Scientific Observations and Reaction Stoichiometry: The Qualitative Analysis and Chemical Reactivity of Five White Powders

Transcription

1 Scientific Observations and Reaction Stoichiometry: The Qualitative Analysis and Chemical Reactivity of Five White Powders Objectives Part 1: To determine the limiting reagent and percent yield of CuCO 3 when Na 2 CO 3 (one of the white powders used in Part 2 of this experiment) reacts with an equal mass of CuCl 2. Part 2: (a) To determine some of the chemical and physical properties of five white solid compounds, and (b) to use these properties to determine the composition of a binary mixture of these compounds. Background In the first part of this experiment, the double displacement (or metathesis) reaction between Na 2 CO 3 (aq) and CuCl 2 (aq) will be studied. As indicated in the reaction shown below, one of the products of this reaction is a precipitate. A precipitate is an insoluble, solid substance that separates from the solution in the course of a reaction. Na 2 CO 3 (aq) + CuCl 2 (aq) 2 NaCl (aq) + CuCO 3 (s) In this experiment, solutions containing equal masses of Na 2 CO 3 and CuCl 2 will be reacted. The solid CuCO 3 that precipitates out of solution (which represents the actual product yield) will be collected and weighed. Using this information and the amount of Na 2 CO 3 and CuCl 2 that were initially reacted, the limiting reagent, the theoretical yield of CuCO 3, and the percent yield of CuCO 3 will be determined. Chemical reactivity and physical properties are characteristic of a substance and may be used for compound identification. Although one or two properties of compounds may be similar, it is those properties which are different that may serve to distinguish one substance from another. In the second part of this experiment, the solubility, solution ph, and chemical reactivity [with HNO 3, the sulfate ion (SO 4 2- ), and the silver ion (Ag + )] of five white inorganic compounds, specifically BaCO 3, BaCl 2, BaSO 4, Na 2 CO 3, and NaCl, will be examined. After completing these qualitative tests, each group will receive an unknown containing only two of these white powders. The binary (2-component) mixtures will be subjected to a battery of similar qualitative tests, and the observed data will be used to identify the constituent compounds. Pre-Lab Questions 1. Name the following compounds: (a) BaCO 3 ; (b) BaCl 2 ; (c) BaSO 4 ; (d) Na 2 CO 3 ; (e) NaCl; (f) CuCl 2 ; and (g) HNO 3. 1

2 2. When solutions containing 1.00 g of BaCl 2 and 1.00 g of Na 2 SO 4 were mixed, solid BaSO 4 formed. The BaSO 4 precipitate was filtered out of the solution, allowed to dry, and then weighed. The mass of BaSO 4 that was isolated from solution was 1.04 g. Use this information to answer the following questions. Pay close attention to the proper use and reporting of significant figures and units. You must show all of your work (i.e., your calculations) for full credit. Safety (a) Write a balanced chemical equation that shows the reaction that occurs when an aqueous solution of BaCl 2 reacts with an aqueous solution of Na 2 SO 4 to form aqueous sodium chloride and solid BaSO 4. In your equation, you should indicate the physical states of each reactant and product. Note: Precipitation reactions are discussed in Section 4.2 of your textbook. (b) Calculate the moles of BaCl 2 that are available for reaction. (c) Calculate the grams of BaSO 4 that should form if all of the available BaCl 2 reacts. (d) Calculate the moles of Na 2 SO 4 that are available for reaction. (e) Calculate the grams of BaSO 4 that should form if all of the available Na 2 SO 4 reacts. (f) Which reactant (BaCl 2 or Na 2 SO 4 ) is the limiting reagent in this reaction? Why? (g) What is the theoretical yield of BaSO 4? (h) What is the percent yield of BaSO 4? (See Section 3.10 of your textbook, pages , for information regarding the calculation of a percent yield.) (i) If this reaction went to completion, then how many grams of the excess reagent should remain after the reaction is complete? Goggles must be worn at all times. Care must be exercised when using nitric acid. Barium, silver, and copper compounds in solid or solution form (and the first washing of the precipitates of these ions) must be collected and disposed of as directed by your TA. Laboratory Procedures Part 1 Reagents: Na 2 CO 3 and CuCl 2 Weigh accurately (correct to g) 1.0 g of Na 2 CO 3 and 1.0 g of CuCl 2. Record the measured masses in your laboratory notebook. Place the Na 2 CO 3 and the CuCl 2 into separate 100 ml beakers. Add approximately 25 ml of water to each beaker. Stir (or agitate) the solutions until the solids fully dissolve. Record the color of each solution. Pour approximately 5 ml of the CuCl 2 solution into the beaker containing the dissolved Na 2 CO 3. Record your observations. Continue adding the CuCl 2 solution a few ml at a time. After pouring out all of the CuCl 2 solution, rinse the emptied beaker with 2

3 approximately 2-3 ml of water and add this rinse to the Na 2 CO 3 beaker. If necessary, repeat this a second time to ensure that all of the CuCl 2 is transferred. Gently stir the reaction mixture for several minutes with a stirring rod. Assemble a vacuum filtration apparatus (see Figure 1 below) to separate the copper (II) carbonate precipitate from the solution. While gravity filtration would allow isolation of the insoluble product, vacuum filtration will be used to expedite the separation process and to help dry the solid residue. Make sure that flexible vacuum tubing is used to connect the filter flask to the water aspirator. DO NOT connect the tubing to the water outlet. After assembling a vacuum filtration apparatus, have a TA verify the setup before proceeding. Figure 1. Different views of a vacuum filtration apparatus. Weigh (correct to g) a piece of filter paper and record the measured mass. Place the filter paper in the Buchner funnel, moisten it with a small amount of water, and then turn on the water aspirator. The suction will seal the filter paper to the funnel. This will prevent your precipitate from flowing underneath the filter paper when filtering your reaction mixture. While the vacuum is on, slowly begin to pour your reaction mixture into the funnel by decanting the liquid along the length of a stirring rod. Avoid pouring the mixture along the edges of the filter paper. This could result in a loss of product. If precipitate remains in the beaker after you have finished pouring, then rinse the beaker 3

4 with a small amount of water and transfer the precipitate residue to the filter paper. Repeat as necessary to make sure that all of the precipitate is collected onto the filter paper. What color is the precipitate? What color is the filtrate (the fluid in the filter flask)? Continue to pull a vacuum until the precipitate is mostly dry. If the precipitate does not dry sufficiently, then carefully transfer the filter paper and precipitate to a clean watch glass and place the watch glass into an incubator oven for additional drying. You should start Part 2 of this experiment while you wait for your solid CuCO 3 to dry. When the filter paper and precipitate are dry, weigh (correct to g) the filter paper and precipitate. Record this mass. Use the mass of the filter paper and the combined mass of the filter paper and precipitate to determine the mass of CuCO 3 that was isolated. Calculate the theoretical yield of CuCO 3 based on the initial amounts of Na 2 CO 3 and CuCl 2 that were allowed to react. Which reactant was the limiting reagent? Which reactant was the excess reagent? Explain your reasoning. How many grams of the excess reagent should remain in solution if the theoretical yield of CuCO 3 is produced? What is the percent yield of your reaction? If your calculated percent yield is not 100%, explain possible reasons for this. Part 2(a) Reagents: white solids - BaCO 3, BaCl 2, BaSO 4, Na 2 CO 3, and NaCl; 3 M HNO 3, 0.1 M Na 2 SO 4, 0.1 M AgNO 3, Universal Indicator Solution This part of the experiment involves the qualitative analysis of five white powders, namely BaCO 3, BaCl 2, BaSO 4, Na 2 CO 3, and NaCl. Each of these solids will be tested for solubility in water. The resulting aqueous mixtures will be tested for ph, treated with nitric acid, and then subsequently treated with sodium sulfate and silver nitrate solutions. Construct a table in your laboratory notebook like the one shown below. Compound Solubility in water ph Reaction with HNO 3 Reaction with Na 2 SO 4 Reaction with AgNO 3 BaCO 3 BaCl 2 BaSO 4 Na 2 CO 3 NaCl Record your observations in this table as you perform the five tests on each of the five white solids. A small amount (pea-sized or less is appropriate) of each of the white solids should be placed in separate small test tubes. Add water to each test tube to dissolve (or attempt to 4

5 dissolve) the solids. Record your observations. Add Universal Indicator to the "solution" or mixture. Note the approximate ph of each. (A chart indicating the color of the Universal Indicator solution at various phs will be available in the laboratory.) Record whether the measured ph indicates an acidic, neutral, or basic solution. Add nitric acid to each test tube in a drop-wise manner until the solution becomes acidic. Record your observations. The resulting solutions should be decanted (if necessary) and divided in half. Add sodium sulfate solution to one portion and silver nitrate solution to the other. Record your observations. Part 2(b) Reagents: 3 M HNO 3, 0.1 M Na 2 SO 4, 0.1 M AgNO 3, Universal Indicator Solution, and unknown binary mixtures of BaCO 3, BaCl 2, BaSO 4, Na 2 CO 3, and/or NaCl A binary mixture of two of the white solids will be subjected to the same tests used in Part 2(a) of this experiment, and the composition of the mixture will be determined. Your TA will give you a small amount of an unknown mixture containing two of the white solids you have studied. Record the number (#1, #2, #3, #4, or #6) assigned to your unknown. Devise a method of analysis to determine the composition of your binary mixture. Discuss this with your group and with your TA before you begin. You may wish to develop a flow chart for your analysis. Perform your analysis. Be sure to identify the components of your unknown binary mixture in your notebook. In your laboratory report, you must justify how your data support your conclusions. 5

6 Post-Lab Questions Solubility tests, not unlike the ones that you performed in this lab, have been used to formulate a list of rules that allow one to predict the solubility of common inorganic compounds. The solubility rules that are given in Chapter 4 of your textbook are summarized below. Solubility Rules for Common Ionic Compounds in Water at 25 C Soluble Compounds - Compounds containing alkali metal ions or the ammonium ion (NH + 4 ) - Nitrates (NO 3 - ), bicarbonates (HCO 3 - ), chlorates (ClO 3 - ) Exceptions - Halides (Cl -, Br -, and I - ) - Halides of Ag +, Hg 2 2+, Pb 2+ - Sulfates (SO 4 2- ) - Sulfates of Ag +, Ca 2+, Sr 2+, Ba 2+, Hg 2 2+, Pb 2+ Insoluble Compounds Exceptions - Carbonates (CO 2-3 ), phosphates (PO 3-4 ), - Compounds containing alkali metal ions or chromates (CrO 2-4 ), sulfides (S 2- ) the ammonium ion - Hydroxides - Compounds containing alkali metal ions or the Ba 2+ ion Use these solubility rules to help you answer some of the following questions. 1. What is gravity filtration? How is this technique different from the filtration method used in Part 1 of this experiment? 2. Explain how the porosity of the filter paper could affect the amount of precipitate that is collected in Part 1 of this experiment. 3. Why is it important to dry your precipitate in Part 1 of this experiment? 4. In Part 2(a) of this experiment, you examined the solubility of each of the five white solids in water. Do your recorded observations agree with the predictions given by the solubility rules? Explain. 5. Acids react with carbonates and bicarbonates to produce a salt (which can be described as an ionic compound made up of a cation other than H + and an anion other than OH - ), H 2 O(l), and carbon dioxide gas (CO 2 ). Did you see evidence of gas formation when you added nitric acid to the two carbonates used in this lab (BaCO 3 and Na 2 CO 3 )? Explain. Write balanced chemical equations for these chemical reactions (see below). In your equations, you should indicate the physical states of each reactant and product. Use the solubility rules to help you predict 6

7 whether the salt formed in each reaction is soluble or insoluble. Do your experimental observations agree with your predictions? Explain. BaCO 3 (aq) + HNO 3 (aq)? Na 2 CO 3 (aq) + HNO 3 (aq)? 6. When an ionic compound dissolves in solution, the ionic substance dissociates into its component ions. Based on solubility rules, which cation(s) in the five white solids would be expected to react with the sulfate anion (SO 4 2- ) to form an insoluble ionic compound? What is/are the precipitate(s) that should form? Explain how your observations agree or disagree with your predictions. 7. Based on solubility rules, Ag + should form silver carbonate (Ag 2 CO 3 ), an insoluble solid, when reacted with CO Indeed, Ag 2 CO 3 (s) readily forms when aqueous solutions of Na 2 CO 3 and AgNO 3 are mixed. Did you see a precipitate form when AgNO 3 was added to an acidified solution of Na 2 CO 3? Explain why you would or would not expect a precipitate to form under these conditions (consider solubility rules and the concepts covered on Question 5). 7