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Activation Rate Law

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Abstract. The objective of this work was to determine the activation energy and rate law for the reaction between hydrochloric acid and magnesium shot. An analysis of the reaction’s initial rate at varying molarities of hydrochloric acid and masses of magnesium shot, along with the method of initial rates was used to determine the rate law for the reaction; rate = k (SA of Mg)a [H+]b. It was hypothesized that the reaction would be second order overall. After determining the rate law, the activation energy was determined by changing the temperature at which the reaction took place. Using the rate constants calculated at the two different temperatures, the activation energy was calculated. It was found, experimentally through the method of …show more content…

The overall reaction order was 2.89. The full rate law for the hydrochloric acid and magnesium shot reaction was: rate = 0.000853 (SA of Mg)0.98 [H+]1.906. The activation energy for the reaction was calculated to be 55.165 kJ/mol. Different types of magnesium were tested in additional lab groups including magnesium ribbon and magnesium powder. For magnesium ribbon, the average magnesium order was 0.97 and the average H+ order was 2.24. The average overall reaction order was 3.21. The average activation energy for magnesium ribbon was 40.08 kJ/mol. For magnesium powder, the average magnesium order was 1.06 and the average H+ order was 1.44. The average overall reaction order was 2.50. The average activation energy for magnesium powder was 12.81 kJ/mol. Each type of magnesium resulted in different results for both the overall reaction order and the activation energy. Magnesium shot had the closest overall reaction order to the accepted value with 2.89, while magnesium power was the furthest off with 2.50. For activation energy, the values ranged widely depending again on the type of magnesium used; the range of activation energy was from 12.81 kJ/mol to 55.165 …show more content…

One of the biggest sources of error was letting some of the hydrogen gas escape during the reaction because the cap of the pressure probe could not be placed on top of the flask quick enough. Not being able to measure all of the change in pressure resulted in a lower rate law because the graph of pressure versus time had a smaller slope. In future experiments, a system in which the reactants could be mixed with the pressure probe already in place would yield much more accurate results because the change in pressure would be higher which would cause the initial rate to be larger. Another source of error in the experiment was keeping the temperature of the trial in the ice bath consistent. If the temperature of the HCl changed from when it was measure to when the reaction took place, the activation energy calculations would have had increased error. Because the HCl was still sitting in the ice bath, if the actual temperature was colder that what we calculated, the activation energy would have been lowered. For future experiments, the temperature of the HCl should be recorded as soon to when the Mg shot is added as possible, or the experiment could be run in a temperature controlled

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