AVC Chem 101 Testing for Cations and Anions Part 01 2023 08 28-2

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Chemistry

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Dec 6, 2023

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Testing for Cations and Anions(Cations: Part 1) Name: Lab Partner: Section: Goals: > Observe reactions of cations with other chemical reagents. > Identify the presence of a cation by its reaction with certain reagents. > Determine the identity of an unknown cation in a dissolved salt solution by testing for its cation using particular chemical reagents. Pre-Lab Questions : Q1 . If there are cations in a solution, should there be any anions present as well? Why or why not? Q2 . What is the name of the cation that would be produced from the element in the third period in group one in the periodic table? What is its charge? Why does it have that particular charge? Q3 . Why do you think many of the transition elements have oxidation numbers of only one or two? Is there anything you can see in their position on the periodic table or their electronic structure? (Hint: electron configuration): Q4 . Where does the name “cation” come from? Can you think of anything in normal everyday life that has a similar or related name? Q5 Are any of these cations found in your home or work place? What are they used for? Is there any hazards or dangers associated with them? Q6 Are any of these cations found in the human body? What are they there for? What are health problems with too much or not enough of these cations in the body’s systems?

Hazards : HCl and HNO 3 are strong acids and are corrosive. NaOH is a strong base and is corrosive. AgNO 3 is toxic, and stains skin and clothing. Be aware of the hazards associated with some of the ions and reagents. Information may be found in the Material Safety Data Sheet (SDS) book located in the shelf in the lab. Flames may cause burns. KSCN is toxic. FeCl 3 may be an irritant. Some chemicals may burn or stain skin, clothing or papers/ notebooks. Chemicals used : 0.1M SrCl 2 , 1M KCl, and 0.5M CaCl 2 , 0.1M SrCl 2 and 0.1M NaCl in dropper bottles, 3M HCl, 6M HNO 3 , 6M NaOH, 0.1M NH 4 Cl, 0.1M FeCl 3 , 0.1M KSCN, 1M CuSO 4 . Concepts : Some useful concepts to review in preparation for this lab include: the octet rule, the periodic table, and the periodicity of the elements. Cations are found in salts or ionic substances. Cations are positively charged ions resulting from the loss of one or more electrons. Analytical chemists are concerned with what chemicals are present, how they are put together and how much. Why would we care what’s there or how much of a substance is there? Can you think of any examples? Here is an example: Selenium is a trace element. We need a small amount in our diet. Too much of it can be toxic. Sometimes elements like selenium or arsenic are present in ground water. Too much of almost anything can be hazardous. The folks at the water district are required by law to test the water that they send to our houses periodically. Wouldn’t you like to be sure that there is not too much lead or calcium or any other element in your drinking water? What are some ways that could be used to test for the presence of certain cations? We can use electronic transitions. We can use precipitations. We can make observations. If you do something to a solution containing a certain cation, it will react or respond in a certain way, depending on what conditions or reagents that you subject it to. We will try several reactions on a series of known solutions and on one unknown solution and one or more household product(s). From the results, you should know whether certain cations are present or not. Procedure: Flame tests for K + , Sr 2+ , Na + and Cu 2+ and Ca 2+ ions: The presence of certain elements can be seen by the distinctive color they give off when their electrons are excited. Flame can be used as an energy source to excite those electrons so that in a flame you can see certain colors if certain elements are present. K + , Sr 2+ Na + , Cu 2+ and Ca 2+ happen to be the ions of elements that give off color in a flame. What do you think it means when electrons are excited then give off light or energy? (Hint: Think of the energy levels in electron configurations.) To test for the presence of certain cations, you can use a flame test. Take a nichrome wire loop and clean it in a Bunsen burner flame. The flame should be good and hot (your instructor should demonstrate the proper technique for igniting and adjusting a Bunsen burner flame). Once your loop is good and clean (or as clean as you can get it in a reasonable amount of time)

put a drop of a known solution on the loop. The first known is 0.1M SrCl 2 . Take the loop with the drop of SrCl 2 solution and place it in the hottest part of the flame. You should see a distinctive color indicating the presence of strontium(Sr 2+ ). Clean your loop again and repeat the test with 0.5M KCl solution and see if you see the distinctive color of potassium(K+). Repeat the cleaning and testing with a 0.5M CaCl 2 solution. Perhaps your instructor will have another ion that you can test in the flame that gives a distinctive color, such as Na + , Li + or Cu 2+ . Record the colors that you see with your known solutions in the corresponding spaces in your table. Now clean your loop and test your unknown solution in the flame. Record what you see in the space provided in the table. Compare your observation of your known solutions with your unknown solution. Table 1: Tests for the Positive Ions(flame test) : Unknown number: Procedure Cation Observations Tested A.1 Flame Test Sr 2+ K + Ca 2+ Na + Cu 2+ Unknown Q7 Does the result from your unknown solution look the same as the result or observation from one of your known cations? Q8 If so, which one? If not, is it safe to conclude that our unknown cation must be some other cation? Explain. Test for the ammonium ion : Take 1ml of a 0.1M NH 4 Cl solution and put it in a test and 1mL of your unknown in a second test tube. Add several drops of 3-6M NaOH to both test tubes. Place the test tubes in a warm water bath for a few minutes. “Waft” the air over the test tubes and see if there is an odor of ammonia. Take 2 pieces of red litmus paper and put a drop of DI water on the bottom of each piece of litmus paper and suspend a piece of wet litmus paper just inside the mouth of each test tube

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without touching the litmus paper on the glass nor residual drops of liquid which may be on the glass. Record what you observe. Be careful not to touch the walls of the test tubes with the litmus paper. Do the pieces of litmus paper remain pink or turn blue? Record your observations in the appropriate place in table 2 for the ammonium ion. Table 2: Tests for the Positive Ions(NH 4 + ) : Unknown number: Procedure Cation Observations for known Observations for Unknown Tested A.2 NH 4 + test NH 4 + Q9 Did you smell any ammonia? Why or why not? Test for the Fe 3+ ion : Take 1mL of 0.1M FeCl 3 solution and place it in a test tube and 1mL of your unknown in a second test tube. Place 3-5 drops of 3-6M HNO 3 in each of the 2 test tubes. Add 1-3 drops of 0.10M KSCN solution to the two test tubes. Record your observations. Dispose of all chemicals as instructed by your instructor (the heavy metals waste container) or use your vast chemical knowledge and do the right thing. Table 3: Test for Fe 3+ Procedure Cation Observations Is Fe 3+ Present? Tested A.3 Iron(III) Test Fe 3+ _______________ Unknown _______________

Post Lab Questions: Q10 . How can you tell if a particular cation is present in a solution? What are some things you can do to show that a certain cation is or is not present? Q11 . You have a solution that is a chloride salt. You think that the cation is either sodium, potassium, calcium or strontium. What is something you could do to show you what the cation is? Q12 . What can you say about your water if you add a few drops of KSCN to your tap water and it turns red? Q13 . Write the symbol for the cation that gives a bright red light in the Bunsen burner flame. Q14 . Why do cations have a positive charge. What causes them to give up or lose electrons? Q15 . Write a summary of your experiments (not a procedure). Generalize what you have seen and give some conclusions about the experiment/lab.