This week began what has become an annual tradition for my AP Chemistry students. Summer Lab! This activity counts as their summer assignment for class. Modeled after the activity that College Board uses to train new AP Chemistry teachers at summer institutes here in Baltimore (thank you Jon Hnatow!) the students do the infamous green crystal lab.

I first ran this activity after attending the APSI at Goucher College in 2011. I had never done the green crystal lab and found it to be a great example of simple lab skills, and a good review of some aspects of chemistry that incoming AP Chemistry students should already have under their belts (stoichiometry, molar dilutions, empirical formulas, formula writing, and redox reactions). This lab experience also checks off several of the old “required” labs that were a part of the AP Chemistry curriculum.

The experience starts before the end of the school year with the Summer Assignments meeting. Students are given the lab handout, a carbon-less copy lab notebook, and the dates for the summer lab. They choose to attend the lab experience or an alternate assignment. Summer lab runs from Monday through Friday, begins around 8:30 AM, and usually ends by 1:15 PM. Students form lab groups (usually pairs) and work together to analyze an iron oxalate compound. The schedule for the week looks like this:

• Monday – Synthesis of the Compound, Introduction to the lab
• Tuesday – Harvesting & cleaning of the crystal, Standardizing NaOH
• Wednesday – Dehydration analysis to determine the % of water, Standardizing KMnO4, Determining the % of oxalate
• Thursday – Determining the % of K+ and Fe3+ through titration
• Friday – Analysis of data

Tuesday afternoon the students also have the opportunity to tie-dye t-shirts. This is a tradition inspired by one of my favorite chemistry professors, who always wore a tie-dyed shirt to class. It makes for a fun team-building activity.

On Wednesday the testing of the crystal really begins. The students determine the percent of water using small samples in crucibles and a drying oven. The nice thing about these crystals is that they change color, from dark emerald green to a light green, when they dry out. Makes it easy to see that the dehydration is working.

Wednesday also increases the skill set with regards to titration. Students titrate a redox reaction between oxalate and permanganate. This one is more challenging than the Tuesday titration for standardizing the NaOH. The redox titration is heated, and acidified. I start the morning, after the first crystals hit the drying oven, with a review of balancing a redox reaction in acid. Then we can connect how water is involved, and how we make this reaction acidic by adding a small amount of 6M sulfuric acid, and 85% phosphoric acid. The students take great care with this titration, as it requires true teamwork to maintain a constant 85-90°C temperature, and hit the color change.

Thursday ratchets up the titration skill. This titration focuses solely on pH and the graph generated by the titration. We use an ion exchange column to substitute H+ ions for the K+ ions in the crystal. Then the students begin a detailed titration using Vernier data acquisition technology, pH probes and LabQuest 2 units. While the ion exchange columns are being re-charged with some 1M HCl, the students gather to learn how to graphically determine the equivalence points on a titration graph with two inflection points. This technique will be used on their own experimentally generated graphs to determine the % potassium and % iron in the crystals. At the end of the day’s work the students are presented with a titration master certificate that they can use in their college portfolios.

Friday is the last day of the summer and the students use their experimental data to determine the empirical formula of the iron oxalate crystal. This part becomes a typical empirical formula problem, where each component is given in percentages, and they calculate the mole ratios for each component. The students turn in their work for each day, showing the calculations for each molarity or percentage that is determined experimentally, and the final empirical formula calculations with a percent yield on the crystal.