A Cool Chemical Reaction Verifying the Law of Conservation of Mass

Transcription

1 3 Verifying the Law of Conservation of Mass OBJECTIVE Students will design an experiment to verify the law of conservation of mass. LEVEL Middle Grades: Chemistry NATIONAL STANDARDS UCP.2, UCP.3, A.1, A.2, B.2, B.3, B.5, B.6 T E A C H E R P A G E S TEKS 6.2(A), 6.2(B), 6.2(C), 6.2(D), 6.4(A), 6.7(A) 7.2(A), 7.2(B), 7.2(C), 7.2(D), 7.4(A) 8.2(A), 8.2(B), 8.2(C), 8.4(A), 8.4(B), 8.9(A), 8.9(C), 8.10(A) IPC: 2(A), 2(B), 2(C), 2(D), 8(B), 8(C) CONNECTIONS TO AP AP Chemistry: III. Reactions B. Stoichiometry 2. Balancing of equations including those for redox reactions TIME FRAME 45 minutes MATERIALS (For a class of 28 working in pairs) several balances 1 box* baking soda (sodium bicarbonate, 14 heavy duty zippered freezer bags (pint size) NaHCO 3 ) 14 plastic pipets 200 ml vinegar** (acetic acid, HC 2 H 3 O 2 ) OR 3M hydrochloric acid, HCl * 1 box of baking soda is sufficient for several class periods. **Household vinegar (which is 5% acetic acid) works well. To prepare 3M HCl: Place 100 ml of distilled water into a 200 ml graduated cylinder. In another graduated cylinder, measure 50.0 ml of concentrated (12.0 M) HCl. Slowly add the concentrated HCl to the distilled water. Fill to the 200 ml mark with distilled water and mix. 266 Laying the Foundation in Middle Grades Chemistry and Physics

2 3 TEACHER NOTES The law of conservation of mass states that the mass of the reactants in any chemical reaction equals the mass of the products. Matter can be neither created nor destroyed. Sometimes the products of a chemical reaction are very different in color, texture, and physical state than the reactants. If a gas is produced its mass also contributes to the mass of the products. The law of conservation of mass is an important natural law. This law holds true for all chemical reactions. The purpose of this experiment is to show that chemical reactions obey the law of conservation of mass. In this experiment students will explore the reaction between baking soda and vinegar. When the two reactants are mixed the following reaction occurs: HC 2 H 3 O 2(aq) + NaHCO 3(aq) H 2 O (l) + CO 2(g) + NaC 2 H 3 O 2(aq) If using HCl instead of vinegar, the reaction is: HCl (aq) + NaHCO 3(aq) H 2 O (l) + CO 2(g) + NaCl (aq) Suggested Teaching Procedure Show students the above chemical reactions and instruct them to design an experiment verifying that the law of conservation of mass is observed for this reaction. Have students check with you before beginning their experiment. Initial their procedure page and make them follow their proposed procedure, as appropriate. They will have an opportunity at the conclusion of the experiment to submit a revised procedure. Guide the students with questions, but avoid telling them what to do. If they make a mistake, allow them to repeat their experiment. The procedure should include at least the following: 1. Place a small amount of baking soda into the plastic bag. 2. Completely fill the plastic pipet with the acid, dry it, and place it into or beside the plastic bag. T E A C H E R P A G E S 3. Seal the bag, removing as much air as possible. 4. Mass the bag + baking soda + acid-filled pipet and record. 5. Squeeze the acid out of the pipet and allow the reaction to occur. 6. Observe the bag (sight and touch) and write down observations. 7. Mass the entire contents of the bag + the empty pipet after the reaction is complete and record. 8. Flush the contents of the bag down the sink with plenty of water. Dispose of the plastic bag and pipet in the waste container. Laying the Foundation in Middle Grades Chemistry and Physics 267

3 3 POSSIBLE ANSWERS TO THE CONCLUSION QUESTION AND SAMPLE DATA Initial total mass of bag + baking soda + acid-filled pipet before the reaction Final total mass of bag + empty pipet after the reaction 5.87 grams 5.87 grams OBSERVATIONS Answers will vary, but should include: Observing the initial appearance, white solid Expanding of the bag, production of gas Dissolving of powder, change to liquid Changing temperature, bag gets cold T E A C H E R P A G E S ANALYSIS 1. Calculate the difference between the initial and final masses in your experiment. Final mass initial mass = g 5.87g = 0 CONCLUSION QUESTIONS 2. Use your data to explain how the law of conservation was proven in your experiment. Students should restate the law of conservation of mass and then state their observed mass before and after the reaction. 3. Account for any differences between the expected and the actual data. Answers will vary, but include some of the following possibilities: All of the reactants did not react. The bag was not sealed properly. 4. A student fails to notice that the bag was not completely sealed. How will this error affect the recorded mass of the products? What erroneous conclusions might be drawn as a result of this error? If the bag were not sealed tightly some of the carbon dioxide gas would escape. With loss of a product, the mass at the end of the reaction would be lower than expected. A student would erroneously conclude that the law of conservation of mass is not obeyed. 5. Was the change that took place a chemical or physical change? Cite at least two pieces of evidence from the experiment to support your choice. This experiment was a chemical change. Students should indicate that a gas was produced and cite evidence that the bag expanded and/or the bubbles were produced. Students should indicate that an energy change occurred and cite evidence that the entire bag became cool to the touch. Discuss with students that a chemical reaction that feels cool to the touch is the result of energy being absorbed by the reaction. When energy is absorbed by the reaction, the reaction is said to be endothermic. Exothermic reactions release energy and feel warm to the touch. 268 Laying the Foundation in Middle Grades Chemistry and Physics

4 3 6. On a piece of notebook paper, write an improved procedure that will minimize or eliminate errors encountered by your lab group. Student answers will vary depending on the type of errors that occurred in their own design. They should explain what changes they are making in order to avoid the initial mistakes. T E A C H E R P A G E S Laying the Foundation in Middle Grades Chemistry and Physics 269

5 3 Verifying the Law of Conservation of Mass The law of conservation of mass states that the mass of the reactants in any chemical reaction equals the mass of the products. Matter can be neither created nor destroyed. Sometimes the products of a chemical reaction are very different in color, texture, and physical state than the reactants. If a gas is produced its mass also contributes to the mass of the products. The law of conservation of mass is an important law of chemistry. This law holds true for all chemical reactions. When the two reactants are mixed the following reaction occurs between baking soda and vinegar. HC 2 H 3 O 2(aq) + NaHCO 3(aq) H 2 O (l) + CO 2(g) + NaC 2 H 3 O 2(aq) If using HCl instead of vinegar, the reaction is: HCl (aq) + NaHCO 3(aq) H 2 O (l) + CO 2(g) + NaCl (aq) PURPOSE Design an experiment to verify the law of conservation of matter using the following materials. MATERIALS balance zippered freezer bag plastic pipet baking soda (sodium bicarbonate, NaHCO 3 ) vinegar (acetic acid, HC 2 H 3 O 2 ) OR 3M hydrochloric acid, HCl Safety Alert 1. Goggles and aprons are required. Acids used in this experiment could cause damage to eyes, clothing and skin. 2. Wash hands thoroughly before leaving the laboratory. PROCEDURE 1. Using an if-then format, write a hypothesis for your experiment that describes the relationship between the mass of the reactants and the mass of the products of this chemical reaction. Record your hypothesis on the student answer page. 2. Using your knowledge of the scientific method, design an experiment to verify the law of conservation of matter. Points to remember: Select your equipment from the list above. Create an appropriate data table. Use only small quantities. Repeat this experiment if time allows. 270 Laying the Foundation in Middle Grades Chemistry and Physics

6 3 3. On a piece of notebook paper, record the steps you will use to perform this experiment. Have your teacher approve and initial your procedure before beginning. 4. In the space provided on the student answer page, record all data and observations. 5. Flush the contents of the bag in the sink with plenty of water. Dispose of the plastic bag and pipet in the waste container. Laying the Foundation in Middle Grades Chemistry and Physics 271

7 3 Name Period Verifying the Law of Conservation of Mass HYPOTHESIS DATA AND OBSERVATIONS Your data table and observations here: ANALYSIS 1. Calculate the difference between the initial and final masses in your experiment. CONCLUSION QUESTIONS 2. Use your data to explain how the law of conservation was proven in your experiment. 3. Account for any differences between the expected and the actual data. 272 Laying the Foundation in Middle Grades Chemistry and Physics

8 3 4. A student fails to notice that the bag was not completely sealed. How will this error affect the recorded mass of the products? What erroneous conclusions might he draw as a result of this error? 5. Was the change that took place a chemical or physical change? Cite at least two pieces of evidence from the experiment to support your choice. 6. On a piece of notebook paper, write an improved procedure that will minimize or eliminate errors encountered by your lab group. Laying the Foundation in Middle Grades Chemistry and Physics 273