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Precipitation Reactions and the Conservation of Mass A CORE learning cycle lab experiment

During Oct. 5-8 (Tuesday-Thursday), the gen chem lab program will offer opportunities to make up experiments. Guidelines were sent to you and are also available at www.interchemnet.com, at the bottom of the ICN News page

(Lab Make-up Guide Fall 2021). Questions? Ask your TA. The lab reports are due 72 hours after performing the lab. Introduction Goals:

1. To observe a precipitation reaction. 2. To use an analogical model to think about chemical interactions and the conservation of mass in a precipitation reaction.

3. To design an experiment that evaluates the conservation of mass concept. The Law of Conservation of Mass We know from data from many experiments, that atoms are conserved in physical or chemical transformations. This experiment will help us explore this concept. The law of conservation of matter states that the number of each type of atom is the same before a chemical reaction as after a chemical reaction. Each atom has a particular mass associated with it and thus, mass is conserved in a chemical reaction. In other words, the total mass of the reactants is equal to the total mass of the products. The conservation of mass seems straight forward, but before atomic theory, the idea that mass is conserved in chemical reactions was not obvious to experimentalists. Different types of chemical reactions, when observed, can appear as if they have created or destroyed matter. Consider a combustion reaction, where a block of wood (made up of a variety of compounds containing hydrogen, oxygen, and carbon atoms) reacts with oxygen. After the wood burns, the resulting pile of ash is lighter than the block of wood. However, in a combustion reaction, atoms in the wood are rearranged to form water (H2O) and carbon dioxide (CO2) which are released as gases into the atmosphere. All of the atoms in the original block of wood still exist. Precipitation reactions can also be misleading. In a precipitation reaction, two solutions containing soluble ions (which are invisible to the eye in the solution) are mixed and react to form a new compound that is no longer soluble. The insoluble compound, called the precipitate, is a solid that settles out of the solution. It may appear as if matter has been created from nowhere. Familiarity with precipitation reactions will help you understand the next experiment in this two-lab sequence about limiting reactants. In this experiment, you will study two precipitation reactions and explore an analogical model for the reaction to help think about what is occurring on the atomic scale. The CORE Learning Cycle Phase 1: Following a procedure, you and a partner in lab will make chemical observations.

Phase 2: You will review materials that develop an explanation about what you observed in lab using an analogical model of the chemical reaction. Modeling is an important skill to help develop insight. Phase 3: Before going back into the lab, you will be asked to apply your understanding by designing an experiment. You and your partner will then go back into the lab to conduct your experiments and gather experimental results.

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An analogical model We will use nuts and bolts and assign them to different types of ions and molecules to model the precipitation reaction. This analogy is useful for helping us to “visualize” what happens to all of the chemical species (atoms, ions, compounds, etc.) present in a solution. In a reaction, when bonds break and new ones form, it is important to have a way to keep track of all of the chemical species. Nuts, bolts, and nails come in different sizes. In a box of nuts, bolts, and nails, some combinations “fit” together securely while others do not. In a similar way, in a collection of chemical species in solution, some combinations may readily interact, while other combinations do not. In the analogy portion of this lab you will explore how various combinations of nuts, bolts and nails connect, and relate what you learn from the analogy to the behavior of different chemical species in the precipitation reaction. Pre-lab Assignment In your lab notebook, please prepare the following information and answer the questions. You must complete the pre-lab before coming to the lab meeting or you will not be permitted to go into the laboratory. 1. Write a 2-3 sentence introduction to the lab.

2. Create a safety information table including the chemicals used in the lab, the hazards

associated with them, and any safety handling precautions. (See example safety table on the Student Resource Page on ICN.)

3. Consider a chemical reaction between 5 Zn2+ (zinc cations in water) and 5 S2- (sulfide anions in water) to form 5 ZnS (solid zinc sulfide). Write a balanced chemical equation for this reaction.

4. Now imagine that you replace Zn2+ with a nut and S2- with a bolt and draw a representation for the reaction described above. Is this a type of analogy? Explain by annotating your drawing.

Laboratory Guide On the following pages, you will find instructions for doing an experiment. In this experiment, pair up with another student when doing lab work. If there is an odd number of participants in lab, one group may have three people. Your lab work will involve individually making observations and recording these in your own lab notebook, and working in partnership on certain activities such as: answering questions, discussing observations, analyzing results, and designing procedures in response to scientific questions. As you go through the experimental guide, you will notice there are questions that are set off in the guide (i.e. “Q:”). For example:

Q: What are the physical/observable characteristics of each of these solutions? To develop best practices, we ask that you respond to every question in your lab notebook. Our expectation is that you write enough to give an indication of your thinking. You do not have to write

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the question AND answer, but you must address the answer, for example: “we found that both solutions were colorless and of similar viscosity….” Part of the purpose of making an entry in your notebook is to allow you to remember later what you were thinking at this point in the lab, which can be very useful when writing your lab report. It also offers evidence of your thinking process.

Safety Suggestions Goggles are required at all times in the lab. If you are outside lab and do not have goggles, obtain goggles before entering he lab. There are no exceptions. Gloves and aprons are available. If you have questions about safety, please do not hesitate to ask your laboratory instructor. Note: This lab has portions of the procedure that must be completed in a chemical fume hood (e.g. see phase 3).

Equipmentandsupplies: 12.5cmfilterpaperLong-stemfunnelRingstandandringclampAcetonewashbottle

Phase 1: Making observations The following activities should be completed in the lab. 1. Obtain 0.1 M NaCl and 0.1 M Ag(NO3) solutions. Q: What are the physical/observable characteristics of each of these solutions? 2. In an Erlenmeyer flask, mix together approximately 10 mL of 0.1 M NaCl solution and 10 mL of 0.1 M Ag(NO3) solution. Record the exact volumes in your lab notebook. Carefully swirl the flask to mix the solutions. Q: What happens when the solutions are mixed? Is there evidence of a precipitate forming? This is the end of Phase 1.

Chemicals:silvernitratesolution:Ag(NO3)sodiumchloridesolution:NaClcalciumchloridesolution:CaCl2acetone

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Phase 2: Exploring the Analogy The following activities should be completed in the breakout room. Thinking about balanced chemical equations A balanced chemical equation has a lot of information encoded into it. Consider the example below:

Ultimately, the balanced chemical equation gives you a recipe for a reaction using ratios of moles of reactants. In the example above, it takes two moles of sodium atoms and one mole of chlorine (Cl2) molecules to create two moles of sodium chloride. The molar ratio of sodium metal to chlorine molecules is 2:1. The molar ratio of sodium to sodium chloride in the reaction is 1:1. The ratio in the recipe will still hold for a much smaller number of atoms and molecules. For example, in the reaction above, we can also say that 2 Na atoms will react with 1 Cl2 molecule to create 2 NaCl. The picture below represents this approach:

In summary, chemical equations provide a lot of information, such as the general ratio of the amount of each reactant needed to create a certain amount of a particular product. Molar ratios become useful in predicting the mass of product that can be expected in a chemical reaction.

2Na(s) + Cl2(g) 2NaCl(s)

+

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Carefulthoughtisalsoneededtointerpretformulas.Asanioniccompound,NaCldoesnotexistassinglemoleculesof"NaCl"butina3-dimensionallatticecontainingmany"NaCl"units.

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The precipitation reaction Now let’s take a look at the chemical equation for the precipitation reaction you performed in Phase 1.

Consider all of the information that you can extract from the balanced chemical equation above. Q: What is the precipitate that is formed?

Q: What are the molar ratios of the reactants to each other? What is the molar ratio of each reactant to the product silver chloride?

Because we are dealing with ionic compounds in the reaction shown above, we also can write the chemical equation a different way:

Analogical model for the precipitation reaction Now you are going to construct an analogical model for the reaction of sodium chloride and silver nitrate that results in the formation of the precipitate silver chloride and aqueous sodium nitrate. In front of you should be a variety of hardware. There are nails, bolts, and two different sizes of nuts:

NaCl(aq) + Ag(NO3)(aq) Na(NO3)(aq) + AgCl(s)

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1. Assign the types of hardware to the different chemical species involved in this reaction.

In this analogical model, we will use nuts to represent the cationic species, and bolts or nails to represent the anionic species. If a nut fits tightly with a bolt, that will represent formation of a precipitate. If a nut slides loosely over a bolt or nail, that will represent ionic species that stay in solution.

We will give you one assignment: the chloride anion is assigned as the bolt (see picture below).

As you assign the other hardware, consider that some combinations will fit together more tightly than other combinations. The chemical interaction between sodium cations and nitrate anions in an aqueous solution is weak compared to the chemical interaction between silver ions and chloride ions. Make sure your hardware assignments reflect this.

Record the assignments of your hardware in your notebook. Show them to your TA. There is only one combination of the hardware representing Ag+ + Cl- which results in a tight connection to represent AgCl(s). If you have any questions about your assignment, please discuss your assignments with your TA before proceeding with the analogy.

2. Now use the hardware to create a representation of 3 sodium cations and 3 chloride anions in a solution. Then, create a representation of 3 silver cations and 3 nitrate anions in a separate solution. These are your reactants.

3. Next, mix the hardware representations of aqueous sodium chloride and aqueous silver nitrate to simulate the reaction by rearranging the reactants to form the hardware representation of the precipitate silver chloride and aqueous sodium and nitrate ions. Carefully keep track of all the hardware.

4. You should fill out the Analog and Target worksheet individually on pages 7 and 8 (remember, you will each need to scan a legible copy of the worksheet and submit it with your lab report electronically). As you fill out the worksheet, you may discuss what you're thinking with your partner, if this helps you think about the chemistry at the atomic scale.

Extra Credit - Extra Challenge If this analogy seems simplistic to you, consider extending the analogy for extra credit. This might

involve thinking about additional aspects at the atomic scale and relating them back to the analog. For example, you might consider how you would modify the analogy to model the precipitation of silver sulfide, Ag2S (2 Ag+ cations combining with 1 S2- anion) or an analogy that includes solvent (water). To receive extra credit, describe your modified analogy in your lab report in a separate section called Extra Credit, Extra Challenge. In this section, you should identify how the modified analogy strengthened your understanding or insight of the chemical phenomena under examination. You should also identify explicitly where in the Analog and Target worksheet these modifications revealed themselves. Extra credit will be worth up to 10% on your entire grade for the experiment.

When you have completed the worksheet, you have finished Phase 2.

Na+(aq) + Cl-(aq) + Ag+(aq) + (NO3)-(aq) Na+(aq) + (NO3)-(aq) + AgCl(s)

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Analog and Target Worksheet (pages 7-8) Name: ___________________ Fill out this worksheet individually as you perform the analogical reasoning activity. Work together to discuss similarities and differences. Each student must include a scanned copy of this sheet with their lab report. Make sure your scan is entirely legible. Please label the components in your drawings.

Assignments: Bolt = _____________________ Nail = ___________________ Large nut = ___________________ Small nut = ___________________

Analog and Target Comparison

Sodium and chloride ions in an aqueous solution compared to your hardware assignments for these species

Silver and nitrate ions in an aqueous solution compared to your hardware assignments for each of these species

The action of rearranging the hardware compared to the chemical reaction

Silver chloride and sodium nitrate in an aqueous solution compared to the hardware products of the reaction, taking into account the conservation of mass

Similarities: What characteristics does the analog (nuts and bolts model) share with the target (ions in solution, precipitate)?

Differences: What features of the nuts and bolts model (analog) are not representative of the ions in solution and the precipitate (target)?

Differences: Now think about the target (ions in solution and precipitate) and describe the features that are missing in the analog (nuts and bolts model).

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Analog and Target Worksheet Continued (pages 7-8) Name: ________________________

Draw representations of the following situations at the molecular level to model the precipitation reaction and the conservation of mass.

Remember that there are many water molecules surrounding each ion in solution.

Sodium chloride in an aqueous solution (don’t forget the

water molecules):

Silver nitrate in an aqueous solution (don’t forget the

water molecules):

The chemical reaction (don’t forget the water molecules):

The products of the reaction in an aqueous solution (don’t

forget the water molecules):

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Phase 3: Designing experiments

Plan your experiments in the breakout room before proceeding to lab to complete them.

Using the analogical model to make predictions

Q: Why is mass conserved in chemical reactions? Explain the law of conservation of mass at the molecular level. Use the analogical model to justify your answer.

The experiment Design an experiment to explore the conservation of mass in a different precipitation reaction

between calcium chloride and silver nitrate and answer the following scientific question:

Scientific Question: Is the number of moles of silver chloride produced in the chemical reaction equal to the number of moles of silver cations and the number of moles of chloride anions in the reactants?

The balanced chemical equation for the reaction of calcium chloride and silver nitrate is shown below. Please note the difference in charges for some of the ions compared to the sodium chloride and silver nitrate reaction you performed in phase 1.

Q: Why is it necessary to start your reaction with twice as many moles of silver nitrate (Ag(NO3)) as moles of calcium chloride (CaCl2)?

In the lab, you will have the following solutions to work with:

0.1M Ag(NO3) 0.1M CaCl2 0.2M Ag(NO3) 0.2M CaCl2

Use the Designing Experiments worksheet (page 11) to summarize this process. You will need to make a copy of this to submit with your lab report. Make sure to also take careful notes of the process in your laboratory notebook. Make sure that you check in with your laboratory instructor before starting your experiment.

Below are some useful techniques and sample calculations to help you plan your experiment. Separating the precipitate A technique that will be useful in Phase 3 is called gravity filtration. Gravity filtration involves using filter paper to separate the insoluble solids (the precipitate) from the liquid to obtain a clear solution (the filtrate) in a new beaker. Obtain a piece of 12.5 cm filter paper. Fold the circle in half to form a semicircle. Then fold the semicircle in half. Open it into a cone and tear off a small piece of the upper, outside corner. This will improve the seal between the filter paper and the funnel. Weigh the filter paper cone (to the nearest 0.001 grams) and record the weight in your lab notebook. The figure to the right demonstrates the folding of filter paper. Place the folded filter paper in a long-stem funnel that is attached to a ring stand. Place a beaker underneath to collect the filtrate. Pour liquid from the reaction into the funnel to wet the filter paper. Then, slowly pour the rest of the reaction through the funnel. The solids (i.e. precipitate) will be trapped in the filter paper and the liquid (i.e. filtrate) will be collected in a beaker.

CaCl2(aq) + 2Ag(NO3)(aq) Ca(NO3)2(aq) + 2AgCl(s)

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Drying the precipitate When you have obtained a precipitate in the filter paper, it will still be wet. The water molecules will add mass, so you need to remove them. In a chemical fume hood, place a clean, dry beaker under the funnel stem (supported in a ring clamp on a ring stand) and rinse the precipitate on the filter paper with acetone using an acetone wash bottle. Transfer the filter paper with the precipitate to a watch glass and then follow your TA’s instructions for drying the precipitate. Acetone is used because it dissolves the water but it does not dissolve the precipitate. When the precipitate is dry, weigh the filter paper and precipitate and record the weight in your lab notebook. Note: When you are finished with this process, transfer the acetone washings to the correctly labeled waste container.Calculating moles from molarity The molarity of a solution tells you how many moles of solute, e.g. CaCl2, are in a liter of that solution. Molarity is a measure of concentration. For example, if the bottle says 0.1M on it, that means there would be 0.1 moles of CaCl2 in a liter of that solution. To figure out how many moles of CaCl2 are in 10 mL of a 0.1M solution, here’s what you can do: remember that 10 mL = 0.010 L and therefore: (0.010&'()*+) -0.1!"#$%&'($) . = 0.001012)+3432*

What volume of solution should I use if I want a specific number of moles? Say you know you want to use 0.002 moles of Ag(NO3) in a reaction. How much solution (what volume) should you use? If it’s a 0.1M solution, you can calculate the volume that would contain 0.002 moles this way: (0.002012)+)/ -0.1!"#$%&'($) . = 0.022'()*+789:+or 20 mLNote: if your precipitate is not completely dry by the end of the lab period, ask your TA’s permission to record an initial mass, write your lab report and then reweigh your precipitate next week. If your TA agrees to this, store your precipitate in your drawer during the week to dry out. Next week you can reweigh your precipitate and use this new value to revise your lab report. (If your TA gives you permission to do this, he/she will give one extra day to submit your final lab report.) Reflections & post-lab discussion (group discussion) To be completed at the end of the laboratory session. 1. In a group, discuss how the moles of AgCl produced compare to the initial moles of Ag+ cations and Cl- anions in the CaCl2 and AgNO3 reactants. What are some possible reasons (e.g., sources of error) for obtaining less or more product than expected.

2. In a group, discuss how the analogical model can be used to explain how your experiment supports the conservation of mass to someone who thinks that precipitation reactions create matter.

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Designing Experiments Worksheet Names: Experiment #:

Signatures: Section:

Scientific Question: Is the number of moles of silver chloride produced in the chemical reaction equal to the number of moles of silver cations and the number of moles of chloride anions in the reactants? Please use this sheet to summarize your lab group’s experiment and findings. Before going into lab, have your lab instructor check and initial it. This worksheet is to be scanned and submitted as part of your lab report. Make sure that it is entirely legible!

Please describe your proposed experiment.

(Check in with your lab instructor before performing experiments)

Instructor’s initials:________

(attach extra pages if needed)

After you perform the experiment, please summarize the success or failure of your experiment and any claims you think you will be able to make in your lab report.

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Rubricforlabreport(duenextlabmeeting)(UsetheGenChemlabreporttemplate)Introduction:

(10pts)

This section provides a short introduction or summary to the report, and should include: • Title of the report • Author name (your name) • Lab partner name (if you had partner(s)) • Date of experiment • Overview: a few sentences describing what the lab experiment was about

WealsorecommendincludingyoursectionandTAname(optional)Data,Results,Evidence:SubmissionoftheAnalogandTargetandDesigningExperimentsWorksheetsarerequired.(25ptstotal)

This section organizes data, results, and evidence. The goal of the section is to describe what you did and what data were collected. Observations can be important data to use in analysis. See additional instructions in the Template for Submission of Lab Reports (in ICN Resources).

This section should include: • Procedure: Reference the laboratory procedure that was downloaded and the date it

was accessed (Precipitation Reactions and the Conservation of Mass procedure, accessed: 10/25/2019). Any changes in procedure should be noted.

• Analog and Target (A&T) Worksheet: This worksheet should be included and discussed in the lab report. (Your scan must be legible)

• Designing Experiments (DE) Worksheet: this worksheet should be included and discussed in the lab report. (Your scan must be legible)

• Observations: include observations and note if they help explain any chemistry. • Notebook data and Calculations. You should present data such as the mass of silver

chloride and a calculation of how it was converted to moles. Note: since patterns are often critical to understanding data, you may want to present data in Tables as well as Figures. This will increase the quality of this section. Note:datafromoutsidesourcescanbeusedinthissectiontosupportdataobtained.

AnalysisofEvidence(Reasoning):Scientificexplanationsthatuseevidenceandappropriatechemistryconceptstoconstructclaims.(30ptstotal)

Inthissection,yourdata,results,andevidence,includingobservations,areanalyzedtohelpexplainthechemistrythatoccursintheexperiment.Youranalysiswillbeamajorsupportforanyclaimsyoumake.See additional instructions in the Template for Submission of Lab Reports (in ICN Resources).

This section should carefully analyze evidence (using reasoning) and include: • Data to Analysis Connection: a goal of this section is to connect data, results, and

evidence to your analysis. This is in preparation for making a claim. • Discuss Data and Observations: include in your discussion data and observations that

can help you explain the chemistry involved in this lab. • Include an answer to the scientific question: Is the number of moles of silver

chloride produced in the chemical reaction equal to the number of moles of silver cations and the number of moles of chloride anions in the reactants?

• Discuss A&T Worksheet: discuss how the analogy relates to your experimental data. Try connecting it to the analogical model to develop your explanations of results and underlying chemical concepts in your discussion

• Discussion of phenomena: you can discuss phenomena at both the submicroscopic (molecular) level & macroscopic (visible to your eyes) level in order to explain the chemistry of this experiment.

Note:Datafromoutsidesourcescanbediscussedinthissectiontosupportanalysis.Claim(s):Statement(s),derivedfromevidence,usingscientificreasoning.(15ptstotal)

This section is to present claims. It is advantageous to directly connect evidence (from your lab investigation and outside sources) that you utilize in your discussion to develop your claims. See additional instructions in the Template for Submission of Lab Reports (in ICN Resources). Addressingthescientificquestionaspartofyourclaimmaybeuseful.