction (1). Only after all of the HSO3 has been consumed by reactions (1) and (2) will the I2 concentration increase and its reaction with the starch begin. This not only explains the sudden appearance of the blue colour, but it also allows us to determine the rate of the reaction. A measure of the rate of HSO3 consumption can be made by dividing the initial concentration of HSO3 by t, the time it takes for the blue colour to appear, [HSO3]o/t.A Sample Lab Report The Iodine Clock Reaction

Introduction:The factors that affect the rate of a chemical reaction are important to understand due to the importance of many such reactions to our health, well-being and comfort. It would be advantageous to slow down some of these reactions such as food spoilage and rust formations, while in the cases of reactions such as the Tums-stomach acid reaction and the conversion of organic matter to fossil fuels, it would be beneficial to them speed up.

The rate of a reaction is governed by the collision theory. In order for a reaction to occur, there must be a collision between reactant molecules. This collision must have enough energy to break and form the appropriate bonds as well as have the correct orientation when colliding.(1) When all of these things happen, a chemical reaction has occurred. If we can increase the amount of these favourable collisions, then we increase the rate of the reaction.(1)

One of the factors that affects reaction rate is reactant concentration. The more reactant molecules in a given area, the greater the number of collisions possible.(1) Whenever we have a larger number of collisions possible, there is a probability of having a favourable collision increases. The opposite is true if we have a lower concentration of reactant molecules.

Another factor that affects the rate of reaction is temperature. In this case, there are not more reactant molecules to collide. Temperature is proportional to the average kinetic energy, which is the energy associated with motion.(2) All reactions have what is called the Minimum Threshold Energy. This is the amount of kinetic energy it takes for reactant bonds to break, re- arrange and form the bonds necessary to make products.(1) This energy is equivalent to the activation energy of the reaction. When we increase the temperature of a reaction, the average kinetic energy of the reactants increase.(1) This change results in two things. First, the molecules show an increase in motion within the confines of the same area causing an increase in the amount of collisions. Second, more molecules now have an energy equal to the Minimum Threshold Energy and can now form products. More collisions, more energy, and a greater probability of favourable collisions leads to an increase in reaction rate. Again, the opposite is true if we decrease the temperature.

Purpose/Objective: The purpose of this lab is to observe the effect of temperature and concentration on the rate of a chemical reaction. (OR The objective of this lab is to observe the effect of temperature and concentration on the rate of a chemical reaction.)

Procedure: Please refer to Heath Chemistry: Laboratory Experiments (Canadian Edition), DiSpezio, Michael, A., et al, 1987, D.C. Heath Canada, Ltd. pg. 197-203