EXPERIMENT 2 Stoichiometry. Introduction

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1 EXPERIMENT 2 Stoichiometry Introduction Stoichiometry is the study of the quantitative relationships in chemical reactions. By studying stoichiometry, you can calculate the quantity of reactants that will be consumed in a chemical reaction, and the amount of product produced. Consider the reaction of vinegar with baking soda. As you may know, this reaction produces carbon dioxide gas which bubbles out of the vinegar. But if you want to know how much gas would be produced from combining a teaspoon of baking soda with a cup of vinegar, you would need to consider the stoichiometry of the reaction. Stoichiometry answers questions about chemical reactions dealing with how much and how many. In the first part of this experiment you will investigate the reaction of copper metal with oxygen in the air. Using stoichiometry, you can predict the amount of copper oxide (CuO) that could be produced if all the copper would react. However, when you perform the reaction yourself, you will discover that only some of the copper reacts to form black copper oxide. A stoichiometric calculation will allow you to determine the amount of copper oxide produced and the amount of unreacted copper remaining. In the second part of the experiment, you will use the techniques learned in the first part to investigate the chemical composition of an unknown compound. The solid compound will contain a certain quantity of water trapped inside it, and your task will be to calculate the amount of water it contains. Based on the amount of water released, you should be able to identify your unknown compound as one of the three possible compounds described in the experiment. You will need to make stoichiometric calculations in order to find out how much water should be released by each of the three unknown compounds, and compare that result with the amount of water actually released by your compound. Discussion Investigation of a Copper Reaction When heated in the presence of oxygen in the air, copper metal reacts to form copper oxide, CuO: 2 Cu (s) + O 2 (g) 2 CuO (s) Because only the surface of the copper metal will react to form copper oxide, you will use finely powdered copper in order to maximize its surface area. You will heat the copper powder in a crucible, which is a small thimble-shaped porcelain container used to heat substances to high temperatures. By heating the copper in a crucible using an intense gas flame from a Bunsen burner, you will be able to make much of the copper react to form copper oxide. However, despite the use of fine copper powder, some of the copper will not be exposed to oxygen in the air, and hence will remain unreacted. It is often the case that chemical reactions will not proceed entirely to completion, and this copper reaction is an example of such a reaction. With such a reaction, the amount of product Experiment 2 1

2 actually formed will be less than what could theoretically be produced in ideal circumstances. The amount of product formed is often reported in terms of a percent yield. The percent yield for a given reaction is defined as: mass of product formed precent yield 100 % theoretical maximum mass of product In this experiment, as the copper reacts to form copper oxide, the mass of the contents of the crucible will increase. This mass increase will correspond to the mass of oxygen consumed during the reaction. A stoichiometric calculation will enable you to determine the mass of copper oxide actually produced based on the mass of oxygen in the final product. The theoretical maximum mass of copper oxide can be calculated using stoichiometry based on the amount of copper used in the reaction. By comparing the mass of copper oxide actually produced with this theoretical amount, you can calculate the percent yield for your reaction. You would hope to get 100% yield in every chemical reaction, but in reality a perfect yield is rarely attained. Investigation of a Hydrated Salt At one time, you may have seen simple humidity indicators that change color to indicate the amount of water in the air. Or perhaps you have seen clothes that change color when wet, or children s bath toys that behave similarly. These items all depend on substances known as hydrated salts. A hydrated salt is a solid substance that contains water bound within the solid. For instance, the natural mineral bieberite has the formula CoSO 4 7H 2 O. This means that, for every atom of cobalt in the solid, there are 7 molecules of water also trapped within the solid. By heating the solid, the trapped water molecules can be released as water vapor: CoSO 4 7H 2 O (s) CoSO 4 (s) + 7 H 2 O (g) Note that the resulting solid will weigh less due to the water lost in the process. The release of the bound water is often accompanied by a color change. In the above example, the hydrated salt CoSO 4 7H 2 O is red-pink, while the anhydrous ( no water ) salt CoSO 4 is dark blue. These types of substances can be used to indicate the ambient humidity, because they will release water in dry environments and absorb water in moist environments, changing color in the process. In the second part of the experiment, you will be given one of the following hydrated salts, but will not be told which one you have: CuSO 4 5H 2 O CuCl 2 2H 2 O CoCl 2 6H 2 O Your task will be to remove all the water from your hydrated salt, determine how much water was removed, and thus discover which of the unknown salts you were given. You will report the quantity of water removed in terms of the ratio of moles of water released per mole of anhydrous salt. For instance, if your salt were CuSO 4 5H 2 O, you would expect to report that 5 moles of water were released per mole of anhydrous CuSO 4. In addition, you should observe any color changes during the course of the reaction. As described above, these color changes will indicate the progress of the reaction. Experiment 2 2

3 Procedure In this experiment you will work in partners. One partner will perform the copper experiment, while the other will independently perform the experiment with the hydrated salt. At the conclusion of the experiment you will swap data and discuss your observations and conclusions. Waste Disposal All waste produced in this experiment will be solid waste. Solid waste must be kept dry in order to dispose of it properly. After you are finished with the substances in your crucible, scrape the solid product into the container in the waste hood. Place the empty crucible in the appropriate dirty glassware container. Remember, do not use water to rinse the waste out of the crucibles. Use of the Crucible The crucible is a thin, porcelain container designed to withstand high temperatures. We will be heating the crucible using the gas flame of a Bunsen burner. Place a ceramic triangle on a metal tripod or ring stand. Obtain a crucible from the center bench, making sure it is not cracked, and place the crucible on the ceramic triangle. Connect a Bunsen burner to the gas outlet with a rubber hose and have your TF come and verify your setup. Your TF will show you how to light the burner and adjust the flame. The blue cone at the center of the burner flame should not be taller than one inch. Experiment 2 3

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5 TF: 1/4 Name: Investigation of a Copper Reaction Data, Observations, and Notes First, take a clean, dry crucible from the center lab bench, bring it to the balance, and weigh the empty crucible. Using the analytical balance, weigh out approximately 1 gram of the copper powder. Add this powder to the empty crucible, and weigh the filled crucible. Return the crucible to the burner flame, and heat the copper powder. Do not heat the crucible so intensely that it glows red hot. Periodically remove the crucible from the flame and gently tap it against the benchtop in order to mix up the copper. While the crucible is away from the flame, note the color of the contents. Continue heating for at least 25 minutes, or until no more color change is observed. Mass of empty crucible: Mass: Mass of crucible with copper before heating: Mass: Color changes observed: Using the crucible tongs, carefully set the crucible aside to cool. When it has reached room temperature, weigh the crucible and its contents on an analytical balance. Final mass of crucible with product: Mass: Experiment 2 5

6 2/4 Investigation of a Hydrated Salt Data, Observations, and Notes This part of the experiment is performed very much like the copper reaction. In your prelab report, you will have to write out a detailed procedure. Your task will be to determine the mass of water released by heating one of the following hydrated salts: CuSO 4 5H 2 O, CuCl 2 2H 2 O, CoCl 2 6H 2 O. You will not know which of the salts you are heating, but should be able to deduce this from your final calculations. As part of your prelab, write up an appropriate data table in this space that you can use while performing the laboratory experiment. Don t forget to leave space in which to note the observed color changes. Here are some helpful hints for designing a successful procedure: You will be able to grind the sample into a fine powder using a mortar and pestle. Heat the compounds very gently. If the compound starts to turn brownish-black, gives off green gases, or starts to bubble and boil, then it is being overheated. Record all color changes observed for the compounds as they are heated. One of the compounds should exhibit more than one color change due to the formation of an intermediate stable hydrate with 2 moles of water per mole of anhydrous salt. The colors of the three anhydrous salts, in no particular order, are light blue, greenish-white, and yellow-brown. Once your entire sample has reached one of these colors, you should stop heating in order to avoid overheating. Experiment 2 6

7 TF: 3/4 Name: Lab Report Results 1. Determine the mass of CuO which was produced in the copper reaction, and calculate the percent yield of this reaction. Mass of CuO: Percent yield: 2. Calculate the mass of water released from the hydrated salt. Mass of water: 3. Determine the ratio of moles of water to moles of anhydrous salt assuming the unknown salt is CuSO 4. Ratio: Repeat this calculation assuming the salt is CuCl 2. Ratio: Repeat this calculation assuming the salt is CoCl 2. Ratio: 4. What is the most likely identity of your hydrated salt? Experiment 2 7

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9 TF: 4/4 Name: Prelab 1. Provide a detailed procedure for determining the mass of water released upon heating a hydrated salt. Be sure you write up a data table in the appropriate space on that page of the experiment. 2. One student weighed gram of copper powder into a crucible and heated it to produce copper oxide. He was able to obtain only a 15% yield. What was the total mass of the crucible contents (CuO plus unreacted Cu) after the reaction? Mass of contents: 3. Another student heated gram of a red-pink hydrated salt until all the water had been removed. She was left with grams of a dark blue solid. Is it possible that the hydrated salt was bieberite, CoSO 4 7H 2 O? Why or why not? Calculations should be shown Provide a list of all the chemicals and equipment needed for this experiment. Be concise. Experiment 2 9

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