Observing Chemical Reactions

Observing Chemical Reactions INTRODUCTION Observing Chemical Reactions INTRODUCTION An integral part of any experimental science is observing how the world behaves and drawing conclusions from the observed behavior. In this laboratory exercise you will mix chemicals and make observations about the resulting solutions. Your observations of these attempts will tell you whether or not the reactions actually occur, and from this data you will be able to plan a procedure for identifying and separating the salts in an unknown. How do you know that mixing two chemicals results in a chemical reaction? Look for as many physical indications as possible. Does the color of the solution change? Does it heat up? Does it cool down? Is gas evolved? Use all of your senses- except smell and taste; remember, don't smell or eat the chemicals! It cannot be emphasized enough that making good observations, and writing them down, is critical to successful investigations in science. Think about how often you have said to yourself, "I'll remember the phone number until I get home," and then promptly forgotten it. It is much easier to forget something you have noted about a new chemical reaction, especially something you did not realize was significant at the time, than something you considered important in the first place. If you note a change or a lack of change, write it down! After determining which chemicals react, you will need to develop a scheme for the separation of a mixture of salts. Check with you TA to ensure that your scheme will work. Once you have an acceptable scheme, you will identify which two of the salts you have worked with are in an unknown solution. In preparation for the Volumetric experiment, each pair must obtain about 3.0 grams of primary-standard grade potassium acid phthalate, KHP, in a vial and dry it in an oven at 110o C for 2 hours. Place the vial of KHP in a small, beaker to keep it from spilling. Label your beaker using a graphite pencil in the white frosted area. Cover the beaker with a watch glass and place it in the oven. After two hours, remove your beaker from the oven. While keeping the watch glass on top of the beaker, let it cool until it is warm but safe to handle. Remove the watch glass and place the beaker containing the uncapped vial in a desiccator. Observing Chemical Reactions PROCEDURE Warning Use care when handling acids and bases. Silver nitrate will stain clothing and skin; wear gloves. Dispose of solutions containing silver in the waste jars in the hood. Wear your goggles. PROCEDURE You will work in pairs on this experiment. Each student will submit a separate postlaboratory report. Stock Chemicals Used Chemical Maximum Amount Used 0.1M Magnesium Nitrate (aq) 5 mL 0.1M Strontium Nitrate (aq) 5 mL 0.1M Aluminum Nitrate (aq) 5 mL 0.1M Silver Nitrate 5 mL 6M Hydrochloric Acid <5 mL 6M Nitric Acid <5 mL 3M Sulfuric Acid <5 mL 6M Sodium Hydroxide <5 mL 6M Ammonium Hydroxide <5 mL Potassium Hydrogen Phthalate 3g Part I. Reactions of the Metal Salt Solutions (KHP) In this part of the experiment you will experiment with four metal salts and five other reagents. The metal salts you will use are silver nitrate, strontium nitrate, magnesium nitrate and aluminum nitrate. The other reagents you will work with are 6 M hydrochloric acid, 3 M sulfuric acid, 6 M ammonium hydroxide, 6 M sodium hydroxide and 6 M nitric acid. The salt solutions will be found on the chemical shelves, and the acids and bases will be found on the trays by your lab station. If the reagent present in the laboratory is more concentrated than this, dilute it. React each reagent with the individual metal salts 1. Using a disposable transfer pipet, transfer approximately 1 mL of a 0.1M metal salt solution to a clean test tube. 2. Using the dedicated transfer pipet attached to the reagent bottles, slowly add a couple of drops of one of the reagents to one of the metal salts. Record your observations. 3. Add a couple more drops and record your observations. Continue until you are sure that you have added an excess of the reagent. It will not take more than 0.5 mL to reach an excess of the reagents. Observing Chemical Reactions PROCEDURE 4. There are twenty possible combinations of salts with reagents. Try them all. You may also want to see what happens if you add more than one reagent to the salt solution. As you work on this portion of the experiment, compare your results with your neighbors. If you seem to get disparate results, talk about why your results are different. Did one of you make an error, or are you just going about things differently? Question A: In performing these reactions and any required dilutions, should you use tap water or deionized water? Why? Question B: For each observed reaction between a reagent and a metal salt write a balanced chemical equation that shows what is occurring. Part II. Analyzing an Unknown In this part of the experiment you will use the data you have accumulated to tell which of the salts are present in an unknown. Your unknown contains two of the four metal salts you have worked with. Develop a procedure that will distinguish between these four compounds and use it to identify the composition of your unknown. You may need to be able to separate a solid precipitate from a solution. Instructions for separation follow. 1. Transfer the solution and the precipitate to a centrifuge test tube. Fill a second test tube with water until the volumes in the two test tubes are approximately the same. 2. Place the two test tubes in the centrifuge. The test tubes should be placed opposite each other so that their weight is balanced as the centrifuge spins. As there are likely to be more people using the centrifuge, make sure that your test tubes are labeled so that you can identify them when the centrifuge stops. 3. Turn on the centrifuge and allow it to spin for a minute. When the centrifuge stops spinning remove your test tube carefully- you do not want to disturb the solid at the bottom of the tube. 4. If you are trying to completely remove a salt from a solution, add a little more of the reagent that caused the precipitation. If more solid forms in the solution, recentrifuge and repeat this step. 5. Decant the supernatant solution from the solid and reserve. Avoid disturbing the precipitate when pouring off the solution. 6. Add a few milliliters of deionized water to the precipitate and stir. This washes any excess reagent away from the precipitate. Recentrifuge. Decant the supernatant and combine with the reserved solution from Step 5. You have now completed the separation of your precipitate from solution. Separate tests may be performed (as needed) on the precipitate or the supernatant solution. Question C: Why do you test for complete precipitation in Step 4 if you are going to do any further chemical tests on the supernatant? Observing Chemical Reactions DATA ANALYSIS Clean up: Pour any solutions or solids containing silver or aluminum into the Cation Metal Waste jar in the fume hood. Pour any other solutions down the drain DATA ANALYSIS Why must you use de-ionized water rather than tap water in performing the reactions and any required dilutions? Which metal salts formed a precipitate when a few drops of 6M hydrochloric acid were added? What is the correct chemical equation for the reaction of strontium nitrate with sulfuric acid? What chemical equation represents what occurs when ammonium hydroxide is added to a solution of silver nitrate? What chemical equation properly describes the reaction of magnesium nitrate solution with added sodium hydroxide? What chemical equation properly describes the reaction of aluminum nitrate solution with added ammonium hydroxide? Why is it necessary to test for complete precipitation if a supernatant solution is going to be subsequently tested for ions? What is the letter of the unknown solution that you analyzed? What metal salt solutions have you identified in your unknown solution? Provide a cohesive summary of the reactions you undertook and the observations you made in this laboratory session. If you did not obtain a perfect score on your unknown, explain what error in procedure or observation you might have made that resulted in a false conclusion.