Limiting Reagent (using an analogy and a learning cycle approach)

Transcription

1 Limiting Reagent (using an analogy and a learning cycle approach) Welcome: This is the fourth of a four- experiment sequence, covering four important aspects of chemistry, and utilizing a learning cycle as well as an analogy activity. Lab 1 Making Experimental Observations: chemical interactions, structure, and reactivity Lab 2 Paper Chromatography: chemical interactions and separation techniques Lab 3 Conservation of Mass Lab 4 Limiting Reagents Learning cycle: to help you better understand the chemical concepts in each lab experiment, we will employ a three- step learning cycle. Step 1: We will ask you and a partner to go into the lab, follow a procedure, and make observations. Step 2: You will then be asked to review material that provides an explanation about what you observed in lab. Step 3: Before going back into the lab, you will be asked to apply your understanding by designing experiments. You and your partner will then go back into the lab to conduct your experiments and gather experimental results. Designing and carrying out your own experiments can be a very challenging part of the lab and requires very careful thought. Please bring a copy of the Step #2 worksheet from the conservation of mass lab. You will need it during this lab. Analogy: to help you better understand the chemical concepts in each lab experiment, we have incorporated an analogy into today s lab activity. Learning Goals: 1. Pre-lab discussion: to understand precipitation reactions and the idea that when there is an imbalance of reactants, some of the reactants will be used up first and those in excess will remain at the end of the reaction. 2. To observe a precipitation reaction with an imbalance of reactants. 3. To construct an analogy using nuts and bolts to gain insight into chemical bonding and the concept of a limiting reagent. 4. To design an experiment to discover which reactant limits the extent of reaction. 5. Post-lab discussion: how did we design our experiments? what was our evidence? what can we conclude? 1

2 Lecture Connections: In Moore et al. (4th ed), see chapter 4 (pg. 137) for information about limiting reactant. In Shultz, see chapter 1 (pg ) for limiting reactant. The law of conservation of mass states that matter can be neither created or destroyed. Balanced chemical equations describe the relative numbers of reactants used in generating products. The states of matter (gas, liguid, solid) are often noted in the balanced chemical equation to help understand the reaction. Reaction types: chemists have found it useful to classify reactions with a label that describes an important aspect of the reaction. In this lab we will describe a precipitation reaction, and in other labs or in your lecture you will hear labels such as acid-base or redox reactions. Precipitation Reaction: A precipitation reaction is a type of reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. The insoluble compound is called a precipitate. Approaching Limiting Reagent Problems: When a reaction occurs with two or more reactants, it can proceed as long as one of the reactants doesn t run out. If this occurs, we say that the reaction is limited by the reactant that runs out first, which is called the limiting reagent. The total amount of products that form in this circumstance, is determined by the limiting reagent. To solve a chemical limiting reagent problem, we first need to identify the limiting reagent. Here s an approach: Write the balanced chemical equation for the reaction. The balanced chemical equation relates the relative number of reactants involved in the reaction. Using the balanced chemical equation and relative numbers of reactants, we determine the limiting reagent. Sometimes we are then asked to calculate the products that can form, given the amount of limiting reagent present. 2

3 An analogy for thinking about limiting reagents: In last week s lab, we used nuts and bolts to explore an analogy to account for all of the chemical species present in a solution, before and after a chemical reaction took place. As a continuation of the analogy activity, we will use the same assignments made last week of different nuts and bolts to represent different types of ions. This week, we will consider what happens if we limit the amount of some nuts and bolts. We will again run a precipitation reaction but this time, we will focus on the consequences of having an imbalance in reactants. Analogies for innovative design: You may recall from the first lab that we offered two explanations for introducing analogies: Chemistry is often very abstract. The phenomena occurring are frequently not obvious. Using analogies helps to make connections between tangible, everyday, real-world phenomena and the often abstract, invisible processes that occur in a chemistry lab. Chemical solutions often are arrived at only after careful thinking. Analogies provide the time to reflect upon a concept during an experiment. This can help process information to develop insights about the chemistry. An analogy is a type of thought experiment. Another reason for introducing analogies is as a way of sparking the imagination. The following is a quote from an article called Exploring Multiple Solutions and Multiple Analogies to Support Innovative Design published in Design Computing and Cognition, Springer Science, 2011, pp Innovation is what drives new product development and engineering. Although some believe creativity cannot be invoked on demand, the presentation of appropriate stimuli greatly enhances the generation of concepts. Analogy is one type of appropriate stimuli that aids in generating new ideas. Analogy acts as a stimulus to generate new concepts and solve design problems. It can trigger breakthrough ideas. Using an analogy to solve design problems helps generate innovative and creative solutions. There have been many instances where analogies have led to breakthrough innovations, for example Velcro was based on a burr and the Speedo swimsuit was inspired from shark skin. 3

4 Pre-Lab Assignment In your lab notebook: a) Please write a 2-3 sentence introduction to the lab. b) Please construct a table of safety information that includes chemicals used in the lab and any safety handling precautions. Use the appropriate material safety data sheets (MSDS). In order to understand the chemical hazards in this lab, you need to carefully review all the chemical procedures. c) The following questions will be asked as part of your pre-lab questions: Please answer them online and record your answers in your lab notebook so you will have them available for the pre-lab discussion: Prelab Q1: Ammonia is formed from nitrogen and hydrogen according to the following balanced chemical equation: N H 2 2 NH 3 If 8.0 moles of H 2 reacts with 2.0 moles of N 2, what is the limiting reagent? a. N 2 c. NH 3 b. H 2 d. There is no limiting reagent. Prelab Q2: Describe the steps used to answer Prelab Q1: Prelab Q3: What is the number of moles of ammonia that could be produced in the reaction? a moles e. 4 moles b. 0.5 moles f. 5 moles c. 2 moles e. 6 moles d. 3 moles f. 8 moles At the beginning of lab, hand in the Pre-Lab Assignment and be prepared to discuss it. 4

5 Techniques for Capturing a Precipitate. Gravity filtration involves using filter paper to separate the insoluble solids (the precipitate) from a solution to obtain a clear solution (the filtrate) in a new beaker. a. For gravity filtration, you must first prepare the filter paper. Obtain a piece of 12.5 or 15 cm ashless filter paper. Fold the circle in half to form a semicircle, and crease the fold carefully (do not make too sharp a crease, as this will be likely to tear). Fold the semicircle in half again and you should now have a quarter circle. 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, which will allow the liquid to create a vacuum in the stem of the funnel as it flows downward. The figure below demonstrates the folding of filter paper. b. Fit the filter paper in a long stem glass or plastic conical funnel by opening the quarter circle into a cone. Make sure that the cone is opened in a way such that three pieces of paper are on one side and one is on the other side. Also, make sure that the torn corner is on the upper outside corner (this will ensure a good fit; it is critical that the filter paper fits snugly so that no product leaks and the filtration proceeds rapidly). c. Place the funnel in a ring clamp attached to a ring stand, and make sure that the stem of the funnel rests against the side of the beaker used to collect the filtrate (the liquid that will pass through the filter paper). Pour some of the solution into the funnel to wet the filter paper. Make sure that there are no air bubbles between the paper and the funnel. Then carefully pour the rest of the liquid through the funnel; do not allow the level of the solution to rise above the top of the filter paper. At the end of the filtration, there should be a quantity of clear filtrate in the beaker. It is normal for the solution to take some time to go through the filter paper. Depending on the needs of the experiment, you may discard the filter paper in the appropriate waste container or you may need to save it and dry it to determine the mass of the solid collected from a precipitation reaction. 5

6 Instructions before going into lab: You should work in pairs for this experiment. If there are an odd number of students in lab, then one group can have three people. You should write the name of your partner in your lab notebooks. Each pair of students should discuss and answer questions as they encounter them in this experiment. These questions are set off in the experiment (i.e. Q: ). The questions offer guidance during the lab. Answers to these questions do not need to be reproduced in your lab report. Each student is to record notes and observations and all other data in their own lab notebook. Each student is responsible for collecting the information needed to complete the Lab Report and Post Lab Portions of the Experiment. Goggles are required at all times in the lab. There are no exceptions. Gloves and foot covers are available. Materials and Equipment silver nitrate Chemicals Equipment and Supplies filter paper sodium chloride unknown solutions A/B or 1/2 (these are aqueous solutions of either silver nitrate or sodium chloride) long stem filter funnel 250 ml Erlenmeyer Step #3: various glassware GLASSWARE SHOULD BE CLEAN TO AVOID CROSS-CONTAMINATION As you have experienced in previous analogy labs, in Step #1 we observe chemical phenomena and collect data in the lab. In Step #2 we use an analogy activity that considers a concept that is familiar and makes a comparison to a less familiar chemical concept that was the focus of Step #1. The analogy activity involves mapping similarities as well as differences. For step #3, we ask you to design an experiment to obtain evidence on the concept of conservation of mass so a claim about it can be made. 6

7 Step 1 of our Learning Cycle: the following activities should be completed in the lab: Health Note: wear gloves to avoid getting these solutions on your hands; they can stain. In lab today, we will employ two different sets of solutions. Your pair will be assigned to one of the sets. One set will be lettered A and B while the other will be labeled 1 and 2. For each set, one solution will contain sodium chloride and the other silver nitrate. The concentrations of sodium chloride and silver nitrate in each of the solutions will be unknown to you. a. Circle and record in your lab notebook which set you have been assigned: A/B OR 1/2 b. Mix together into a single beaker approximately 10.0 ml of each of your assigned solutions. Record the volumes for each solution in your notebook. Please label the beaker as the "Reaction of A + B?" or "Reaction of 1 + 2?". Record your observations in your lab notebook. Q: What balanced chemical equation is consistent with the facts you know about the unknown solutions and your observations? c. At this point, you should let your solution sit for about 15 min. This is a convenient time for you to do the next activity. Please use the Step #2 Worksheet as you work on the next activity. 7

8 Step #2 Limiting Reagent Worksheet: this worksheet is to be used in the Step #2, Part 2 activity. Please have it available when doing this activity and refer to instructions in the activity for how to use the worksheet. Q: Interactions: (a) in the case of nuts and bolts what leads to a strong or weak interaction? (b) with a positively charged ion (cation) and a negatively charge ion (anion) pair, what leads to strong or weak interactions? Draw what a strong and weak chemical interaction might look like at the atomic level. Q: Why did we group Na + (aq) Cl - (aq) together? Q: What is the significance of the line ( ) before Na + (aq) Cl - (aq)? Q: Write out all of the chemical species present after running the reaction. Which species was your limiting reagent? Q: If there is a limiting reagent, what does this mean for some of the reactants that are in excess at the start of the reaction? Please explain. Q: If there is a limiting reagent, what does this mean for some of the reactants that are in excess at the start of the reaction? Please explain. 8

9 Step 2 of our Learning Cycle: the following activity should be completed outside the wet lab: Step 2 Part 1: For the next activity, we d like you to stay with your partner, i.e. work in groups of two. Please use the breakout room or somewhere you can sit together away from chemicals. a. Round robin reading and thinking out loud: take turns reading these examples out loud to each other. When finished, please ask the person listening to answer the questions, responding out loud also. Reading out loud to each other is not the same as reading alone, and has been shown to be beneficial to thinking about what you re reading. First person in pair: i. Simple Limiting Reagent Example. The idea of a reactant that runs out first or a limiting reagent is probably familiar to you. For example, let s suppose you purchase a package of hotdog buns, which contains 10 buns, and a package of hotdogs, which contains 8 hotdogs. If you buy one package of hotdogs and one package of buns for a cookout, it s rather straightforward to calculate which would be your limiting reagent (i.e. hotdogs). If you then purchased another package of hotdogs, the buns would become your limiting reagent. Second person in pair: Questions for listener: Can you write out an equation for making hotdogs from hotdogs and buns? What would the limiting reagent be if we purchased 3 packages of hotdogs and 4 packages of buns? Can you think of another example where the limiting reagent is straightforward to determine? Second person in pair: ii. Another Limiting Reagent Example. The idea of a limiting reagent is more complicated when there are different numbers of objects in what you need to assemble. For example, let s suppose we want to assemble ham and cheese sandwiches containing 2 pieces of bread, 3 slices of ham and 1 slice of cheese. If we purchase a loaf of bread with 18 slices, a package of ham with 22 slices, and a package of cheese with 10 slices, it becomes a little harder to keep everything straight to answer the question what ingredient limits making these ham and cheese sandwiches? One idea to make it easier to calculate the limiting reagent is to think about each ingredient of the sandwich separately. Then, we could write the following out: 1 loaf of bread with 18 slices of bread could make a total of 9 sandwiches (18 pieces of bread divided by 2 slices per sandwich). 1 package of ham slices with 22 slices could make a total of 7 sandwiches (22 slices of ham divided by 3 slices per sandwich we round down since we re making complete sandwiches). 1 package of cheese with 10 slices could make a total of 10 sandwiches (1 slice per sandwich). Surprisingly, the package with the most items (i.e. the 22 ham slices) is the limiting reagent! First person in pair: Questions for listener: Does it make sense that the highest number of items is the limiting reagent? Explain. Can you think of another example where the limiting reagent is difficult to determine? b. Staying with your lab partner, now please work on the next activity below. 9

10 Step 2 Part 2: Limiting Reagent. The activity this week uses the same nuts and bolts analogy that was used in last week s lab. Refer to the Analog to Target Worksheet filled out last week when doing the following activity. The Step #2 Limiting Reagent Worksheet has space to answer the questions asked below in this activity. To refresh your memory, fill in the assignments sheet below, using the same assignments made in last week s lab. Answer the following questions using the Step #2 Limiting Reagent Worksheet. Q: Interactions: (a) in the case of nuts and bolts what leads to a strong or weak interaction? (b) with a positively charged ion (cation) and a negatively charge ion (anion) pair, what leads to strong or weak interactions? Running the reaction with a limiting reagent. Last week, we ran the reaction with just the right numbers of "ions" to have the reaction go to completion. Now let us run the reaction with an imbalance in our reactants. First, let s consider all of the reactants we have after we place them into solution, but before any have reacted. Na + (aq) Cl - (aq) + Ag + (aq) NO 3 - (aq) Q: Why did we group Na + (aq) Cl - (aq) together? Q: What is the significance of the line ( ) before Na + (aq) Cl - (aq)? To run the reaction with an imbalance, select a certain numbers of NaCl and AgNO 3 pairs you d like to explore. Remember, one pair must limit the reaction! You can record this in your lab notebook in the form of: Na + (aq) Cl - (aq) + Ag + (aq) NO 3 - (aq) Now run the reaction with your nuts and bolts in a beaker. Run the reaction so that everything reacts as completely as it can. Q: Write out all of the chemical species present after running the reaction. Which species was your limiting reagent? Q: If there is a limiting reagent, what does this mean for some of the reactants that are in excess at the start of the reaction? Please explain. Q: If you were given someone else s beaker with nuts and bolts, how would you go about predicting which complex is the limiting reagent? Is the stoichiometry in the chemical reaction essential to your prediction? Please explain. 10

11 Step 3 of our Learning Cycle: reform your pairs. You may start this step in the breakout room before proceeding to lab. In Step 1, you mixed together solutions that contained different ionic compounds: sodium chloride and silver nitrate. One of the solutions limited the amount of precipitate that formed. Design an investigation to obtain evidence to determine a) the identity of your unknown solutions and b) which was the limiting reagent. You will have available to you various solutions of known concentration. Good luck! 11

12 Step #3 Designing Experiments Worksheet Hand in at end of lab You have been asked to design an investigation to obtain evidence to determine a) the identity of your unknown solutions and b) which was the limiting reagent. Lab Partners: Lab performed on date: Signatures: Before going into lab: Designing Experiments: Procedures, Observations, and Results Describe experiments & predictions: (attach extra pages if needed) Please describe your proposed experiments before performing any experiments Any predictions? We recommend explaining your experiments to your lab instructor before performing them. While in lab: Observations and results: (attach extra pages if needed) Describe data in lab and changes in procedures. Can you support any claims? While in lab: Any special issues to report? 12

13 AFTER LAB: Post-Lab Discussion and Assignments: Before starting the post lab discussion, groups should discuss the evidence regarding the identity your unknown solutions and which one was the limiting reagent. Q: What claims can be made? Q: Were their any issues with the way the experiments were designed or performed? Groups should discuss with the lab instructor and other groups how they designed their investigation and their claims. Particular care should be made to communicate how the experiments were conducted and the reasoning behind the analysis to make any claims. Groups may want to note variations in how experiments were conducted and if some experiments worked out better because of their design. Q: How do the activities in step #2 contribute to our understanding the concept of limiting reagent? Q: Can you list any limitations of the analogy used in step #2? Remember to hand in your Step #3, Designing Experiments Worksheet before leaving lab. Laboratory Report Instructions for completing your lab report are posted at: Here are some points to include in your lab report: a) An introduction, which includes a description of your experiment to identify your unknown. b) Your claim, underlined, following the introduction. c) The reasoning your group used to make your claim. d) Explain how the nuts and bolt analogy can be used to understand conservation of matter in terms of a precipitation reaction. Remember to attach the Step #2 Limiting Reagent worksheet to your lab reports. 13