Introduction
The purpose of this investigation was to find the quantitative data for the reaction between a solution of Sodium iodide and a solution of Lead (II) nitrate. This was done by finding when the limiting reactant changed from one solution to the other. To achieve this, one of the two reactants in the experiment was chosen to maintain the same measurement throughout and the second was gradually increased.
Stoichiometry is the branch of chemistry that deals with measuring the interactions between chemicals that go in and then come out of any given chemical reaction (Collins 2019, Green 2013). Calculations using stoichiometry assist scientists and engineers from varying industries to determine the quantity of product obtain from a particular combination, this can aide in telling the potential profitability or not of a product. (Kailasa 2019). This use of stoichiometry is also very important as it helps to minimise waste by calculating the exact amount of each substance required to produce the end product (Washington University 2005). As some chemicals can be extremely expensive, it is paramount to limit their use to exactly the right amounts.
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The limiting reagent is the substance in a chemical reaction that will be completely used up, Therefore, limiting the final product. The excess reagent, which is the substance that the chemical reaction has more than enough of to use up all of the limiting substance, Therefore, is in excess.
Using stoichiometry to calculate the theoretical yield with the balanced equation, the mass, the molar number and the molar weights of each compound are used. By taking the weight of each reactant and divining it the corresponding molar weight from the periodic table the molar number of both quantities is determined. Then in considering the number of moles per compound in the balanced equation, multiply the lower of the two molar numbers with the molar weight of the product to get the theoretical yield. It would be a perfect world if the actual yield were expected to be the same as the theoretical yield, as there are so many variables, even inside a chemistry lab. The main causes for the differences in this experiment are lack of experience in a lab and transfer loss.
The experiment conducted took 1.66g of Lead (II) nitrate and a gradually increased amount of Sodium iodide starting a 0.75g moving up by increments of 0.15g to 2.10g. Each was diluted into deionised water and then mixed together and let react for 5 minutes, the solution, a yellow precipitate, was then remixed and filtered to separate the resulting products. The balanced equation: 2NaI + Pb(NO3)2 → PbI2 + 2NaNO3, indicates the molar ratio of the reactants is 2 to 1, this is used along with the molar weights to find the maximum theoretical yield of 1.66g of Lead (II) nitrate which is 2.31g.
Aim
The aim of this investigation was to find the quantitative data for the reaction between a solution of Sodium iodide and a solution of Lead (II) nitrate to determine the limiting reactant in a chemical reaction between Pb(NO3)2.
Hypothesis
It can be hypothesised that if reacting a fixed amount of Lead (II) nitrate, being 1.66g, with different amounts of Sodium iodide, then the greatest amount of Sodium iodide that can be reacted is 1.50g and the greatest amount of the resulting product, Lead (II) iodide, is 2.31g. This is due to the balanced equation of the reaction giving a 1 to 2 mole ratio between the Lead (II) nitrate and Sodium iodide. It can be calculated that 1.66g of Lead (II) nitrate will react with a maximum of 1.50g of Sodium iodide to result in a maximum of 2.31g of Lead (II) iodide (refer to appendix).
Discussion
The aim of the investigative experiment was to determine the limiting reactants when combining 1.66g of Lead (II) nitrate with 10 quantities of Sodium iodide, starting at 0.75g increasing by 0.15g to a maximum of 2.10g.
The overall results of the experiment show that as the Sodium iodide was increased the Lead (II) iodide product increased, 1.36g, 1.38g, 1.52g, 1.83g, 1.93g, 2.21g, until it plateaued at about the 2.31g mark, which corresponded with the Sodium iodide reaching 1.50g (refer table 3 & graph 1). This indicates that prior to the Sodium iodide reaching 1.50g the Lead (II) nitrate was the limiting reactant but from 1.50g onwards the Sodium iodide became the limiting reactant this overall trend of the investigative experiment supports the original hypothesis that, as the balanced equation in the reaction between Lead (II) nitrate and Sodium iodide has a 2 to 1 molar ratio, and given the respective molar masses for each reactant, the maximum weight of Sodium iodide that could react with 1.66g of Lead (II) nitrate is 1.50g and the maximum Lead (II) iodide produced would be 2.31g.
In analysing the results, it can also be observed that there was some fluctuation in the relation between the actual yield and the theoretical yield. In sample numbers 2,4 & 7 the actual and theoretical yields were extremely close to one another, with the percentage yield being less than 0.5% out. For 3 of the samples, the actual yield, were more than 4% under the expected amounts. This could be attributed to errors in weighing the original reactants, transferring them from weigh boats to beakers, or from beaker to the filter. The amount of resulting Lead (II) iodide in some cases was actually higher than expected. In the specific case of sample number 1, where 0.75g of Sodium iodide was used, there was a significantly higher actual yield, 1.36g, compared to the expected theoretical yield of 1.15g, resulting in the percentage yield by mass being 118%. This could be the result of errors in measuring the original reactants.
In analysing the theoretical and actual yields in a graph (refer graph 1) it shows how the actual yield follows the theoretical yield relatively closely. The actual yield increased as expected, as the Sodium iodide was increased from 0.75g to 1.50 g, from which point onwards the Lead (II) iodide stayed around about the same. In addition, the outliners don’t stand out much.
As the students taking part in this experiment were mostly at a novice level, there were possible errors in measuring, weighing and transferring of the substances. Therefore, further training and more time allowed to conduct the experiment could be of assistance in accuracy.
This experiment has helped demonstrate how stoichiometry can accurately determine limiting reactants and how the limiting reactant can change as the ratio between reacting substances vary. Using stoichiometry to find limiting reactants can be used in a wide range of situations to minimise waste Therefore, limiting costs. For example, food technologist use these kinds of calculations and investigations to make all kinds of food that we eat every day.
Conclusion
The investigative experiments purpose was to determine the limiting reactants in a reaction between Lead (II) nitrate and Sodium iodide. This was achieved using a fair test, where the Lead (II) nitrate remained constant along with all other facets of the experiment whilst the Sodium iodide was altered. The results from the experiment showed that as the Sodium iodide was increased from 0.75g to 1.50g the Lead (II) iodide weight increased, but from that point onwards the product weight maintained a relatively constant level. This supported the hypothesis that the limiting reactant is the Lead (II) nitrate until the Sodium iodide reaches 1.50g at which point it then becomes the limiter. Nevertheless, there were some outliers that show inaccuracies in the way the experiment was conducted. The experiment could have benefited with more training of the skills needed to complete it and further care taken in the measuring, weighing and transferring of the substances involved.
References
- Collins Dictionary 2019, ‘Stoichiometry’ viewed 8 August 2019, https://www.collinsdictionary.com/dictionary/english/stoichiometry
- Green, H 2013, ‘Stoichiometry: chemistry for massive creatures - crash course chemistry #6’, online video, viewed 5 August 2019, https://www.youtube.com/watch?v=UL1jmJaUkaQ
- Kailasa, A 2019, ‘Stoichiometry – a necessary tool in chemistry’, viewed 12 August 2019, https://teachers.yale.edu/curriculum/viewer/initiative_08.06.05_u
- Key stage wiki 2019, Filtration diagram, diagram, https://keystagewiki.com/index.php/Residue
- Washington University 2005, ‘Stoichiometry’, viewed 12 August 2019, http://www.chemistry.wustl.edu/~coursedev/Online%20tutorials/Stoichiometry.htm
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