Fractional Precipitation

Transcription

1 Why? Fractional Precipitation Can one type of cation be removed from an aqueous mixture of multiple cations by precipitation? In both industry and research there are often times when one particular component of a mixture needs to be separated from a solution. Maybe it is a rare metal that is dissolved in a mixture of minerals. Maybe it is a particular protein from lysed plant cells. If the desired component is volatile, distillation could be used. But if the goal is to separate ions in solution, fractional precipitation is preferred. Model 1 A Precipitation Experiment Solution B 1.00 M Sodium Carbonate Solution A M Zinc Nitrate M Copper(II) Nitrate 1.00 L Copper(II) Ion Selective Electrode Zinc Ion Selective Electrode 1. Consider the experiment setup in Model 1. a. What cations and anions are present in solution A? b. What is the starting molar concentration of solution A for both zinc and copper(ii) ions? c. Identify the cations and anions present in solution B. d. What is the concentration of carbonate ions in solution B? Fractional Precipitation 1

2 2. When solution A and solution B mix, there is potential for two precipitates to form. Write double replacement reactions to show the formation of the two precipitates. 3. In the experiment setup in Model 1, how will the concentration of zinc and copper(ii) ions in solution A be recorded during the experiment? 4. If solution B is added dropwise to solution A, eventually a precipitate will begin to form. What do you expect to happen to the concentration of either zinc or copper(ii) ions in the solution? Model 2 Concentrations of Zn 2+ and Cu 2+ Ions in Solution A Concentration of Solution A ( 10 6 M) Conc. Zn 2+ Ions in Solution Conc. Cu 2+ Ions in Solution Volume of Solution B added to Solution A (ml) 5. Consider the graph in Model 2 that was produced by the experiment in Model 1 as solution B was added dropwise to solution A. Which line in the graph represents the concentration of zinc ions? 2 POGIL Activities for AP* Chemistry

3 6. Initially there is no change in the concentration of either metal cation in solution A. Explain why this is the case. 7. According to the graph in Model 2, which precipitate is formed first during the experiment in Model 1 zinc carbonate or copper(ii) carbonate? Justify your reasoning. 8. Which solute has the higher solubility zinc carbonate or copper(ii) carbonate? Justify your reasoning. 9. Suppose a researcher wanted to pull out as much zinc ion as possible from solution A without any copper(ii) impurity. a. How many milliliters of solution B should the researcher add to solution A? b. What is the maximum percent of the original zinc ion that could be removed from solution A without any copper(ii) impurity? 10. Suppose a researcher wanted to pull out as much copper(ii) ion as possible from solution A without any zinc impurity. Could this be achieved by adding solution B? Explain your reasoning. 11. The solubility product constants for the two precipitates that formed when solution A and solution B are mixed are and at 25 C. Assign these K sp values to the two precipitates based on their relative solubilities. 12. Write the Q sp and K sp expressions for both zinc carbonate and copper(ii) carbonate in solution. Be sure to indicate when initial concentrations are being used in the equation and when equilibrium concentrations are being used in the equation. Fractional Precipitation 3

4 Model 3 Comparing Q sp and K sp Volume of Solution B Added (ml) [CO 3 2 ] Initial (M) [Zn 2+ ] Initial (M) ZnCO 3 ppt? (Yes/No) ZnCO 3 Reaction Quotient [Cu 2+ ] Initial (M) CuCO 3 ppt? (Yes/No) CuCO 3 Reaction Quotient Calculate the initial concentration (before precipitation) of carbonate ions after the addition of each 0.05 ml of solution B to the 1.00 L beaker of solution A. Divide the work among group members and write the answers in the table in Model 3. Assume the volume change as solution B is added is negligible. 14. Notice the initial concentrations of Zn 2+ and Cu 2+ in the table in Model 3. a. Explain how these were obtained from the data in Model 2. b. As solution B is added and precipitates form, do these initial concentrations change? 15. Use the data in Model 2 to indicate the presence of precipitate (either ZnCO 3 or CuCO 3 ) after each 0.05 ml addition of solution B in Model 3. 4 POGIL Activities for AP* Chemistry

5 16. Use the initial concentrations of carbonate ions and zinc ions to calculate the reaction quotient, Q sp, for the zinc carbonate scenarios in Model 3. Divide the work among group members and write the answers in the table in Model Use the initial concentrations of carbonate ion and copper(ii) ions to calculate the reaction quotient, Q sp, for the copper(ii) carbonate scenarios in Model 3. Divide the work among group members and write the answers in the table in Model Refer to Question 11 for the K sp value for zinc carbonate and copper(ii) carbonate. When a precipitate forms in solution, how do Q sp and K sp compare? Use data from Model 3 to support your answer. 19. Consider the point at which zinc carbonate just begins to precipitate in Model 2. a. Use the K sp for zinc carbonate to calculate the concentration of carbonate ions required in the 1.00-L beaker of solution A for a precipitate to form. b. Calculate the volume of solution B that would need to be added to solution A to achieve the carbonate concentration calculated in part a. c. Is your answer in part b consistent with the graph in Model 2? Fractional Precipitation 5

6 20. Consider the point in the experiment when 0.20 ml of solution B has been added to solution A. a. Calculate the concentration of carbonate ions in the 1.00-L beaker of solution A at this point. Consider the volume of solution B negligible. b. Use the K sp of zinc carbonate to calculate the concentration of Zn 2+ ions remaining in solution at this point. c. Is your answer in part b consistent with the graph in Model 2? 21. Use the K sp for copper(ii) carbonate to calculate the volume of solution B that must be added to solution A to cause a precipitate. Check your answer against the graph in Model 2. Note: Look back at Question 19 for steps on how to solve this question. 22. Use the K sp of zinc carbonate to calculate the concentration of Zn 2+ ions remaining in solution when copper(ii) carbonate begins precipitating. Check your answer against the graph in Model 2. Hint: Look back at Question 20 for steps on how to solve this question. 6 POGIL Activities for AP* Chemistry

7 23. A solution of 2.00 M sodium sulfide is added drop-wise to a solution that is M in both cobalt(ii) ion and nickel(ii) ion. The K sp of cobalt(ii) sulfide and nickel(ii) sulfide are and , respectively, at 25 C. a. Which precipitate will form first when the sodium sulfide is added? Explain your reasoning. b. What concentration of sodium sulfide will be required for the first precipitate to form? c. What concentration of sodium sulfide will be required for the second precipitate to form? d. When the second precipitate begins to form, what is the concentration of metal ions that remains unprecipitated from the first precipitate? Fractional Precipitation 7

8 Extension Questions 24. A 1.00 M solution of lead(ii) nitrate is added drop-wise to 1.00 L of M sodium iodide and M sodium fluoride. The K sp values of lead(ii) iodide and lead(ii) fluoride are and , respectively, at 25 C. a. Write the solubility product equations for the precipitates that are expected to form. b. State the initial concentrations of I 1 and F 1 in the solution. c. Which precipitate will form first as lead(ii) nitrate is added? d. What volume of lead(ii) nitrate will be required for the first precipitate to form? (Assume the total change in solution volume is negligible.) e. What volume of lead(ii) nitrate will be required for the second precipitate to form? f. When the second precipitate begins to form, what is the concentration of halide ion that remains unprecipitated from the first precipitate? 25. A 0.85 M solution of silver nitrate is slowly added to a solution containing M chloride ions and M chromate ions. What is the concentration of chloride ions just as silver chromate begins to precipitate? The K sp values for silver chloride and silver chromate are and , respectively, at 25 C. 8 POGIL Activities for AP* Chemistry