Limiting Reagents
Amy George, Kayme Backstrom, Danielle Ranko
Performed October 23, 2013 in Chem. 111, Wednesdayat 10am at College of Western Idaho.
Abstract
The limiting reagents were determined using stoichiometry, mole to mole ratios. Copper (II) chloride, (CuCl2)(s) was dissolved in 25 milliliters of water and reacted with Aluminum, (Al)(s) in three separate ratio proportions. Observations validated the theoretical yield of each experiment. The first experiment was performed in a proportion that produced excess aluminum. The second reaction had excess copper(II) chloride making aluminum the limiting reagent. In the third reaction based on stoichiometric calculation of a balanced equation: 3CuCl2(aq)+2Al(s) 2AlCl3(s)+3Cu(aq), it was determined that by adding 0.0936 g of Al to 0.70g of CuCl2 in 50ml of water that there would be a compete reaction without any limiting reagent.
Introduction
The purpose for this experiment is to use a balanced equation to determine exactly how much product could be made. It is necessary to understand mole to mole ratios, (i.e. stoichiometry), limiting and excess reactants in a chemical reactions, and theoretical yield vs. actual yield and the comparison of those calculations.
Stoichiometry is basically a ratio of ingredients needed to make a product. Just as the product is not important, neither is the measurement of units used. The important part is units can all be compared using ratios. For the work performed mole to mole ratios were used. The balanced equation 3CuCl2(aq)+2Al(s) 2AlCl3(s)+3Cu(aq) was used in our stoichiometric math calculations.
The knowledge of redox reaction would also be helpful in understanding these specific reactions also. A redox reaction is an oxidation- reduction reaction. As stated in the lab manual,”electrons are exchanged between the two metalsas they change their oxidation states.” Through this process it is possible to actually see the effects of limiting and excess reactants.
This type of information appears in many types of real world, everyday experiences. Many people are going “green” and trying to limit waste. By knowing the limiting reagent , it would help to determine the amounts of other reagents in needed to perform a task. Chemists are certainly no exception to this, even chemistry is going green. As Barry Trostdescribes percent atom economy in his publication, Green Chemistry, you could see that similar stoichiometric mathematics is used and the benefit to that is less harmful waste put into the environment. Making something new from waste so that it is no longer waste or as much waste would also be a way to go green.
Methods and materials
Materials needed to reproduce this experiment are for the most part found in the lab manual, An Atoms First Approach to Chemistry. There were a few modifications however. In our experiment we used dehydrated CuCl2(s) and rehydrated with approximately 50mls of water. So in doing so, we set up three beakers as follows:
Beaker one
I the first beaker 0.53 g of CuCl2(s) was added to approximately 50 mls of water, H2O. Next 0.25 g of aluminum was added. The initial observation was and aquablue colored solution witha green precipitate. During the reaction the precipitate formed into a rusty looking substance and the solution became cloudy white. An as the reaction sat for some time the solution was now clear with obvious rusty looking aluminum flakes floating in it. Finally the substance was a cloudy purple without precipitate in it.
Beaker two
The second Beaker 0.72 g of CuCl2 was added to approximately 50 mls of water. Next 0.72g of aluminum was added. The initial reaction turned the solution the same aqua blue with green precipitate. Again the solution became a cloudy white color, however it was not quite as cloudy as beaker one was. The reaction ended with the solution looking cloudy and purple with many black specs of precipitate floating in it.
When the reaction was complete we followed the procedure on how to filter the precipitate. After recording the mass of the filter paper, which was 1.24g, we assembled the described equipment to start the filtering and rinsing process. The manual states that, we rinsed the solid with small amounts of ethanol and acetone. Although we followed this step of the procedure we first rinsed the solid with a small amount of HCl. Following the rinsing process we transferred the sample to a crucible dish and placed in the oven for drying at approximately 170. The sample remained in there for about 15 minutes until it was dry. After removing the sample from the oven we collected the final mass, which was 1.57g.
Beaker three
The third beaker contained 0.75g of CuCl 2 added to approximately 50 ml of deionized water. Using the following calculation it was determined that 0.0936g of aluminum should be added for a complete reaction of both substances.
0.70g CuCl2=
We added 0.09g of aluminum. As the reaction started the solution was aqua blue with a green looking precipitate. The green precipitate turn into a rusty color and the solution became cloudy purple. After some time the solution was completely cloudy purple without precipitate.
Results
The results were as follows:
Using these mass measurements we calculated the number of moles by using an equation
Mass measured
As our results reflect the limiting reagent in beaker 1 was CuCl2. In beaker 2 there was excess CuCl2 and the limiting reagent was aluminum. In this reaction we were also able to check to see how well our experiment went by the following equation: . We had a percent yield of 96%. In beaker 3 our calculations were slightly off because we calculated that we should have, some of our error could have come from inaccuracy in measurement of aluminum added, the scale was acting really weird.
Interestingly enough although beaker 1 and 2 had approximately the same total mass of reacts of ~0.75g the combination of the reactants was different. We found that beaker 1’s total number of moles was 5.62 X 10-3moles, (theoretically). The total number of moles of beaker 2 was 5.93 X 10-3moles, (theoretically). This would suggest that beaker 2 had the potential and made more product than beaker 1, although not by much. Along the same thought, theoretical grams produced shows that beaker 1 with 0.4954g produced more product by mass than beaker 2’s 0.4154g. This shows that there is a difference in the products and reactants molar masses and molecular masses. In other words, the same elements combined together in different ways could produce different theoretical masses and mole numbers. Not that any product was actually lost, but it shows error, part of which may be that reactions may have not been completely reacted.
We concluded that each reaction was complete in various observational ways depending on the limiting reagent. In beaker 1 we gave it time, as it appeared no more changes were occurring even though there was remaining aluminum. Beaker 2 all of the aluminum reacted so it was easy to tell when the reaction was complete because aluminum was the limiting reagent. In beaker 3 we observed all the alluvium react until there wasn’t any visible. Although there was not supposed to be a limiting reagent in beaker 3, it is very unlikely that our measurements and calculations were so spot on that there really wasn’t a limiting reagent.
Conclusion
As discussed in the above, the ability to determine liming reagents reduces waste and predicts the amount of product possible. This relates to real life in many areas such cooking, assembly, along with chemistry going “green”. Although theoretical yields are only a prediction of what you might end up with based on your ratios, it is better to see that if everything goes well, what you might get as a product. You can also use percent yield to “check” how well everything went.
References
- Dieckmann, Gregg R. ; Sibert, John w., An Atoms First Approach to the general Chemistry Laboratory, McGraw Hill ,2012, pp 81-88
- Barry M. Trost;The Atom Economy-A Search for Synthetic Efficiency; Science 1991, (254), pp 1471-1477.
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