Chemical Reactions: Making Table Salt

Transcription

1 Chem 100 Section Experiment 5 Name Partner s Name Chemical Reactions: Making Table Salt Introduction During chemical reactions, substances combine with each other in a definite proportion by mass, meaning that only a certain amount of one reactant will react with a given amount of another reactant. The amounts of reactant species can be expressed in a variety of ways: grams, pounds, tons, and liters. However, no matter what units are used, they are all related to the ratio of moles of one species reacting with a certain number of moles of another species. If you are unclear about the concept and definition of a chemical mole, you should review the discussion of moles in Chapter 3 of Chemistry in Context - 5 th Ed. In this experiment, you will investigate in more detail a reaction you have already used in Experiments 1 and 4, i.e., the reaction of sodium bicarbonate, NaHCO 3, with acid (vinegar, HC 2 H 3 O 2 or hydrochloric acid, HCl). Sodium bicarbonate is also known as sodium hydrogen carbonate, but it is most familiar as baking soda. The reaction of sodium bicarbonate with hydrochloric acid produces salt (sodium chloride, NaCl), and water, and carbon dioxide. NaHCO 3 + HCl NaCl + H 2 O + CO 2 In Experiment 1, you used this reaction to make carbon dioxide in a plastic bag. In Experiment 4 you prepared CO 2 in a microscale gas syringe. Now you will do quantitative measurements to find out how many moles of salt (NaCl) are formed from 1 mole of NaHCO 3. You cannot measure moles directly, but you will determine the mass of NaHCO 3 used and the mass of NaCl formed by weighing samples on a balance. To further improve the accuracy of the experiment, data will be assembled from the whole class, and you will be asked to decide on a "best" value for the ratio. After completing this study with sodium bicarbonate, you will try a similar study with sodium carbonate (Na 2 CO 3 ) to see how the results may or may not differ. The experimental procedure is the same and the calculations are very similar. When completed, you should be able to propose a balanced chemical equation for the reaction of sodium carbonate with hydrochloric acid. Overview of the Experiment 1. Label and weigh six clean, dry, large test tubes. 2. Add a weighed quantity of sodium bicarbonate or sodium carbonate to each test tube. 3. React the sodium bicarbonate or sodium carbonate with 6 M hydrochloric acid. 4. Evaporate the liquid remaining in each test tube after the reaction has taken place. 5. Determine the weight of sodium chloride produced. 6. Calculate the ratio of moles of NaCl formed to moles of NaHCO 3 or Na 2 CO 3 used. 5-1

2 Materials Needed Chemicals Sodium bicarbonate, NaHCO 3 Sodium carbonate, Na 2 CO 3 6 M hydrochloric acid, HCl Equipment 6 test tubes test-tube holder Fischer or Bunsen burner SAFETY NOTES 6 M HCl is corrosive to the skin and other materials. Avoid spilling it on yourself, your partner, or your work space. Be extremely cautious about flammable materials near an open flame. Be alert to the possibility of hot solution accidentally splashing out of a test tube if it is heated too rapidly. Experimental Procedure: 1. Obtain three test tubes that are clean and completely dry. Label them A, through F, putting the labels near the tops of the tubes. Add a small boiling chip to each test tube. 2. Take the test tubes and data tables to one of the laboratory balances. Weigh each of the test tubes (including the boiling chip) to the nearest ten thousandth of a gram (0.0001g) and record the masses on the appropriate lines in the data tables. (If the last decimal place is a zero, be sure to record it, e.g., g) 3. To test tubes labeled A, B and C add just enough NaHCO 3 to fill the curved bottom of the tube. To tubes labeled D, E and F add a similar quantity of Na 2 CO Again, weigh each test tube to the nearest ten thousandth of a gram ( g) and record its mass (tube + contents) in the data table. Note: The masses of the three solid samples of NaHCO 3 or Na 2 CO 3 do not need to be identical. Typical masses of added NaHCO 3 or Na 2 CO 3 be about 0.3 to 0.7g. 5. Fill a plastic pipet with 6 M hydrochloric acid solution. Add this acid dropwise to tube A. Let the liquid run down the wall of the test tube and gently "spank" the tube after each drop reaches the bottom. Continue to add acid slowly until all of the solid has dissolved. (It is important to add only the minimum amount of acid needed to get the solid dissolved.) Save the tube and its contents for further work. 6. Repeat step 5 with each of the remaining test tubes (B through F). 7. Gently heat test-tube A and its contents over a Fischer or Bunsen burner flame, holding the tube at an angle and pointed away from you (as well as pointed away from anyone else in the immediate vicinity). See Figure 5.l. The idea is to evaporate the water in the tube without spattering anything out of the tube. 5-2

3 Figure 5.1 The correct way to heat a test tube over a Bunsen burner Caution: Too rapid heating of the tube, especially if it is held in an upright position, will cause the hot contents to splash out of the tube and will necessitate starting over with a fresh sample. Boiling chips should help to produce smooth boiling. Your instructor may provide additional advice on how to minimize the problem. Continue heating until all of the liquid has evaporated and solid NaCl remains. (It is crucial to the success of this experiment to be sure that all of the water has evaporated from the upper part of the tube.) 8. Remove the tube from the flame and test for the evolution of water vapor from tube A by inverting a clean, dry test tube over the upright mouth of test-tube A. If condensation occurs in the cold test tube, continue the drying and testing process until no condensation occurs. Then set test-tube A with its dried contents aside to cool. 9. Repeat the procedures in steps 7 and 8 with test-tubes B through F. 10. Allow the three test tubes to cool (at least 5 minutes), check to be sure there are no water droplets left, and then weigh each with its contents. Record the masses in the data table. 11. If time permits, confirm that the tubes were fully dried by reheating them for 1-2 minutes, cooling for 5 minutes and reweighing. Record the second mass for each by modifying the data table. If they were dry the first time, there should be a negligible change in mass. Clean Up Rinse the test tubes thoroughly with tap water, then with distilled water, and leave them upside down to drain and dry in your drawer. 5-3

4 Calculations 1. For the six columns in the data tables, subtract the numbers on line 2 from line 1 to obtain the masses of NaHCO 3 used (line 3, page 5-5) or line 11 from line 10 to obtain the mass of Na 2 CO 3 (line 12, page 5-6) 2. Add up the molar masses of the atoms in NaHCO 3 to find the mass of 1 mole of NaHCO 3. Then use this result and the masses on line 3 to calculate the number of moles of NaHCO 3 used (line 4). Similarly, determine the number of moles of Na 2 CO 3 used (line 13). 3. Line 6 (mass of tube) is the same as line 2. Subtract line 6 from line 5 to obtain the masses of NaCl formed by the reaction of HCl with NaHCO 3 (line 7). By similar reasoning, obtain the mass of NaCl formed by the reaction of HCl with Na 2 CO 3 (line 16). 4. Proceeding the same way as in step 2, calculate the moles of NaCl formed in each type of reaction (lines 8 & 17). 5. Finally, calculate the ratio of moles NaCl to moles NaHCO 3 (line 9). This should be recorded to two decimal places (i.e., 2 digits after the decimal point). In like fashion, calculate the ratio of moles NaCl to moles of Na 2 CO 3 (line 18). 6. Calculate the average of your three experimental results (tubes A, B, C) for this ratio of NaCl to NaHCO 3 and record it to two decimal places in the space provided. Also, perform a similar calculation for tubes D, E and F, and record the average in the space provided. Questions To Be Answered After Completing This Experiment Write out answers to the following questions on separate sheets of paper and turn them in along with the entire experiment (procedures and data sheets). 1. If class results have been assembled, what was the average of the mole ratios for the whole class? What was the range of experimental values (from lowest to highest)? Do you think any results should be excluded from calculating an average ratio for the whole class? If so, explain why and then calculate a revised average ratio. 2. Copy the balanced equation for the reaction of sodium bicarbonate with hydrochloric acid from the introduction to this experiment. What ratio for moles NaCl per moles NaHCO 3 does this balanced equation predict? 3. Do your experimental results and those from the class agree with the balanced equation? Write a brief discussion of the agreement (or lack of agreement). 4. Experimental error is a part of any scientific experiment. Suggest two possible sources of error in this experiment. (Do not include weighing errors.) Indicate whether each error would increase or decrease the experimental value for the ratio of moles NaCl to moles NaHCO Now answer similar questions (#1 - #4) for sodium carbonate, Na 2 CO 3, instead of sodium bicarbonate (i.e., tubes D-F). Balance the equation Na 2 CO 3 + HCl NaCl + CO 2 + H 2 O. What mole ratio, i.e., moles NaCl to moles Na 2 CO 3, does this balanced equation predict? 5 4

5 Experiment 5 Name Partner s Name Date Chem 100 Section Chemical Reactions: Making Table Salt Data Sheet Tube A Tube B Tube C 1. Mass of tube + NaHCO 3 2. Mass of tube 3. Mass of NaHCO 3 4. Moles of NaHCO 3 5. Mass of tube + NaCl 6. Mass of tube (cf. #2) 7. Mass of NaCl 8. Moles of NaCl 9. Ratio: Moles NaCl Moles NaHCO 3 Average ratio for these three results: Class average for these results: Range: (high) (low) 5 5

6 Experiment 5 Name Partner s Name Data Sheet (continued) Tube D Tube E Tube F 10. Mass of tube + Na 2 CO Mass of tube 12. Mass of Na 2 CO Moles of Na 2 CO Mass of tube + NaCl 15. Mass of tube (cf. #11) 16. Mass of NaCl 17. Moles of NaCl 18. Ratio: Moles NaCl Moles Na 2 CO 3 Average ratio for these three results: Class average for these results: Range: (high) (low) 5 6