By: Howard Andres

In this laboratory experiment, our lab group’s purpose was to use different amounts of reactants for a reaction and determine their rates, order, and constant values so to come out with an average rate constant(k).  In this experiment, the reactants of acetone, HCl, and I₂ dissolved in water were used.  In order to determine the rates, orders of reactants, and constant values, our lab group’s job was to produce four mixtures in which in each mixture, the amount of one reactant differed from the rest, but the total volume of each mixture had to equal 50 mL.  Once all of the reactants were combined, the reaction was timed until all the color disappeared and the mixture became clear.  By obtaining these results, our lab group was able to find the rate of the reaction, order of the reactants, and an average constant value.

In each of our four different mixtures, our lab group decided to make one of the reactants have a 20 mL volume while the remaining reactants maintained a 10 mL volume so to have a total volume of 50 mL.  Once all of the reactants were mixed together, it was observed that the reaction produced a yellowish color which proceeded to fade away as the reaction progressed.  When the reaction in the test tube became clear, this was the indication that the reaction had stopped.  Two runs of the reaction of each mixture were to be completed.

As noted in the lab book, the I₂ was the limiting reactant, since when there was a lesser amount of this reactant when compared to the other reactants, the reaction proceeded more quickly.  The fastest reaction was the mixture in which there was 20 mL of the HCl and 10 mL of the other reactants, with a speedy time of 77 seconds in its first run, and 75 seconds in its second.  This was followed by the mixture with 20 mL of acetone with  times of 135 seconds and 140 seconds, and then by the mixture with 20 mL of water with times of 302 and 295 seconds.  The mixture with the longest reaction time was, as noted, the one with 20 mL of I₂ and 10 mL of the remaining reactants, with times of 385 and 375 seconds.  From using this information, the molarity of each reactant was found in total volume, and the values for the rates, orders, and average constant(k) value were obtained through calculation. The rates were 3.4 x 10^-6 (Mixture I), 7.3 x 10^-6 (Mixture II), 1.3 x 10^-5 (Mixture III), and 5.3 x 10^-6 (Mixture IV).  The order of the acetone was found to be 1, the order for H⁺ was 2, and the order for I₂ was 1.  The average constant(k) value was found to be 0.10.

Overall, the experiment proceeded quite smoothly.  A note for error though can be found in the order with respect to I₂.  The lab book states that I₂ is supposed to be in zero order, while the calculations in this experiment allowed I₂ to be in first order.  As a result, the constant(k) value is affected because the value for I₂ in the equation should be 1 instead its molarity of 0.0010 M or 0.0020 M.  Thus, the average constant value is probably incorrect.  Besides this though, the experiment went quite well.

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