An Investigation Into Finding The Limiting Reagent In A Chemical Reaction

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Introduction

Stoichiometry is the section of chemistry that contains relationships between either reactants and/or products in a chemical reaction to assist in determining the required quantitative data (Nijmeh 2008, p1). Stoichiometry is used by chemists to give a detailed explanation on how much of certain materials are required in order to make a certain number of given products (Kailasa 2005, para.1). No products would be able to be made without chemical stoichiometry using balanced equations to calculate the number of products needed in order for a certain amount of product to be formed. This can often be seen through the use of limiting and excess reagents and how much of one substance is needed to produce this end amount of solution formed with most minimal expense possible (Kailasa 2005, para.13).

Limiting reagents are the reactants that determine how much of the products are made in a chemical reaction. The other reactants in the chemical reaction are often giving the term, excess reagents due to the leftover product after the limiting reagent is completely used up, also determining when the reaction stops (Khan Academy 2015, para. 1). If the equation is not balanced then using the right ratios based on the stoichiometric coefficients then it will result in an incorrect result of product formed (Khan Academy 2015, para. 3).

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The theoretical yield is the number of products created by a completed chemical reaction given that none of the reactants were wasted and everything was balanced (Kennan 2018, para. 1). Theoretical yield calculation of an equation takes into account the molar amounts of the reactants and products and clarifies that enough of each reactant is present in order for the reaction to be done completely and correctly (Kennan 2018, para. 1).

In the investigation aimed at testing different amounts of aqueous Sodium Iodide being reacted with a fixed amount of aqueous Lead Nitrate and the insoluble yellow Lead Iodide precipitate formed is measured to study the influence of the limiting reagent in the products. The balanced chemical equation of the reaction between the aqueous Lead Nitrate and Sodium Iodide shows that the mole ratio between the two is 1:2 (refer to equation 1).

Aim

The aim of this investigation was to test the limiting reagent in a reaction where numerous different masses of Sodium Iodide is reacted with a fixed amount of Lead Nitrate and the Lead Iodide precipitate formed is gathered and analysed.

Hypothesis

It can be hypothesised that if 1.66 grams of Lead (II) Nitrate is reacted with different amounts of Sodium Iodide, then the maximum amount of Sodium Iodide that can be reacted to produce the perfect amount of Lead Iodide will be 1.5 grams and the maximum amount of Lead Iodide that can be formed is 2.31 grams (refer to appendix for calculation). This is due to the balanced chemical equation showing the mole ratio of 1:2.

Discussion

The aim of this investigation was to test the limiting reagent in a reaction where numerous different masses of Sodium Iodide is reacted with a fixed amount of Lead Nitrate and the Lead Iodide precipitate formed is gathered and analysed.

The experiment results showed a steady increase of the Lead Iodide when using the masses 0.75g, 0.90g, 1.05g, 1.20g, 1.35g, 1.50g. When the quantity of Sodium Iodide had increased from 1.50g to 1.65g and 1.95g to 2.10g, the Lead Iodide obtained 0.13g and 0.75g more than 2.31g making these results outliers as well as a disproving the hypothesis. Contradictory to human fault, the hypothesis was otherwise overall supported by the trend in Lead Iodide for the experiment maintaining less or equal to 2.31g.

For experiments 7 (2.44g) and 10 (3.06g), the actual yield for Lead Iodide was higher than the theoretical yield for Lead Iodide by a fair bit. For experiment 7, the expected mass of Lead Iodide should have been 2.31g and same with experiment 10. The possible reasons for these inaccuracies include errors that might have occurred during measuring the amount of Lead Nitrate and Sodium Iodide and not getting all of the Lead Iodide precipitate into the funnel or improper drying which all could have resulted in increases in the mass of the product.

From Experiment 1 (0.75g of NaI) to Experiment 6 (1.50g of NaI) a gradual increase of Lead Iodide was evident in the results. In these masses, NaI was the limiting reagent as the mole ration suggested that 1.66g of Lead Nitrate will require 1.50g of NaI for it to react completely. This resulted in why the mass of the Lead Iodide gradually increased as the mass of NaI was increased from 0.75g to 1.50g.

Experiments 7 (1.65g of NaI) to experiment 10 (2.10g of NaI) the limiting reagent was Lead Nitrate. Since the mass of Lead Nitrate had been controlled to 1.66g, the Sodium Iodide mass above 1.50g should not increase the mass of the product. Experiment results also showed a similar result in the mass of Lead Iodide sitting at around 2.31g. These results support the hypothesis however, the results in experiment 7 and 10 do not support the hypothesis due to mistakes in the measuring or improper drying of the precipitate.

To look further into the results, it can be seen that experiments 7 and 10 were both outlier experiments and were incorrect due to being subjected to human error but all the other results show stable trends according to the hypothesis.

During the experiment, there were many errors that could be seen when calculating this data including the measuring of the Lead Nitrate as well as the Sodium Iodide. The pouring and cleansing of the beakers into the funnel, the way the funnels were made and weighed, how long they were stirred and left to set for as well as the temperatures they were kept in and how long they were left for as some experiments were done well before others. Some other errors seen throughout the experiment include when pouring precipitate into beaker some may have splashed out resulting in loss of some substances meaning incorrect readings would occur. The equipment used had certain limitations as well as limitations to that of the classroom and the number of certain pieces of equipment available. Excess rinsing using the deionised water spraying the water into the beaker or on the stirring rod spraying some more of the precipitate onto the bench and not in the beaker. Some examples of this can be best seen when comparing the theoretical and actual yields of the PbI2’ outliers in the experiment.

For future investigations and experiments, some areas for improvement include being more accurate with the measuring of the Lead Nitrate and the Sodium Iodide as well as being more accurate with the time stirring the mixtures and the time in which the mixtures are left overnight to set into the funnel. The filter paper should have no open creases where the precipitate can seep through as well spotting tears in the filter paper sooner rather than later to also make sure no precipitate can seep through.

This investigation assisted in giving a proper investigation into the importance of what a limiting reagent is and how important it is in an experiment. Limiting reagents can be applied to all forms of things including real life scenarios making them valuable to know and even more valuable to understand. Limiting reagents allow for the best possible outcome at the best possible cost and avoids wasting materials unnecessarily.

Conclusion

In conclusion, the aim of the investigation was to obtain enough quantitative data in the experiments where the fixed amount of Pb(NO3)2 is reacted with varying amount of NaI to find out whether the limiting reagent has any influence on the amount of product formed in a reaction. Experiment results showed that as the NaI increased from 0.75g to 1.50g, there was a clear linear trend being followed. It wasn’t until the NaI amount hit 1.65g and 2.10g that the trend was broken due to these outliers. These outliers should have been closer to and below if not equal to 2.31g of PbI2. Despite these outliers, it can be concluded that the hypothesis of these experiments is supported due to the linear trends but could be improved by the experiments being conducted with modifications to the method, ensuring that all the products are measured accurately.

Reference List

  1. Byjus.com 2019, Limiting Reagent, viewed on the 24th Feb. 2019, https://byjus.com/chemistry/limiting-reagent/
  2. Kailasa A 2005, Yale National Initiative, viewed on the 25th Feb. 2019, https://teachers.yale.edu/curriculum/viewer/initiative_08.06.05_u
  3. Kenning M 2018, Sciencing, viewed on the 25th Feb. 2019, https://sciencing.com/calculate-theoretical-yields-2658.html
  4. Kotz, J. C., P. M. Treichel, J. R. Townsend, and D. A. Treichel 2015, 'Stoichiometry: Quantitative Information about Chemical Reactions.' In Chemistry and Chemical Reactivity, Instructor's Edition, 139-49. 9th ed. Stamford, CT: Cengage Learning, viewed on the 25th Feb. 2019, https://www.khanacademy.org/science/chemistry/chemical-reactions-stoichiome/limiting-reagent-stoichiometry/a/limiting-reagents-and-percent-yield
  5. Nijmeh J 2008, Chem Libre Textbooks, viewed on the 24th Feb. 2019, https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Chemical_Reactions/Stoichiometry_and_Balancing_Reactions
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