Stoichiometric Relationships in Chemical Reactions

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0.92g NaOH • 1mol NaOH/40.0 • 1mol Cu(OH)2/ 2mol NaOH = 0.0115 mol Cu(OH)2

The calculation of these formulas shows that in reaction 1 NaOH is the limiting reactant. (0.92g is the amount of NaOH after the four pellets were added). Once the limiting reactant was calculated the theoretical yield was calculated for the product of Cu(OH)2 using the amount of miles of Cu(OH) produced by the limiting reactant. Equation:

0.0115mol Cu(OH)2 • 97.57 Cu(OH)2/ 1mol Cu(OH)2

By this calculation the theoretical mass is 1.1221g Cu(OH)2. The precipitants left in the filter paper took approximately 48 hours to dry. Once dry the filter paper for reaction 1 was largely blackened, any part of the filter paper that was not black was a dark blue color. The black portion of the filter paper was due to the oxidation of Cu producing CuO2, the oxidation as caused by the violent reaction of Na with H2O even when not in pure form. This oxidation means that there were two driving forces in reaction one; formation of a solid and oxidation. The actual yield of Cu(OH)2 produced by the experiment was 2.5137 grams. The percent yield of Cu(OH)2 was calculated with the formula theoretical yield divided by actual yield multiplied by 100 percent.

[ 1.1221g Cu(OH)2 / 2.5137g Cu(OH)2 ] • 100% = 224%.

By this calculation the percent yield was 224%. This means that there was around double the amount of Cu(OH)2 produced than calculated, but the production of copper (II) oxide, CuO2, which added more oxygen molecules to the precipitant left on the filter paper Cu(OH)2. The heavy oxidation shows that there was too much NaOH in the solution.

Mass Table for Rxn 2:

Mass of NaI

2.6670g

Mass of Pb(NO3)2

0.9522g

The second reaction was a double replacement reaction between lead nitrate and sodium iodide in distilled water to produce aqueous sodium nitrate and solid lead iodide.

Balanced chemical equation:

Pb(NO3)2 (aq) + 2NaI (aq) ––> 2NaNO3 (aq) + PbI2 (s)

The mass of the scoop of Pb(NO3)2 was 0.9522 grams but some of the NaI was 2.6670 grams but some of the the NaI stuck to the weight boat it was weighed in. After the Pb(NO3)2, and NaI, and 25mL of distilled water was combined in the test tube the solution turned a bright yellow (the yellow color of the solution was because of the iodine reacting with water). After being stirred then left in the test tube holder for 3-5 minutes some part of the solution had settled at the bottom. Unlike the first reaction, the only indicator a reaction even occurred was the change in color. The process of the filtering was the same for reaction 2 as it was in reaction 1. The mass of the filter paper in reaction 2 was 0.7587 grams. When the solution was poured into the funnel through the filter the water that was filtered off was completely clear. Just as in reaction 1 an extra 25mL of distilled water was used to wash as much residue of the precipitant as possible off the walls of the test tube and into the filter. While the solution was filtering the limiting reactant and the theoretical yield for the precipitant produced in the reaction (PbI2). The limiting reactant was calculated using this formula:

0.9522g Pb(NO3)2 • [1mol Pb(NO3)2 / 331.22g] •[1mol PbI2 / 1mol Pb(NO3)2] = 0.002875 mol PbI2

2.6670g NaI • [1mol NaI / 149.89g] • [1mol PbI2 / 2mol NaI ] = 0.008897mol PbI2

From this calculation it is shown the theoretical yield of PbI2 is 1.325g. The precipitants left in the filter, like in reaction 1, take approximately 48 hours to dry. The precipitants left on the filter paper from it. The actual yield of PbI2 from the experiment was 1.2362g. After the theoretical yield were found the percent yield was calculated using this equation:

[1.2362g PbI2 / 1.325g PbI2] • 100% = 93.4%

The percent yield of reaction 2 was 93.4%, which means that the experiment for reaction 2 was very efficient.

In this experiment error as very easily identified, for the most part, by sight or calculations, and were determinate errors. The first major error in this experiment was the mistake in the amount of NaOH needed in reaction 1. This error caused some of the product to be lost in the filtration of reaction 1. This in turn caused another error to be made. After it was noticed that there was not enough NaOH in the solution, pouring of the solution stopped and the four extra pellets of NaOH were added. This increase in sodium caused the heavy oxidation of some of the copper in reaction 1 turning it black due to sodium an agitation in water. This is the reason for the 224% percent yield of reaction 1. Another cause of error in the experiment was some of the CuCl2 and NaI lost in the weight boat which could have had an effect on the percent yield.

Conclusion- 8/20

This lab involved stoichiometric concepts within in two reactions. Improvements on this experiment could have been made in several areas. The first improvement could have been made when the CuCl2 and the Pb(NO3)2 were being measured and some of both chemicals stuck to the weight boat, but not as much would have been lost if it had been allowed to scrape any remaining reactant from the weight boat. Accuracy of each experiment was connected to percent yield. These two are connected through the fact that the accuracy of the actual performance of the experiment determined the percent yield because the percent yield is also a reflection of how close the experimenter was able to get to the correct formula. There was no precision in this lab.

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Works Cited:

T.E. Brown, H.E. LeMay, B. Bursten, C. Murphey. Chemistry: The Central Science. Practice, Jan 8, 2008.

J.C. Kotz P.M. Trenches, J. Townsend. Chemistry and Chemical Reactivity

University of California-Davis CHEMWIKI

Science in Motion. “Stoichiometric Relationships in Chemical Reactions.” UAH., 2013.

Zumdahl, et al. World of