Rationale
The main component of marble chips is calcium carbonate, CaCO3, which is an alkaline compound. With this property, marble chips are generally used as an acid neutralization material in many real-life applications, such as streams, lakes and soils where there is a certain level of acidity (Pol, 2020). When CaCO3 reacts with acid, for example hydrochloric acid (HCl), a salt (CaCl2), carbon dioxide (CO2) and water (H2O) are formed: CaCO3(solid) + 2HCl(aqueous) CaCl2(aqueous) + H2O(liquid) + CO2(gas) (Equation 1)
The rate of reaction is defined as the speed at which the reactants interact with each other to form the products (BYJU'S, 2020). Since the above reaction(Equation 1) is generally known to occur over a period of minutes, it is relatively reasonable to use this for an investigation on reaction rate. As the reaction has taken place, it produces gaseous carbon dioxide, thus allowing the rate of reaction to be observed by measuring the volume of gas produced or the mass loss due to liberation of CO2 overtime.
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The collision theory states that for a reaction to occur, reacting particle must be broken and rearranged to form new bonds. This process requires all reactants to collide in a proper orientation and adequate energy to break the bonds between atoms (OpenStax, 2016). Therefore, reaction rate is dependent on several variables that can alter the activation energy or the molecular collision such as reactants concentration, surface area, pressure, temperature and catalysts application (BYJU'S, 2020). In the reaction above(Equation 1), by increasing the HCl concentration, there will be more HCl molecules present in a certain volume of solution which increases the frequency of collision between HCl and CaCO3 molecules to form the products faster.
Methodology
Original experiment: The online simulation (Veerendra, 2017) quantitatively investigated the effect of total exposed surface area of solid reactant on reaction rate. This was examined with 2 sizes of marble chips, small and large pieces, being placed in an amount of Hydrochloric acid and measuring the volume of carbon dioxide liberated over time. The result revealed the smaller-size marbles, thus higher exposed surface area, produced CO2 at a faster rate due to the increased collision chance. Since the HCl concentration also relates to the density and colliding of reactants, this led to the following research question being developed.
Research question: What effect does change in concentration of Hydrochloric acid have on the rate of mass loss due the loss of carbon dioxide over time in the reaction with marble chips?
Trends, patterns and relationships
All trend lines in both graph 1&2 represent a positive correlation between mass loss and time, suggesting mass was lost as time increased. The trends in graph 1 both depict the initial masses were lost the most rapidly and the subsequent masses loss follow the subtle curves, thus indicates the reaction rate slowing down as the time increased. This supports theory that as time increases, the reaction is towards its completion, thus decreasing the reaction rate that results in an exponential decline between mass loss and time (Thakur, 2017). Therefore, trendlines in graph 1 were valid indications of the results. However, graph 2 does not have the same pattern as there is a linear correlation between mass loss and time, thus not a valid indication for the mass loss over time which might be due to both systematic and random errors.
In both graph 1&2, the trendline of mass loss with the higher HCl concentrations in each graph display steeper gradients as all the data points relatively above the trend of other lower concentrations, thus indicating higher HCl concentration produces a higher reaction rate. Nevertheless, there are large areas of error bars overlap between 0.2M and 0.18M HCl (graph1), and between 0.16M and 0.14M HCl (graph2) which suggest there is no significant different between these subsets, thus contradicting with the theory and decreasing the experimental validity. However, error bars between 0.12M and 0.16M HCl(graph2) has no overlapping area and total mass loss with 0.2M HCl is 0.327(g) higher than that in 0.12M HCl, meaning CO2 is liberated at faster rate with higher HCl concentrations.
The equation y=3.15x-0.4125 in graph 3 represents a positive relationship in which as the HCl concentration increased from 0.12M-0.2M, the average reaction rate at 30s increases from 0.007-0.233(gmin-1). However, the R2 value of 0.8198 suggests the trendline not reliable indication of the data which illustrates a low correlation between HCl concentration and reaction rate.
Limitations of the evidence Reliability and validity of the experimental process
The mass loss recorded for each trial was inconsistent, thus the average time was unreliable. Different mass loss at a specific time interval within the same concentration (Appendix 1-5) indicates the raw data for mass reading was inconsistent and imprecise The measurement uncertainty for mass loss based on equipment was ±0.002(g) while the SD and SE for average mass loss was up to ±0.047(g) and ±0.027(g) respectively (table 2). Also, the SD and SE tended to increase as mass loss increased over time, suggesting the experiment was unreliable due to random errors that consistently accumulated throughout the whole process.
The initial mass readings were inaccurate. As CaCO3 added into the flask, the mass varied for 2-3(s) before it fixed for a friction of second then start to lose mass, therefore the initial masses were not accurately at 0s. As the initial masses were not directly determined at the beginning, this created the inconsistency in the method of collecting that caused high imprecision of mass readings(appendix 1-5). This random error decreased both validity and reliability of the experiment.
The recorded mass loss was inaccurate As CO2 liberated, the mass decreased. However, there was potential loss of liquid spray which might further reduce the mass. This could be seen in the in trendlines for 0.2M(graph1) and 0.12M(graph2) which had the masses loss at 300-360s abnormally higher and out of the trendline pattern. Since the mass decreased due to both loss of CO2 and potential escape of liquid, the experiment lacks validity due to random error.
The diluted [HCl]s might be inaccurate. Each HCl concentration was the result of multiple steps of dilution which accumulated the measurement uncertainty. This might cause the [HCl] to be much lower or higher than the actual [HCl]. The significant difference in mass loss (table2) between the two subsets of [HCl], 0.18-0.2M compared to 0.12-0.16M, might be due to the inaccurate [HCl], meaning the [HCl] from 0.12-0.16M was too low because of subsequent dilutions Since the mass loss recorded with inaccurate [HCl], the experiment lacks validity due to both random and systematic errors in diluting [HCl].
Conclusion
In conclusion, the general trend from graph 1 and 2 suggests increasing the [HCl] in the reaction between HCl and marble chips increased the CO2 production that resulted in faster rate of mass loss. Although there were error bars overlap in the mass loss trendlines between 0.2M - 0.18M, and between 0.16M - 0.14M that reduced the experimental validity, the significant difference in average rate of mass loss between 0.2M and 0.12M(graph3) supported the theory which states as the reactant concentration increases, there will be more chance of collision of molecules which increases the reaction rate. Nevertheless, the experimental results had high SD and SE, suggesting the high variability and imprecision, thus low reliability of data. Therefore, the proposed improvements and extensions are recommended to enhance the validity and reliability of the experiment. Further investigation can look at other factors that affect the rate of reaction, such as temperature or different enzymes as a catalyst, with different reactants to gain a deeper insight about the collision theory.
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