Identification of an Unknown Compound through Mass Correlations

Transcription

1 EXPERIMENT Identification of an Unknown Compound through Mass Correlations PURPOSE To carry out a series of decomposition reactions for five different unknown, and use stoichiometry in order to identify each one of them through mass correlations calculations. To confirmed your result analytically using the Flame Test method. MATERIALS AND EQUIPMENT NaHCO 3, Na 2 CO 3, KHCO 3, K 2 CO 3, (NH 4 ) 2 CO 3, 6 M HCl, 0.1 M NaCl and 0.1 M KCl. Crucible and lid, iron ring, ring stand, clay triangle, Bunsen burner, nichrome wire, and analytical balance. BACKGROUND When chemical reactions occur, there is a stoichiometric relationship between the masses of the reactants and products that follow directly from the balanced equation for the reaction and the molar masses of the species that are involved. In this experiment we will use this relationship to identify an unknown substance. For selected ions we can confirm the result by the Flame Test. The flame test is used to visually determine the identity of an unknown metal ion based on the characteristic color that the metal turns the flame of a Bunsen burner. The heat of the flame excites the electrons of the metals ions, causing them to emit visible light (E=hc/λ). Every element has a signature emission spectrum (fingerprint) that can be used to differentiate between one element and another. Your unknown will be one of the following compounds, all of which are salts: NaHCO 3 (s) Na 2 CO 3 (s) KHCO 3 (s) K 2 CO 3 (s) (NH 4 ) 2 CO 3(s) In the first part of the experiment you will be heating a massed sample of your compound in a crucible. There is a distinct difference in behavior of these carbonates in decomposition reactions. Therefore the following will occur depending on the identity of your unknown: 1) If your sample is ammonium carbonate, heating will decompose it to NH 3 (g) and HCl (g). Therefore nothing will be left in the crucible after decomposition. (NH 4 ) 2 CO 3(s) 2NH 3 (g) + H 2 O (g) + CO 2 (g) Page 1

2 2) If your sample is any other carbonate, there will be no chemical reaction that occurs, but any small amount of adsorbed water (moisture) will be driven off upon heating in the first part of the experiment. Therefore there may a slight mass change. In order to differentiate between two possible carbonates; sodium and potassium, you need to continue with second part of the experiment (addition of HCl and heating) to form the corresponding chloride salt and use mass correlations to establish the identity of the unknown. 3) If your sample is a hydrogen carbonate, it will decompose to form a mixture of gases and salt by the following reactions (pattern I): 2 NaHCO 3 (s) Na 2 CO 3 (s) + H 2 O (g) + CO 2 (g) 2 KHCO 3 (s) K 2 CO 3 (s) + H 2 O (g) + CO 2 (g) In these cases there will an appreciable decrease in mass, since some of the products will be driven off as gases. Therefore if such a mass decrease occurs, you can be sure that your sample is a hydrogen carbonate. In the second part of the experiment, we treat the solid carbonate in the crucible with concentrated hydrochloric acid. There will be considerable fizziness as CO 2 gas is evolved; the reactions that occur are (pattern II): Na 2 CO 3 (s) + 2 HCl (aq) 2 NaCl(s) + H 2 O (l) + CO 2 (g) K 2 CO 3 (s) + 2 HCl (aq) 2 KCl(s) + H 2 O (l) + CO 2 (g) We then heat the crucible strongly to drive off any excess HCl and any water that is present, obtaining pure, dry, solid NaCl or KCl as our product. To identify your unknown, you will need to find the molar masses of the possible reactants and final products. For each of the possible unknowns there will be a different relationship between the mass of the original sample and the mass of the corresponding chloride salt that is produced in the second reactions. If you know your sample is a carbonate, you need only to be concerned with the mass relationships in the reactions involved in the second part of the experiment (pattern II), and should NOT use the original mass of your unknown. Instead you should use the mass of the dry carbonate after it has been heated. On the other hand, if you have a hydrogen carbonate, you should consider the overall reactions in both parts of experiment (pattern I& II), because your sample undergoes both reactions. From your experimental data you will be able to calculate the ratio of the mass of the solid chloride to the mass of either the original hydrogen carbonate or the mass of the anhydrous carbonate in your sample of unknown. From your calculation of the relative masses of solid chloride to solid hydrogen carbonate or solid carbonate in the two corresponding equations (pattern I& II) you can calculate what the theoretical ratio of those masses should be. Your observed value should match one of the theoretical values and thus allow you to identify the compound your unknown contains. SAFETY Hydrochloric acid is corrosive. Several of the chemicals are toxic. Be cautious with the handling of 6M HCl acid. If you spill these reagents on yourself, rinse the affected area with water. Wear gloves and goggles. Page 2

3 PROCEDURE 1. Mass and record a clean and dry crucible with lid to the nearest milligram using an analytical balance. YOU MUST USE ANALYTICAL BALANCE FOR THIS EXPERIMENT! 2. Obtain an unknown compound. The unknown compound may be NaHCO 3, Na 2 CO 3, KHCO 3, K 2 CO 3, or (NH 4 ) 2 CO Mass and record about 0.5 g of the unknown compound directly into the crucible. 4. Secure an iron ring on a ring stand, but make sure to leave enough space underneath the iron ring for a Bunsen burner. 5. Place a clay triangle on top of the iron ring and then place the crucible containing the sample on the clay triangle. Make sure the lid is placed partly open on the crucible. 7. Place a Bunsen burner underneath the iron ring and gently heat the sample for 6 minutes. 8. Increase the intensity of the flame and heat for an additional 8 minutes. The high intensity flame should cause the bottom of the crucible to glow reddish in color. 9. Allow the crucible to cool to room temperature (~ 12 minutes). 10. When the crucible is no longer hot, weigh and record the mass of the crucible with lid and sample. 11. Repeat the heating and cooling process until constant mass is reached. (Note: If the sample is sodium bicarbonate or potassium bicarbonate, it would decompose into sodium or potassium carbonate respectively, water, and carbon dioxide and you should continue with step 12. However if the sample is ammonium carbonate, it will decompose into ammonia gas and hydrogen chloride gas and it does not require any further experiment to determine its identity since there will nothing left in crucible. 12. Slowly add 6 M HCl, one drop at a time, to the sample inside the crucible. (Note: both sodium carbonate and potassium carbonate react with HCl to produce corresponding chloride salt). 13. Make sure when you add each drop to swirl the mixture until there is no appearance of a chemical reaction before adding another drop. 14. Continue adding drops and swirl until all the solids dissolves, but do not exceed a total of 30 drops. 15. Gently heat the crucible with the lid very partly open for about 12 minutes. It is important that the flame is set at a very low intensity to prevent any HCl excess from boiling out of the crucible. 16. After 12 minutes, turn off the flames and check to see if the sample appears dry. 17. Continue to gently heat if the sample is not dry. If the sample appears dry, set the flame at a high intensity and heat for an additional 12 minutes. The high intensity flame should cause the bottom of the crucible to glow reddish in color. 18. Allow the crucible to cool for at least 12 minutes. 19. When the crucible is no longer hot, mass and records the crucible with lid and sample. 20. Repeat the heating and cooling process until constant mass is reached. 21. Dispose of any residues in a waste container, wash and dry the crucibles. 22. Using the masses data obtained, determine the unknown compound. Page 3

4 Flame Test: 1. First, you need to clean the nickel-chromium (aka nichrome) wire loop by dipping in 6M hydrochloric followed by heating to a bright glow in the Bunsen burner. Make sure to flame the nichrome wire at the top of the blue inner cone (hottest part) of the flame. End the cleaning with rinsing the wire loop with distilled or deionized water. 2. Transfer small quantity (~10 drops) of each known solution (0.1 M NaCl and 0.1 M KCl) to a small spot plate pre-labeled. 3. Dip the clean loop in one of the solution in the spot plate and then preform the flame test by inserting the wire loop into the outer cone of a Bunsen burner. 4. After cleaning the wire loop with acid again repeat the flame test for another known solution. The wire loop must be cleaned between tests for different ions. 5. Write down your observation and indicate the color of each solution. 6. Perform the flame test for each of your unknown solid by dissolving about 0.02g of your solid in 1ml distilled water in a small test tube and then transferring adequate amount to the small spot plate. Write down your observations. Questions 1- If 2.0 grams of sodium bicarbonate was added into a crucible and heated with the lid partly open to constant mass, how many grams of product will remain? 2- In the experiment, what would happen to the mass data if not enough hydrochloric acid is added to dissolve the solid? Page 4

5 Unknown Heat Ammonium Carbonate (Nothing will be left) Carbonate (Slight mass change) Hydrogen Carbonate (Noticeable mass change) HCl & Heat to constant mass HCl & Heat to constant mass Figure 1: Flow Chart to identify the unknown FORMS NaCl or KCl Use mass Correlations & Flame Test To identify the Unknown FORMS NaCl or KCl Use mass Correlations & Flame Test To identify the Unknown Page 5

6 LAB REPORT Unknown Mass correlations Name Section Date Unknown # 1 Unknown # 2 Unknown # 3 Unknown # 4 Unknown # 5 Page 6

7 LAB REPORT Unknown Mass correlations Name Section Date Show your calculations including related reactions for each unknown: Unknown # 1 Unknown # 2 Unknown # 3 Unknown # 4 Unknown # 5 Page 7