THEORETICAL YIELD vs EXPERIMENTAL YIELD IN A COMBUSTION REACTION

THEORETICAL YIELD vs EXPERIMENTAL YIELD IN A COMBUSTION REACTION Suzanne Abou Shalah (Year 10) Science Faculty, The Illawarra Grammar School, Western Avenue, Mangerton, 2500

Abstract

Stoichiometry in Chemistry is the study of quantitative relationships between the amounts of reactants used and the amounts of products formed. It is based on the Law of Conservation of Mass, which states that matter cannot be created or destroyed. In chemical reactions, the amount of matter present at the end of a reaction must be the same as the amount present at the beginning. The calculations depend on balanced chemical equations, which indicate the molar ratio of the reactants and products. In this study, stoichiometry was used to determine the amount of product produced of MgO in a combustion reaction. This was tested in an exothermic, combustion reaction of magnesium. The experiment was repeated multiple times, and the results found that although the experimental yield was similar to the theoretical yield, the mass of the products varied due to experimental errors.

Introduction

This study aimed to investigate how experimental yield compares to theoretical yield in a combustion reaction of magnesium. The theoretical yield was calculated using stoichiometry and was used to calculate the percentage error when the experimental yield was found. Based on stoichiometry and the Law of Conservation of Mass, it was predicted that the amount of matter in the reactants will be the same as the amount of matter in the products.

Method

The following equipment was used in the experiment: crucible with lid, tongs, gauze mat, bunsen burner, magnesium. Firstly, the experiment was set up as shown in Figure 1 and the mass of the empty crucible was measured. Secondly, the piece of magnesium was coiled tightly and also measured along with the crucible and lid. To ensure consistent mass throughout the trials, the crucible was heated over the bunsen burner, until the mass was stable, and all moisture was released. Subsequently, the magnesium was placed in the crucible and heated over the bunsen burner with the lid remaining closed. Oxygen was periodically allowed in the crucible to aid combustion and to react. Combustion was complete when the Magnesium no longer ignited when exposed to oxygen; the product was white and flaky. Thereafter, the crucible with the lid was weighed once again and the mass was recorded. The following steps were repeated another 2 times and the results were recorded.

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Figure 1 - Equipment set up

Results

Yield Trial 1 Trial 2 Trial 3

Theoretical yield 0.48097 0.3815 0.3317

Actual yield 0.37 0.35 0.32

Percentage error 23.1% 8.2% 3.5%

Discussion

The experiment was repeated multiple times to test the Law of Conservation of Mass using stoichiometry in a combustion reaction by burning magnesium. To ensure the reliability of the experiment, five trials were conducted, in which the results varied significantly from the first and the last trials. Of course, in such an experiment, burning magnesium in a crucible will not have 100% accuracy when it comes to the amount of yield produced. This is due to various errors, predominantly due to the loss of matter whilst allowing oxygen into the crucible to react with the magnesium and aid combustion. Moreover, in the first few trials, such as trial 1 in figure 2, the crucible was not heated over the bunsen burner; there would have been excess products from previous experiments, and the mass of the crucible was not stabilised. Meanwhile, when the crucible was heated in trial 2 and 3, and when the crucible was weighed after it was heated, its mass remained stable and steady; the percentage error decreased, thus the results were more accurate.

The theoretical yield was calculated using stoichiometry and molar ratios. The reason stoichiometry was used is because it allows us to predict the mass of the product after it undergoes a combustion reaction, given the mass of the reactants. With these calculations, I tested the magnesium combustion reaction, found the experimental yield/ mass of the product, and compared it to the theoretical yield. As seen in figure 2, the percentage errors in the three trials varied from 3.5% to 23.1%.

To improve this experiment in the future, the magnesium ribbon could be coiled loosely rather than very tightly. Since the magnesium was coiled very tightly in some of the trials, the magnesium took longer to react, and in some cases, it did not completely react, hence less yield was produced, with a higher percentage error. Error can also occur due to the moisture in the crucible prior to experimentation. To prevent this in the future, the crucible should be heated over the bunsen burner until all moisture is released, which was done in trials 2 and 3. In conclusion, the results did not support the hypothesis, which predicted that the same amount of matter in the reactants will be the same amount of matter in the products. As

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addressed in the discussion, there were various errors leading to the inaccuracy of the results. Nonetheless, the experiment itself was successful in determining and calculating theoretical and experimental yield using stoichiometry.

References

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