Saved the Day! Pt 3 and beyond

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Objective

SWBAT interpret their lab results for percent yield and perform a basic error analysis before practicing their mass to mass conversions.

Big Idea

Most labs do not go perfectly, but good scientists acknowledge and analyze their errors to do better.

Background

This lesson is the third of three, today the students will be executing their lab procedures. The focus is on working safely in lab and filtering their product carefully, as students will be graded on accuracy.

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 experiment on the same topic where the students design their procedure and data collection. 

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:

  • HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • Science and Engineering Practice 3: Planning and carrying out investigations
  • Science and Engineering Practice 5: Using mathematics and computational thinking
  • Scale Proportion and Quantity Cross-cutting Concept: Scientific relationships can be represented through the use of algebraic expressions
  • Energy and Matter Cross Cutting Concept: Matter is conserved because atoms are conserved in physical and chemical processes.

Pre-Lab and Safety

10 minutes

When the class begins I distribute the packets of work back to each table, including their lab procedures.  I also provide each table a piece of filter paper.  Once everyone has their papers, I move to the lab part of the room to discuss important parts of the lab.

I hold up a piece of filter paper, and demonstrate how to fold the paper to make it fit into the funnel.  I walk the groups through it again so everyone has the paper properly folded.  I tell them to label it with pencil now, so they don't forget when they come into the lab.

I move to a lab station and show students what each chemical is packaged in, as well as the beakers at each station.  I explain how they will use a different piece of filter paper as their "weighing dish" for the lab.  I remind students when they are weighing chemicals to tare or zero out the balance after adding our "weighing dish."

Students ask about safety equipment, and I refer them back to step 1 of everyone's procedure "Put on goggles and apron."

I check for individual questions, and once they are answered, I invite students back to the lab.

Executing the lab

30 minutes

Attached is my Procedure which some groups used.  Most groups are using their own from our jumble sort.  Due to the potential variations in the procedures, my circulation of this lab is more oriented on safety and lab techniques.

I urge groups to check off steps of the lab as they complete them, both to keep their place and to make life easier for me if they have a question about what to do.

Students fawn over the yellow precipitate, and some connect it to having done it earlier in the replacement reaction lab.  When students begin filtering, some haven't wet the filter paper enough, and some precipitate leaks around the filter.  I ask these groups how they can re-capture that lost product.  Most have enough time to re-filter it, which will impact their percent yield.

When groups are in the middle of filtering, I indicate to them how they will leave the product at the side of the lab table to dry overnight.  All groups are careful to rinse as much of the precipitate into the funnel as possible.

As groups are winding up, I encourage them to write down a couple observations about what happened in the lab and how they handled any mistakes.

Closure/Clean-Up

10 minutes

As groups are finishing, I encourage them to wash their hands and properly clean up the lab.  Cleanup takes a few minutes this time as students have to scrub any remaining precipitate from the beakers.

I remind students to attach their data papers to their packets so we can't lose them.

I walk the lab to make sure everything is cleaned up and properly stored.  I found one group had thrown out their product by accident.  Luckily, we find it and get it set out to dry so they will have results to measure.

Post Assessment

Whether it was the jumbled procedure, or the simplicity of the lab, this was the smoothest day in lab all year.  Students worked efficiently and handled the adversity of the precipitate sneaking around like real problem solvers.

Two days later, students measured their precipitates and calculated their percent yield.  Most groups were 80% or higher.  Some groups as high as 95%.  For our first experience with quantitative labs, I was thrilled. 

More so, when I was prompting students to explain why they were under 100%, groups were able to analyze that there was precipitate still in the mixing beaker that didn't go into the filter, or stuck to the plastic of the filter when they cleaned up, or passed through the filter.  Students attributed the errors to actual lab techniques they had evidence for, versus just assuming "human error" or "the balance was off".  The error analyses were generally more insightful than the first lab I did with AP chemistry this year.

The language below is less than scientific, but shows their results and that the precipitate didn't all get into the filter.

This example is one of the best conclusions I had this year.