Today's lesson consists of a quiz on the material from previous lessons, especially the parts of a chemical equation and counting the number of atoms in a formula. These are the precursor skills to balancing chemical equations, so I like to teach them early and reinforce them well in advance of balancing equations.
The second part of the lesson is a mini-lab where students will be dissolving copper II chloride into distilled water, and then adding aluminum foil. The aluminum replaces the copper in the solution and the copper precipitates out. The reaction has a strong color change and evolution of heat, and is a nice bridge into replacement reactions, which is our next topic. The chemicals are fairly safe, and the change is dramatic, so you will notice the pre-lab is much less involved here.
In the lab, students will carry out an investigation (SP3) and then argue from the evidence (SP7) of their lab data whether or not the various steps were chemical reactions. This lesson also has an alignment to the Scale, Proportion and Quantity Cross Cutting Concept, as all chemical labs are macro-scale observations to allow us to mentally model what is happening at the nanoscale.
While this lab isn't as explicit in the scale differences, I am hoping students will discuss how the atoms must be re-arranging from the evidence of the aluminum foil reacting and producing solid copper. If I am not seeing the arguments I was hoping for, I will tweak the analysis questions for future years to focus students on that aspect of the lab.
Lab Prep (per table):
When students have entered the room, I remind them that is has been two school days since they were warned of their quiz. This elicits some choice feedback, given our weather day off and late start in between. I promise students a chance to look over their notes, but need to explain what we are doing after the quiz.
I walk to the back of the room and explain that after the quiz they will get goggles and aprons to do our mini-lab. I show them how the copper II chloride crystals are packaged in jars, and how to get a spoonful to react. I remind them that they MUST record their observations of all the chemicals before beginning so they can accurately talk about the changes they observe.
I then give the students about five minutes to quiz each other or look over their notes and previous assignments prior to the quiz. Most students take advantage of the time given, especially those who happened to miss the day before and are out of sync with school. I make sure to pass back all papers to students who have been absent.
After five minutes, I ask students to put away all papers and keep out a pencil and a periodic table if they need one.
Students take between five and twenty minutes to complete the quiz. The quiz is focused on the difference between reactants and products. This is asked three different ways on the quiz. Many students get the first and last correct, but when it comes time to write an equation in order, they flip and put the products first.
Students do well with the number of atoms when there are no parentheses. My students who are in a lower track math class consistently only apply the number outside the parentheses to the closest element. For example, in Calcium hydroxide they label as 1 calcium, 1 oxygen, and two hydrogen instead of 1, 2, and 2. I notice some students still struggling with identifying elements, looking for NH instead of N and H separately.
When I grade the quizzes, I'll be watching to see which students are making these mistakes and provide feedback such as "Every capital is a new element!" or showing the number outside the parentheses distributing into each element in the formula.
When students finish, they transition back to the lab. Students handle this transition better than I expected, as we haven't yet done a lab, but many have a station they used last semester that they gravitate back to.
When students finished their quiz and turned it in, they picked up their Reactions Mini-Lab investigation sheet. Students seem to be very comfortable with the steps of the lab, especially with taking the time to write their observations directly embedded in the procedure.
Having procedure step 5 is really important to my students. Too often, they just write down what a reaction looks like AFTER, and not how the chemicals looked before. By forcing them to write the observations out now, they don't have to worry about it later. This also provides students who finished the quiz really quickly a chance to work ahead and let their lab partner catch up later.
Step 3a and 4a are also important. This semester of chemistry begins to introduce more quantitative processes. Rather than having students record that the reaction "got warm" we have them measure the temperatures, so they will be able to say how warm it got. This same lab can be repeated later in the year when we do thermochemistry to measure the heat of reaction.
Students working through the lab can be observed below:
When students are finished and have cleaned up, they work on the analysis questions. The questions are focused on using the signs of a chemical reaction to determine which steps of the procedure contained chemical reactions. Prior to teaching this, I needed to decide if I was ok with students labeling dissolving as a chemical reaction.
I decided to see what their evidence was: many students cited the color change of the liquid as a sign of a reaction. When they did, I wrote feedback to ask if the crystals changed color as they dissolved. When I observed students writing this, or was able to insert that question into their partner discussions, most students decided that since the color was approximately the same as the crystals, there was no color change and therefore no reaction.
The leader of my AP conference two summers ago discussed that dissolving could be justified as a reaction as bonds are broken in the ionic compounds and you may get an energy change, either endo- or exothermic. This level of nuance is beyond all but my most advanced students, but I might expect an honors student to ask about it.