Soomin Bae
Conclusion
The aim of this experiment was to see the impact of the concentration of HCl in the rate of reaction with magnesium metal. As shown in table 1 above (table 1 would have been a processed data table that shows the rate of reaction for the 3 different HCl concentrations), the data collected in this experiment suggests there is no correlation between the concentration of HCl and the rate of reaction; in other words, the concentration of HCl did not affect the rate of reaction with magnesium metal. However, based on the knowledge of Collision theory, it can be deduced that this conclusion drawn from the experiment is wrong. The concentration of a reactant should affect the rate of reaction – a greater concentration of HCl should have resulted in a faster rate of reaction. This is because a higher concentration of HCl means there are more moles of HCl in the 50.0 cm3 solution. With more moles, HCl molecules would be closer together and thus have an increased frequency of collision. Consequently, the number of successful collisions – with the appropriate orientation and energy greater than or equal to activation energy – would theoretically increase as well. Thus, according to the Collision theory, the 1.0 mol dm-3 HCl solution should have had the fastest rate of reaction compared to the other solutions. However, the data collected clearly shows that the reaction rate does not adhere to the collision theory. In fact, it even contradicts it – the 0.5 mol dm-3 HCl had a faster rate of reaction than the 1.0 mol dm-3 HCl, which is theoretically inaccurate. From these observations, it follows logically that the procedural errors of the experiment would have resulted in such inaccurate data collection. 3 methodological weaknesses that could have caused the inaccurate conclusion are described in the section below.
Evaluation
Limitation of the measuring tool: Digital balance
Change in mass of the HCl and Magnesium mixture over time was measured using a digital balance that showed up to 2 decimal places. Considering H2 gas leaving is the reason for the mass loss, a measuring tool with only 2 decimal places leaves quite a room for error. This is because the molar mass of H2 is extremely small – about 2.02 g mol-1 –, and given the maximum number of moles of H2 that can be produced by each concentration of HCl, the range of mass change that can happen is from about 0.013g to 0.05g. As such, a digital balance only measuring up to 2 decimal places is not suitable for this experiment since the small changes in mass would not be able to be detected. This could lead to a misconception that the rate of reaction for different concentrations are the same when they are different. For instance, the change in mass was 0.05g over 140 seconds for both 1.0 mol dm-3 HCl and 0.25 mol dm-3 HCl. Given only 2 decimal places, the rate of reaction seems to be the same – 3.5710-4 g s-1. However, it is possible that the rates were in fact different – with one being 3.9210-4 g s-1 (0.0549g mass loss) and the other being 3.5810-4 g s-1 (0.0501g mass loss). This could result in an inaccurate comparison between the rate of reaction of different concentrations and thus an inaccurate conclusion.
As such, one improvement for this limitation would be to use a digital scale with more decimal places available – according to the maximum mass change that can happen, up to 4 decimal places would be good enough. This way, there would be a more pronounced change in mass in each trial for different concentrations and thus noticeable difference in rates, which helps compare the rate of reactions and see which one is more relatively faster or slower. Having more decimal places would result in more accuracy when comparing the rate of reactions.
Different initial temperatures for HCl solutions & Exothermic Reaction
Temperature is a significant factor that affects the rate of reaction. Since higher temperature means an increase in average kinetic energy of all molecules in a sample, it would make a greater portion of the molecules to have energy that exceeds the activation energy. Thus, with increased collision frequency, successful collisions would also occur more frequently, leading to a faster reaction rate. In the experiment conducted, there are 2 ways in which temperature could have affected the rate of reaction.
The first way is by having different initial temperatures for HCl solutions in different trials as shown in (imaginary) Figure 1. Due to the theory explained above, HCl solutions with a relatively higher initial temperature would have had a faster rate of reaction than those with a relatively lower initial temperature. As a result, it becomes ineffective to compare the rate of reactions of those since temperature might have accelerated the rate of reaction of one of them. This can lead to an inaccurate conclusion derived since the calculated rate of reaction is not a direct reflection of the impact of the concentration of the HCl solutions. However, since the temperature only varied by a small amount, with the greatest being 0.04°C, it can be concluded that although temperature would have an effect on the rate of reaction, it would only by a small extent. This error can be improved by conducting the whole experiment in a temperature-controlled water bath. This would ensure that the initial temperatures of the HCl are fairly constant throughout all trials. By doing so, the impact of different initial temperatures accelerating the rate of reaction would be minimized.
The second way is by an increase of temperature during the reaction itself. The reaction between HCl and Mg is exothermic. In other words, it releases energy in the form of heat during the reaction, resulting in an increase in temperature of the surrounding area. This is clearly shown in (imaginary) Figure 1 as for all trials, an increase in temperature was observed after the reaction. Again, due to the theory explained above, a rise in temperature during the reaction would have accelerated the rate of reaction itself. This leads to an ineffective measurement of the rate of reaction since we are no longer measuring the impact of concentration solely. However, based on Figure 1, the change in temperature was the greatest by 4°C for 1.0 mol dm-3 HCl and the smallest by 0.5°C for 0.25 mol dm-3 HCl. This actually aligns to the collision theory since a greater concentration of reactant led to a greater amount of heat (energy) being released. Thus, the impact of the increase in temperature due to exothermic reactions seems to be minor since it would not affect the overall trend. However, since the amount of heat released is unclear, it would still lead to some inaccuracies of the correlation between HCl concentration and rate of reaction. Heat being released and surrounding temperature increase is a natural component of the reaction that can not be eliminated. However, one way to minimize this would be to change the parameter of this reaction – reducing the investigated concentration. With a reduced initial concentration, the extent of the reaction would be reduced and thus the heat generated by the exothermic reaction would decrease as well. As a result, the impact of the temperature increase due to it being an exothermic reaction would be able to be minimized.
Different sizes of the Magnesium Strips
Another important factor impacting the rate of reaction is the surface area. In this case, the surface area of Magnesium would affect the rate of reaction in addition to the difference in concentration of HCl. A greater surface area of magnesium means that there are more places where HCl molecules can be in contact with Mg, and thus have a more frequent collision with each other. The experiment does not specify the shape or size of the Magnesium strip for each trial, and I observed an inconsistency of the size and thus surface area of Mg in each trial. Since the size did not vary by a huge amount and the Mg strip was not cut into smaller pieces, the impact of this error would not have been major. However, it could have still accelerated the rate of reaction by a small extent.
One improvement could be to have a specific size stated for Magnesium strips to be used. Since the width of the Mg ribbon would be fairly constant for all Mg strips, one way would be to measure the length of the Mg strip and make sure all of them are about 15mm long. That way, all of the strips would have similar surface area, minimizing the impact of surface area in changing the rate of reaction.
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