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Experiment 2 Ans EXPERIMENT 2 TF Notes Experimental Notes Copper Oxide: 1. Watch out for both inadequate heating and overheating of the copper sample. The burner flame should never touch the crucible. Students should heat their sample for at least 10 minutes after the sample has turned an even black color. Every 3 minutes, students should use crucible tongs to gently tap the crucible on the table surface. 2. The yield for this part of the lab will be low, usually around 15% to 20%. If students report a high yield (anything greater than 50%), it is usually due to an error in their calculations. Go around and ask your students for their yield. If it seems high, double-check their calculations. 3. Students should never look directly down into the crucible, especially when the crucible is being heated over an open flame. 4. Demonstrate how to light the Bunsen burner in your Prelab talk and to individual groups as needed. 5. Do not allow students to grind the copper powder. Copper powder (Cu), some of which has been partially oxidized (the lower darker half). Copper oxide (CuO). Experiment 2 Unknown Salts: 1. It is even more important in this part of the lab that students gently heat their crucible. 2. Make sure students note which salt they choose in the data table they generate. 3. Identity of unknown hydrates. A is CoCl2⋅6H2O, reddish-purple in color, melts at 86°C, loses all water at 100°C. Since it must boil before it loses water, some bubbling is permissible. Before becoming anhydrous, there is a stable dihydrate that is purple in color. Do not let students stop at this point. The anhydrous salt is light blue and melts at 724°C. B CuSO4⋅5H2O, dark blue in color, melts and loses 4 waters at 110°C, loses the 5th water at 150°C. The anhydrous salt is greenish-white and melts and decomposes at 200°C. C CuCl2⋅2H2O, aqua blue in color, melts and loses all its waters at 100°C. The anhydrous salt is yellow- brown, melts at 620°C, and loses Cl2 at 993°C. Hydrated salts: Left to right, CoCl2⋅6H2O (reddish purple), CuSO4⋅5H2O (dark blue) and CuCl2⋅2H2O (aqua blue). Stable, purple dihydrate of CoCl2⋅6H2O. Do not let students stop at this point! Anhydrous salts: Left to right, CoCl2 (light blue), CuSO4 (greenish-white), CuCl2 (yellow-brown). Experiment 2 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) + O2 (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 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 CoSO4·7H2O. 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: CoSO4·7H2O (s) CoSO4 (s) + 7 H2O (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 CoSO4·7H2O is red-pink, while the anhydrous (“no water”) salt CoSO4 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: CuSO4·5H2O CuCl2·2H2O CoCl2·6H2O 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 CuSO4·5H2O, you would expect to report that 5 moles of water were released per mole of anhydrous CuSO4. 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 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 (This Page Intentionally Left Blank) Experiment 2 4 TF: __________________ Name: __________________________ Experiment 2: Procedure, Lab Report and Prelab Before You Come to Lab: • Read the entire lab report, including the previous introduction and discussion, and the entire procedure. • Complete the Prelab, which is the last page of the lab report, and turn in the prelab to your TF as you enter the lab. Safety in the Laboratory • Safety glasses and lab coats must be worn at all times while in the lab.
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