Nathalie Dagmang
Date Performed: December 9
Co-worker: Annjaneth Briones Group 8
Results and Discussion Report: Kinetics of Reaction Between Thiosulfate and Hydrochloric Acid Using the conventional conventional method of ratio and proportion, proportion, it was found out that the rate rate law for the reactio reaction n betwee between n HCl and thiosu thiosulfa lfate te is R = 0.3680 [S2O32-]. This This impl implie ies s that that the the rate rate of the the react reactio ion n only only chang changes es proportionally with the concentration of thiosulfate and is not affected by the concentration of hydrogen ion. Sinc Since e the the rate rate of the the reac reacti tion on is inve invers rsel ely y rela relate ted d to the the time time the 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 1 st 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 run’s rate ) It can be inferred from the second part of the experiment (Part B) that as the tempe temperat rature ure increa increases, ses, the rate rate of the reaction reaction also also incre increase ases. s. As stated in the collision theory, a chemical reaction occurs when the molecules involv involved ed collid collide. e. Hence, Hence, the rate rate of the reaction reaction depends depends on how much coll collisi ision on happe happens ns per per unit unit of time time.. So when when temp temper erat atur ure e is incr increa ease sed, d, 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 frequency factor (A), another another concept, is the number of collision collisions s 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 react reaction ion (to be consid considere ered d fast, fast, a react reaction ion should should finish finish in 10 seconds 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 infrequen infrequent. t. Therefor Therefore, e, a smaller smaller frequency factor and higher higher activatio activation n 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+
3: HSO3- + H+
HSO3- (fast)
SO2 + H2O (fast)
This is because because the rate law implies implies that the rate of reaction reaction depends on the concentration concentration of thiosulfate thiosulfate.. The slowest step should should be the elementary 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 may have not been fast enough to press 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)
ln k
0.0035587 2
5.837730 4
0.0033167 5
4.569024 8
0.0030959
2
8
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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