The Eight Solution Problem Exploring Reactions of Aqueous Ionic Compounds

Transcription

1 The Eight Solution Problem Exploring Reactions of Aqueous Ionic Compounds About this Lesson This activity allows students to mix a variety of known ionic solutions while making careful observations. After completing the set of known solutions, a set of unknown solutions is presented to the students. The students must design an experiment to determine the identity of the unknown solutions. The analysis includes writing balanced formula equations and net ionic equations for each of the reactions and answering related questions regarding the experiment. This lesson makes a nice addition to a discussion of net ionic equations. This lesson is included in the LTF Chemistry Module 9. Objectives Students will: Mix various ionic solutions in a grid-like fashion to generate a set of chemical reactions. Record observations and then use these observations to identify eight unknown solutions. Level Chemistry Common Core State Standards for Science Content LTF Science lessons will be aligned with the next generation of multi-state science standards that are currently in development. These standards are said to be developed around the anchor document, A Framework for K 12 Science Education, which was produced by the National Research Council. Where applicable, the LTF Science lessons are also aligned to the Common Core Standards for Mathematical Content as well as the Common Core Literacy Standards for Science and Technical Subjects. Code Standard Level of Thinking (LITERACY) RST (LITERACY) RST Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text. Translate quantitative or technical information expressed in words in a text into visual form (e.g., a table or chart) and translate information expressed visually or mathematically (e.g., in an equation) into words. Apply Apply Depth of Knowledge II II T E A C H E R P A G E S Connections to AP* AP Chemistry: III. Reactions A. Reaction types 1. Acid-base reactions 2. Precipitation reactions *Advanced Placement and AP are registered trademarks of the College Entrance Examination Board. The College Board was not involved in the production of this product.

2 Materials and Resources Each lab group will need the following: aprons calcium chloride, 1 M copper(ii) sulfate, 1 M copy of laminated data sheets for Eight Solution Lab goggles hydrochloric acid, 3 M iron(iii) chloride, 1 M lead(ii) nitrate, 1 M nitric acid, 3 M paper towels 12 pipettes, thin stem potassium iodide, 1 M scissors silver nitrate, 1 M sodium carbonate, saturated sodium hydroxide, 3 M sodium phosphate, 1 M sulfuric acid, 3 M toothpicks Additional teacher materials: 150 labels for pipettes Assessments The following types of formative assessments are embedded in this lesson: Assessment of prior knowledge Guided questions during the process of experimentation Visual assessment of procedure and conclusions as students perform the experiment The following additional assessments are located on the LTF website: Chemistry Assessment: Equations 2010 Chemistry Posttest, Free Response Question 3 Teaching Suggestions This laboratory exercise enhances a unit on chemical reactions and is designed to follow Net Ionic Equations. Students will be more successful in identifying their unknowns if they are familiar with solubility rules and precipitation reactions. Students must rely on their recorded observations regarding the knowns of Part I to aid in the identification of the unknowns in Part II. Students apply their logic, deductive reasoning, and problem-solving skills as they decide which solutions to combine with the unknowns in Part II. Students will be asked to write net ionic equations as a part of this exercise. T E A C H E R P A G E S This is a qualitative lab, so the molarities of the solutions need not be exact. It is important that the sodium carbonate solution be 3 M or greater and that the acids be 1 M or greater so that bubbling of carbon dioxide is observed. For the salts, any equimolar concentration between 0.1 M and 1.0 M will suffice.

3 Photocopy and laminate the Data Collection Sheet for Part I and Part II found at the end of these teacher pages. Students will perform their reactions on the laminated grids. Instruct students to place the drops of reagents so that half of the drop rests on the white background and the other half of the drop rests on the black background. It is essential that microtip pipets are used to keep drops from running together. The microtip pipets can either be purchased or made from a thinstem pipette. The stem of thin-stem pipets can be pulled and trimmed so that it delivers very small drops (see Figure 1). Do not discard the cut-away portion; it can be used as a micro-stirrer. Toothpicks may also be used as stirrers. Caution students against contaminating their solutions as they apply the drops. T E A C H E R P A G E S Figure 1 Disposal consists of soaking up the drops of reagents (once all 66 reactions have been completed) with a paper towel and disposing of the paper towel in the trash. Use a second, clean, damp paper towel to wipe off the grid so that it is ready for the next class or storage. You can choose any of the solutions for your unknowns but the following list is recommended: nitric acid, potassium iodide, sodium phosphate, silver nitrate, sodium hydroxide, lead(ii) nitrate, and sulfuric acid. You will need to make a numbered set of unknown pipets for each lab pair along with a key for the scheme. To prevent students from figuring out they all have the same unknowns, create a numbering system such that any number ending in zero is sulfuric acid, any number ending in 1 is silver nitrate, and so on. Be sure to keep record of your scheme for refilling pipets and grading students. You may want to allow students more than one evening to complete the conclusion questions for this lab as the students will be writing equations for all 66 of the reactions and net ionic equations for all of the reactions that exhibit a driving force.

4 Data Collection Sheet Pb(NO 3 ) 2 HCl Na 3 PO 4 KI CuSO 4 H 2 SO 4 NaOH AgNO 3 CaCl 2 HNO 3 Na 2 CO FeCl Na 2 CO HNO CaCl AgNO NaOH H 2 SO CuSO KI Na 3 PO 4 66 HCl

5 Unknown Number: Data Collection Sheet

6 The Eight Solution Problem Exploring Reactions of Aqueous Ionic Compounds INTRODUCTION Chemical reactions that occur spontaneously do so as a result of several types of driving forces. Writing the net ionic equation for a reaction often makes it evident which driving force caused the reaction. The most common driving forces for chemical reactions are formation of a precipitate, formation of a molecular compound such as water, and formation of a gas. PURPOSE The goal of this laboratory exercise is to positively identify eight unknown solutions. You and your partner will first collect data by observing reactions between various solutions of known ionic compounds. You will then use your recorded observations to determine the identity of eight unknown solutions. You will apply the solubility rules to write net ionic equations for all reactions producing gases or precipitates. MATERIALS Each lab group will need the following: aprons calcium chloride, 1 M copper(ii) sulfate, 1 M copy of laminated data sheets for Eight Solution Lab goggles hydrochloric acid, 3 M iron(iii) chloride, 1 M lead(ii) nitrate, 1 M nitric acid, 3 M paper towels 12 pipettes, thin stem potassium iodide, 1 M scissors silver nitrate, 1 M sodium carbonate, saturated sodium hydroxide, 3 M sodium phosphate, 1 M sulfuric acid, 3 M toothpicks Safety Alert 1. Wear your safety goggles and lab aprons. 2. If your hands come in contact with any of the chemicals, make sure to rinse them under water and notify your teacher. 3. If the chemicals come in contact with your eyes, use the eye wash according to your teacher s instructions.

7 PROCEDURE PART I The Eight Solution Problem 1. Obtain your laminated data collection sheet and orient it on the table so that you can read the numbers in each box clearly. You will be mixing a pair of solutions in each numbered box. 2. Begin by placing 2 drops of iron(iii) chloride in the center of box 1. Place the 2 drops in combination so that half of the solution is on the white portion of the box and half is on the black portion. This will allow you to accurately record the color of any precipitates that may form on your student answer page. In some cases, no visible reaction occurs. In those cases, write NVR in the box for that reaction. Repeat this procedure for boxes Place 2 drops of the solution listed above each column next to the iron(iii) chloride solution in boxes As you begin to add the second solution, make sure to hold the pipette so that its tip does not touch the other solution. Any contamination of the pipette will cause error in subsequent reactions. 4. Break a toothpick in half and use it to swirl the solutions together until mixed. Record your observations in the appropriately numbered box of the grid found on your student answer page. Discard the toothpick so as to not contaminate other reactions. 5. Repeat steps 2 4 until all 66 reactions have been conducted. 6. When you finish (all 66 reactions have been completed), soak the drops of reagents up with a paper towel and dispose of the paper towel in the trash. Use a second, clean, damp paper towel to wipe off the grid so that it is ready for the next class. Dry your laminated data collection sheet.

8 PART II The Eight Solution Problem 1. Flip your data collection sheet over to conduct this part of the lab. These solutions are a subset of the 12 solutions you have already tested. 2. Record the number of each of the eight unknowns in the blanks provided across the top of the grid found on your student answer page. Reverse the order of the numbers as you write them in the blanks provided down the vertical column. Use a water-soluble marker to do the same on your laminated data collection sheet. 3. Place 2 drops of the unknown solutions in each numbered box just as you did in Part I. Mix them with a toothpick and record your observations. 4. Compare the results obtained by mixing the numbered unknowns with the results recorded in Part I. Identify all of the unknowns that had distinct reactions. 5. Once you have identified the eight unknown solutions, write the unknown number, chemical name, and chemical formula in the spaces provided on your student answer page. 6. When you finish, soak the drops of reagents up with a paper towel and dispose of the paper towel in the trash. Use a second, clean, damp paper towel to wipe off the grid so that it is ready for the next class. Dry your laminated data collection sheet.

9 The Eight Solution Problem Exploring Reactions of Aqueous Ionic Compounds DATA AND OBSERVATIONS

10 The Eight Solution Problem

11 ANALYSIS Do the following for each of the 66 reactions numbered on your grid: Write the balanced formula equation for each of the reactions. Include the state symbols for each reactant and product. Write the net ionic equation for all reactions producing a precipitate, gas, or other molecular compound such as water. Include the state symbols for each reactant and product. If there was no visible reaction taking place, write NVR for no visible reaction in the margin to the left of that reaction s number. Be careful here: you may have recorded NVR because no precipitate was formed but that does not mean there was no reaction. Some reactions produce gases or other molecular compounds such as weak acids or water. Double-check that all chemical formulas are correct and that all equations are balanced. Skip a line on your paper between equations. You may wish to type the equations for neatness

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16 CONCLUSION QUESTIONS 1. List the reactions in which a gas was formed. Use their original reaction numbers from the grid and write the net ionic equation for each reaction. What do they have in common? 2. Some of the reactions produced distinct color changes but no precipitate. Did a chemical reaction take place? Justify your answer. 3. List the acid-base reactions. Use their original reaction numbers from the grid and write the net ionic equation for each reaction. What do they have in common?

17 4. Many of the equations you wrote earlier were labeled NVR for no reaction. Choose one of these reactions and show ion cancellation to explain why no visible reaction occurred.