This is the first of two stoichiometry labs the students will do. This follows the AP guided inquiry model of a structured activity which is followed by a less structured scenario where the students will need to design their procedure and data collection.
This lab investigation involves a precipitation reaction to produce a target amount of lead (II) iodide. Students were introduced to the process the day before and began to calculate the amount of reactants to produce exactly 1kg of precipitate.
This lesson is the second of three, today the students will be scaling the calculations down to a lab setting and planning their lab procedures. Since this is the first lab students will need to design their procedure, I have taken a cookbook procedure, removed the numbers from the steps, printed it and cut the steps up. Students will have to sort them into the correct order. There are three variations on "correct" orders for the procedure.
This experiment is strongly guided, with all students using the same reaction, and being able to collaborate and check ideas.
This lesson addresses a plethora of standards:
When students come in, I return their table packets from yesterday to them. Students were expected to have created their objective, sequenced the calculations, and worked through the calculations: balancing the equation, finding molar masses, and performing two stoichiometric calculations to determine the amounts of reactants needed. I have checked their calculations, and most students are still not complete.
I ask students how they feel about this process. They respond "Confused" so we circle back to their task statement. They need to figure out the number of grams of EACH reactant. So I ask how many conversions they need to complete to be done, and they realize they need two.
I ask what their starting information in each problem will be. Students freeze, because they aren't used to working backwards. It is counter-intuitive that the product of the reaction would be the starting point for the conversion. I realize now that I needed to do more examples like this next year prior to the activity. One of the groups that did well on day 1 says "We need 1000g of PbI2, so that's our starting information" The other groups pause, and I ask a different group, "How are we sure that's the starting information, what is the criteria?"
"It has a number, unit and chemical."
This gets students unstuck for the calculations, and I explain that when they are done they need to get the jumbled procedure and sort it into order.
At this point, I release the groups back to work and start moving around the room to assist as needed.
When students are ready to move on, I have them read the beginning information and determine from the text and their calculations what it is they are proving in the lab. Students are tasked with finding the amount of each reactant to make exactly 1.00g of the precipitate. A common mistake here is that students miss that they are scaling down the lab by a factor of 1000, and the 720 g they calculated in the previous part now means .72 g for the actual lab.
The scale down is fairly simple:
Once students have their purpose statement written, I provide them with the Lab Procedure Jumble. This procedure has all the steps for the lab typed and cut out, then mixed up so students have to determine the best order. This technique came out from a discussion in my building chemistry team as we were trying to brainstorm a scaffold for the first time students are planning a lab. This is the first time I have tried this with my classes this year.
Students are immediately overwhelmed, so I ask them "What is the first and last thing we do in lab?" "Get goggles and aprons" "Cleanup" So students get those two tiles in place. I then ask them to look for things that go together and to group them with each other. Students do this by chemical (KI or Pb(NO3)2) or by action (weighing, pouring water, dissolving). There is room for variation here, and I have to guard against over-helping the students and directing them all to the exact same procedure.
When students believe they have the steps in the proper order to achieve their purpose, they need to check with me. If the steps are correct, I tell them so, and give them time to either write out the procedure or paste it onto their Filmstrip. The filmstrip is a pre-lab technique for students to process the steps of the experiment prior to executing the actual experiment. Students create a storyboard of the steps, either in text or via drawings. While one partner is writing the procedure, I tell the other partner to look at each step and create a materials/equipment list for their group.
Here are two student results from the jumbled procedures, they vary slightly in terms of how things are measured and combined, but not enough to influence the results of the investigation.
While students are working, I am circulating the room to answer questions and provide feedback to their sorting. I am also checking that students aren't overly frustrated. In recognition of this being their first attempt at planning a lab, I may provide a little more overt direction in this lab than in the next one, to ensure students don't quit out of frustration.
I give the class a 10 minute warning, and explain what I need before they leave the room.
I spend the last 10 minutes checking in with each group, figuring out their progress and where they are stuck. I give some directed help on the procedure sorts to help groups finish.
It is obvious some groups are not going to finish because the inquiry aspect of the lab overwhelmed them. I will provide them the Ordered Procedure for the lab so they can still work and complete the lab.
Most groups are growing in confidence about their ability to do theoretical calculations. However, they are skeptical that they will actually be able to make this chemical in the lab the next day. Also, since we have done some contextual investigations without the lab follow-up in the past (particularly in the nuclear unit), there is a disconnect in their minds between the pencil-and-paper activity of today and the idea that will be making the precipitate the following day. Most students do not remember already making it in the Replacement Reactions Lab in the last unit.