LA + B --> L + AB

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Objective

SWBAT work safely in lab to identify single and double replacement reactions.

Big Idea

Knowing the type of reaction helps in predicting the outcome.

Background

This lab is to give students experience with single and double replacement reactions (HS-PS1-2), and to also become more familiar with the concept of precipitates.  It is a very teacher-intensive lab in terms of cleanup, because you need to do a lot of filtering of the waste products.  It took an additional three days of filtering during my prep periods to completely dispose of the waste.

I had an error in my planned experiments: Experiment 4 did not produce a precipitate, but made an activated complex with a deep color change.  I recommend using barium hydroxide and sodium sulfate instead, which will give you a strong white precipitate of barium sulfate.

This lab allows students to carry out an investigation (SP3) as to the results of the reactions.  They also will be deciding on the type of reaction based upon the reactants provided and the evidence (SP7) from their reaction.  Due to another day off from inclement weather, this lab ran on a 42 minute period instead of 50.  In a full 50 minute period, the extra ten minutes would have given students more time in lab, and I would have extended each station to 10 minutes.

I have the students perform the lab in stations, to reduce contamination of waste products, and to make it easier to transport from room to room.  Since this is a non-sequential lesson, the narrative below will be station by station.

For experiment 1, you will need:

  • .25 M silver nitrate
  • Small pieces of copper (I use foil, but wire works as well, so long as it is uncoated)
  • Large test tubes
  • Test Tube rack
  • Waste beaker

For experiment 2, you will need:

  • .1M potassium iodide
  • .1M lead (II) nitrate (I present this in a dropper bottle to control amounts)
  • Large test tubes
  • Test tube rack
  • Waste beaker

For experiment 3, you will need:

  • 1M hydrochloric acid
  • Mossy zinc
  • Large test tubes
  • Test tube rack
  • Waste beaker

For experiment 4, you will need:

  • .5 M barium hydroxide
  • .5 M sodium sulfate
  • Large test tubes
  • Test tube rack
  • Waste beaker

For cleanup post lab, the leftover acid from step three will need to be neutralized before disposal.  You can decant the liquids from experiments 1-3 to reduce filtering time of the solid products.  All solids can be disposed of in the garbage.  However, I keep them a couple of days to show students how the solids can be collected and stored, particularly the silver from experiment 1.

Lab Prep

5 minutes

When class begins, I ask students to get our their Replacement Reactions Lab sheet.

I point out our two safety concerns, the stain potential of the silver nitrate, and the corrosive nature of the hydrochloric acid.

I ask for questions, and the first one is "How do we measure out centimeters of liquids?"

This is a great question, and something I have them do for labs which are purely qualitative.  Since the measurements don't matter, I don't want students wasting time trying to precisely measure amounts in a graduated cylinder, or rinsing the cylinder between every measurement.

I ask all students to hold up two fingers together, and explain that each finger is approximately a centimeter wide, so when the instructions call for "2 cm" of solution, they can be approximate.  I tell them that all places where cm is used, they do not have to be too concerned with precise measurements.

I remind students of our steps, record observations before, during and after the reaction.  I then point out the station rotation, and tell students they will be staying on the same side of the room the entire lab.

I then release students to get goggles and aprons and come to lab.

Reaction 1 & 2

16 minutes

Students immediately sorted out the materials and began their Pre-Reaction observations.

Many students had difficulty differentiating between a "Reaction Observation" and a "Post-Reaction Observation", particularly for the very fast precipitate reactions.  I had to explain to students that they needed to let it sit a minute or two before making their final observations.

This is the third time this year students have seen or done the lead (II) iodide reaction, but students are still enthralled with the bright yellow precipitate.

Below is a sample of student work, you can see the observations are somewhat incomplete and unclear.  This will be the focus of the next lesson.

Reaction 3 & 4

16 minutes

Students are wary of the hydrochloric acid.  I bring over a funnel to help them pour it, and remind them that is about 7 times stronger than their stomach acid, but not enough to hurt them badly if they spill.  I try to get them to respect the chemicals, while not fearing it.

This is a video of the reaction, you can hear my student more concerned with why I am videoing their work instead of observations.

Student observations again were poor for these steps.  Many students did not feel the side of the test tube, and missed the heat generated by the acid and zinc.  They also tended to miss the fact that it bubbled, as seen above.  Students also did not label the shade of blue in experiment four, so the "color change" read as blue to blue.

Students were slightly short on time, so they had some difficulty in completing the types of reaction.

Closure/Clean-Up

5 minutes

As students are finishing, I stop them and make sure that everyone washes their hands today after handling the hydrochloric acid and the silver nitrate.  I ask students to work on labeling the type of reaction for each experiment for homework, using their returned practice from the day before as a model.

Students did not generally complete the types of reactions, so we went over them the following day.

Overall, I was very unhappy with the observations my students were making, so the next lesson will be on how to make better observation skills, repeating the lab sheet on a new paper.