What Do You Think? Investigate GOALS. Part A: Precipitation of Calcium

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1 H 2 Woes Activity 6 Water Softening GOALS In this activity you will: Investigate the equilibria behind water softening. Determine which reduces water hardness more, precipitation reactions or ion-exchange resins. Learn about any drawbacks connected with softening water. What Do You Think? Hard water can cause more problems than leaving a soap-scum ring around the bathtub. It can leave scaling inside water pipes. Hard water must be softened in order to avoid long-term system damage. What reaction can be used to remove the Ca 2 and Mg 2 ions from water? Show the chemistry and explain your reasoning. Can all of the Ca 2 and Mg 2 ions be removed? Explain your reasoning. Record your ideas about these questions in your log. Be prepared to discuss your responses with your small group and the class. Investigate Part A: Precipitation of Calcium Using the same water sample you analyzed in Activity 3, add sodium carbonate to remove calcium ions by precipitation of calcium carbonate. Since calcium carbonate is slightly soluble in water, you will have to account for this in your calculations and the assessment of the amount of calcium left in the solution. This activity is a good review of all of the chemistry you have applied thus far in the chapter, so it would be wise for you to look back in your log if you are unsure about how to make a calculation! 902

2 Activity 6 Water Softening Calculation/Observation total water hardness of your water sample (ppm) amount of Ca 2 (aq) in ml of your water sample (mol) amount of Na 2 CO 3 (s) needed to precipitate the Ca 2 (aq) in your sample (g) theoretical amount of CaCO 3 (s) formed (mol) amount of Ca 2 (aq) remaining in solution due to slight solubility of CaCO 3 (s) (mol) percent of Ca 2 (aq) remaining in solution theoretical amount of CaCO 3 (s) formed, accounting for the slight solubility of CaCO 3 (s)(g) actual amount of CaCO 3 (s) formed (g) results of soap test Result 1. You will use sodium carbonate to remove calcium ions by precipitating calcium carbonate. a) Write the reaction for the formation of calcium carbonate using sodium carbonate. Be sure to include the states of matter and properly balance the equation. 2. Copy the table above into your log. 3. Your teacher will provide you with a sample containing about 500 ppm dissolved calcium ions. Perform the following calculations and record your answers in the table in your log. a) How many moles of Ca 2 (aq) are in ml of your particular sample of water? b) Theoretically, how many grams of Na 2 CO 3 should be added so as to cause all of the calcium ions to precipitate into solid calcium carbonate if the above reaction goes to completion? c) How many moles of CaCO 3 would be formed? 4. The reaction to precipitate Ca 2 does not go to completion since CaCO 3 is slightly soluble. Use the solubility value for CaCO 3 (K sp at 25ºC) to perform the following calculations. Record your answers in the table in your log. a) Write the solubility expression for CaCO 3. b) Calculate the moles of Ca 2 when CaCO 3 (s) is in equilibrium with equimolar amounts of Ca 2 (aq) and CO 3 2 (aq). c) Calculate the percent of Ca 2 remaining in the solution after precipitation. (Hint: Use your amount from Step 3(a) as the original total.) d) Calculate the theoretical mass (g) of CaCO 3 (s) formed, taking into account the slight solubility of CaCO 3 (s). 5. Using the batch method, treat the ml sample of water with the quantity of Na 2 CO 3 (s) that you calculated is necessary to precipitate CaCO 3 according to the initial reaction stoichiometry. Safety goggles and a lab apron must be worn at all times in a chemistry lab. Mix chemicals only as directed by your teacher. 903

3 H 2 Woes Na + Na + Na + Na + Na + Na + Na + water containing Ca 2+ and/or Fe 3+ Ca 2+ Fe 3+ Na + Na +? 5Na + Dispose of the materials as directed by your teacher. Clean up your workstation. Wash your hands and arms thoroughly after the activity. 6. Find the mass of the batch precipitate. a) Measure the mass of a dry piece of filter paper and record this value in your log. Filter according to your teacher s directions. Dry the precipitate on the filter paper as directed by your teacher. b) Calculate the mass of your CaCO 3 (s) precipitate. Record this value in the table in your log. 7. Test the filtrate qualitatively for water hardness by adding some soap to it and observing whether the solution is cloudy. a) Record your results in the table in your log. Part B: Ion-Exchange Resins 1. Choose either the batch or flow technique that you used in Activity Create a table in your Active Chemistry log to collect all of the necessary data. In addition to the data collected for the particular method in Activity 5, you will want to include the amount of resin used and have a column for the analysis of the filtered water Load the mixing flask or filtration column with the recorded amount of acidic anion ion-exchange resin. An acidic ion-exchange resin is a polymeric material that has negative sites on its surface. These negative sites attract and absorb positive ions such as Ca 2 and Mg 2, removing them from water. As the water sample comes into contact with the resin beads, the Ca 2 and Mg 2 ions are attracted to the negative sites, replacing the Na ions originally on each bead. 4. Process a sample of untreated water with the ion-exchange resin. 5. Analyze your filtered solution for total water hardness. For a quantitative analysis, follow the experimental design in Activity 3 for the analysis of total water hardness (Investigate, Part A). For a qualitative analysis, add some soap to a portion of your water sample before and after the treatment with ion-exchange resin. Shake the solution to see if it becomes cloudy. a) Record the results in the table in your log.

4 Activity 6 Water Softening TREATING HARD WATER Water Hardness As you learned in Activity 1, natural waters can differ a great deal in the amounts and types of minerals they contain. The most common positive ions are Ca 2 and Mg 2, and they are responsible for making water hard. They are not dangerous to consume in the usual quantities contained by natural waters. However, they lower the quality of the water and may create expensive complications when present in elevated amounts. They cause the formation of scale. This gradually builds up and closes the inside of iron water pipes. Scale is caused by calcium and magnesium ions when they precipitate as CaCO 3 and Mg(OH) 2. The soap scum that appears in bathtubs and sinks can be traced to hard water as well. Soaps are salts of long-chain molecules called fatty acids. The non-polar end of the soap molecule allows greasy molecules to dissolve. Ca 2 and Mg 2 bond strongly to the carboxylate ions ( COO ) of soaps to form insoluble residues. Considerably more soap is needed to wash laundry with hard water than soft. Much of the residue remains in fabrics after rinsing and prevents thorough cleaning. Many detergents, on the other hand, can work better in hard water than soaps because they contain phosphate ions that can attach to Ca 2 and Mg 2 ions without forming an insoluble residue. Sodium salts of alkylbenzenesulfonic acids are also widely used as detergents, because like the phosphate ions, they are able to form soluble salts with the Ca 2 and Mg 2 ions. Treating Water Hardness: Precipitation Water-treatment plants typically use a mixture of soda ash, Na 2 CO 3, and slaked lime, Ca(OH) 2, to remove the Ca 2 and Mg 2 ions. As you observed in your investigation, Na 2 CO 3 causes the precipitation of calcium carbonate, Ca(CO 3 ). Slaked lime, Ca(OH) 2 causes the precipitation of magnesium hydroxide, Mg(OH) 2. This is commonly 905

5 H 2 Woes Chem Words soda-lime process: a process adopted on a large scale in water-treatment plants for removing Ca 2 and Mg 2 by the addition of Na 2 CO 3 (soda) and Ca(OH) 2 (lime). ion-exchange resin: a polymeric material that has negative sites on its surface where various positive ions are adsorbed. Positive ions of higher charge are more strongly adsorbed, but still may be replaced by ions of lower charge having a sufficiently high concentration. referred to as the soda-lime treatment, a chemical treatment based on a location-specific balance of the following chemical reactions. Na 2 CO 3 (s) Ca 2 (aq) 2Na (aq) CaCO 3 (s) Mg 2 (aq) 2OH (aq) Mg(OH) 2 (s) While the chemistry behind the soda-lime treatment is complicated, it is a preferred method. It is convenient to use with a settling basin followed by filtration, and the chemicals are cheap. Soda ash is currently about $1.20 per kilogram and slaked lime is about $0.50 per kilogram. In this activity, you encountered some of the drawbacks of the sodalime treatment. The Ca 2 cannot be completely removed because precipitation is not complete. At equilibrium, some calcium ions remain dissolved. This is true for the Mg 2 ions as well. The equilibrium concentrations of these ions are determined by the solubility products of the precipitates and the concentrations of added CO 2 3 and OH. It is unwise to add too much slaked lime, because it adds to the Ca 2 removal problem. The addition of Na 2 CO 3 is limited by the health hazard of a high concentration of Na, which can lead to high blood pressure in some individuals. Hence, water softening is not complete in watertreatment plants and may not be performed at all in some areas. In those places, homeowners may choose to soften water that is to be used for cleaning by installing commercial water softeners. Treating Water Hardness: Ion-Exchange Resin Water softening by ion-exchange resins is based on a very different principle. Instead of being removed in precipitates, the positive ions are attached to a polymerby being attracted to negatively charged sites on the surface of the polymer. The polymer cannot be prepared with a net negative charge, but rather starts with Na ions adhering to the negative sites. Since the Ca 2 and Mg 2 are more strongly attracted to the negative sites than the Na, they readily replace the Na. The net effect is the same as in the soda-lime treatment. The stream of water ends up with Na ions instead of Ca 2 and Mg 2. However, instead of separating a precipitatefrom the stream of water, it is necessary to reload the ionexchange resin with Na ions by temporarily exposing it to a high concentration of sodium chloride solution. 906

6 Activity 6 Water Softening During the reloading process, dissolved CaCl 2 and MgCl 2 are carried away and the Na adheres to the polymer. As you may have noticed in your investigation, water softened by this technique can have considerably lower Ca 2 and Mg 2 concentrations than is possible with the soda-lime process. The drawbacks to ion-exchange resins were not as easily observed. In a water softener s flush cycle, Na is washed from the resin down a drain. Some may remain and be carried into the home s water lines. Therefore, some homeowners choose to have lines used for consuming water bypass the softener. In addition, at about $3.00 per kilogram, ion-exchange resins are more expensive than soda and lime. However, they are not consumed in the full treatment and reloading cycle and can be reused for years. Checking Up 1. What causes hard water? 2. What are the disadvantages of hard water? 3. Why is the soda-lime treatment of hard water a preferred method? 4. How do ion-exchange resins work to remove Ca 2 and Mg 2 ions from hard water? 5. What are the drawbacks of ion-exchange resins? What Do You Think Now? At the beginning of this activity you were asked: What reaction can be used to remove the Ca 2 and Mg 2 ions from water? Show the chemistry and explain your reasoning. Can all of the Ca 2 and Mg 2 ions be removed? Explain your reasoning. Were any of the reactions you predicted in the What Do You Think? section part of this activity? You were asked if you could completely remove the ions causing water hardness. How would your answer to this question change after performing the two techniques? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC When sodium carbonate solution was added to hard water, what evidence did you see that confirmed that the hard water contained calcium ions? The surface of the resinexchange column is saturated with sodium ions. Explain what is taking place when the hard water that contains calcium ions comes in contact with the sodium-charged resin-exchange column. Na 2 CO 3 (s) Ca 2 (aq) 2Na (aq) CaCO 3 (s) and CaCO 3 (s) Ca 2 (aq) CO 3 2 (aq) From the equations that are shown, explain why you cannot totally remove all of the calcium ions. 907

7 H 2 Woes How do you know? How did you see that the water had been softened? Explain. Why do you believe? Why are water softeners so popular in the homes of people living in the Midwestern states like Ohio? Why should you care? Is it important to remove the water hardness from your water sample? Explain. Reflecting on the Activity and the Challenge In this activity, you have tried procedures that can be used to soften water. Softening water does not make water any safer to consume. It does make water a better cleaning solvent. It may end up saving a lot of money by reducing soap purchases and avoiding the need to replace appliances and supply lines. In designing water treatment procedures for different communities, you should decide whether a water-softening step is essential. Consider whether attempts to remove other contaminants will be as successful if they have not been preceded by water softening. 1. In Part A of this investigation did the treatment with Na 2 CO 3 noticeably reduce the water hardness? What was your evidence? 2. Suppose that you use CaCl 2 to precipitate F in a water sample. a) Write the reaction as if it would go to completion. b) How many moles of CaCl 2 should you add to 10.0 ml of water containing 12 mg/l of F (aq) if the reaction goes to completion? Show all of your work. c) When the resulting precipitate comes to equilibrium, not all of the F will be removed from solution. Write this equation. d) Find the equilibrium concentration of F using the K sp for CaF 2 ( ). Show all of your work. 3. If twice the stoichiometric amount of Na 2 CO 3 calculated in Part A is added to a ml sample of water, will more CaCO 3 be formed than before? If so, how much more will form? Show the work behind your answers. 908

8 Activity 6 Water Softening 4. Why is Ca(OH) 2 added to water as part of the overall softening process even though it contains the hard-water Ca 2 ions? 5. Why are Ca 2 and Mg 2 ions more strongly attracted than Na ions to negative sites in ion-exchange resins? 6. What is the purpose of periodically adding highly concentrated sodium chloride solutions to ion-exchange resins? 7. Nitrate is commonly removed from water through the use of ion-exchange resins. a) Propose how you think these resins would work on the nanoscale. b) How would you be able to test to see if the ion-exchange resin you proposed would be effective in removing nitrate from your water supply? 8. Preparing for the Chapter Challenge Continue your diagram for the water purification from the previous activity. Decide if water softening is necessary for your particular region. If so, decide whether you will use ion-exchange resins or precipitation. Be prepared to justify your decision. Inquiring Further 1. Magnesium removal Removing magnesium from a water sample involves the consideration of additional chemical equilibria, including the bicarbonate equilibrium. First, research ways in which this precipitation occurs in water purification systems. Under the guidance of your teacher, apply your methods to your water sample and record your results. Write a report for your class on the complex chemistry behind these steps and your results in purifying your water sample. Do this activity only under adult supervision. 2. Nitrate removal Nitrate is removed from water sources through two common methods: ion-exchange resins and biological treatments. Research each of these methods as they relate to removing nitrate from the water supply. Present your findings to your class, including the advantages and drawbacks of each method. 909

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