Nathalie Dagmang Co-worker: Annjaneth Briones Group 8

Date Performed: December 9

Results and Discussion Report: Kinetics of Reaction Between Thiosulfate and Hydrochloric Acid Using the conventional method of ratio and proportion, it was found out that the rate law for the reaction between HCl and thiosulfate is R = 0.3680 [S2O32-]. This implies that the rate of the reaction only changes proportionally with the concentration of thiosulfate and is not affected by the concentration of hydrogen ion. Since the rate of the reaction is inversely related to the time the reaction was completed, the initial reaction rate of each run was determined by getting the values of 1/t. It serves as a value for comparing the rates of reaction at different times of completion. (i.e. if the 1st run is completed in 1 second, then the rate is 1, and if run 2 is completed in 3 seconds, then the rate = 1/3, one-third of the first runs rate ) It can be inferred from the second part of the experiment (Part B) that as the temperature increases, the rate of the reaction also increases. As stated in the collision theory, a chemical reaction occurs when the molecules involved collide. Hence, the rate of the reaction depends on how much collision happens per unit of time. So when temperature is increased, therefore adding energy to the molecules, the molecules move faster and it is likely that these collide more frequently and with higher intensity. The minimum energy needed to make these molecules collide hard and frequently enough to initiate the reaction is called the Activation energy (Ea). The frequency factor (A), another concept, is the number of collisions that occur per unit of time. For this experiment, the calculated Ea is 12,548 J/mol while its A is 1.091. The large value for Ea indicates that the reaction requires a lot of energy for it to occur, which is consistent with the previously calculated slow rates of reaction (to be considered fast, a reaction should finish in 10 seconds or less). The value for the frequency factor is also consistent with the previous data. Having a small value like 1.091 means that the collisions that happen are infrequent. Therefore, a smaller frequency factor and higher activation energy implies that the reaction is harder to occur and its rate slower.1

Based on the rate law, it can be inferred that the correct mechanism for the reaction between HCL and thiosulfate is: 1: S2032- SO32- + S (slow) 2: SO32- + H+ HSO3- (fast) 3: HSO3- + H+ SO2 + H2O (fast) This is because the rate law implies that the rate of reaction depends on the concentration of thiosulfate. The slowest step should be the elementary reaction which involves thiosulfate because all other steps can only follow its pace. Hence, this step is the rate-determining step of the reaction. Errors and some inconsistencies may have been due to personal errors and miscalculations. The timer or the one observing if the mark had already been covered by the solution was late to realize that it is time to stop the timer or may have not been fast enough to press the timer. The temperature recorded can also be inaccurate because in some set-ups, it is hard to maintain the temperature like the set-up for high temperature in part B. The measured temperature of the solution while on the hot plate may be 323, but once it was taken off from the heater, the temperature started to decrease and become unstable. The following is the graph of the kinetics of the reaction between thiosulfate and zhydrochloric acid:

1/T (K-1) 0.0035587 2 0.0033167 5 0.0030959

ln k 5.837730 4 4.569024 8 2

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2.836736 5

Sources: http://www.thestudentroom.co.uk/showthread.php?t=1222805 Skoog, West and Holler, Fundamentals of Analytical Chemistry

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