The previous day, students did a short lab where they performed a single replacement reaction of aluminum with copper II chloride. Today, we will build upon that and students will learn about replacement reactions.
In a manner similar to that in which I introduced synthesis and decomposition reactions, students will observe demonstrations and then record some notes. They then will have some independent practice time.
The first demonstration is the reaction they carried out the previous day. I have the chemical waste from all four classes yesterday to show them the final results. This also reinforces the idea of potential chemical leftovers from our PhET simulation as there is a significant amount of aluminum foil remaining in the waste beaker.
The second demonstration combines 3 mL each of colorless, aqueous silver nitrate and colorless, aqueous sodium (or potassium) chloride to create a white precipitate of silver chloride. Students have done precipitate reactions already this year, but have not had to name the process, nor to connect it as a sign of a chemical reaction.
This lesson is rooted in HS-PS1-2 where students should be able to understand the outcome of a simple chemical reaction. We will focus here on "like replacing like" with metals replacing metals and non-metals replacing non-metals.
We continue to build student understanding of chemical equations as models for what is happening at the molecular level (SP2) and that matter is conserved in these reactions (Energy and Matter Cross Cutting Concept).
For the demonstrations you need:
When students enter the room, I have the Reactions Part 2 paper on their desk. I ask them to fill in the review at the top, with our states of matter notation for chemical equations. While they are doing this, I pass back their quiz from the previous day, giving some verbal feedback to accompany the written feedback.
I remind students that they can correct their quizzes on a separate sheet of paper for half the points they missed. Since most of the incorrect answers were more inconsistencies than errors, about 50% of students took advantage of this opportunity.
We then reviewed our symbology, that (g) stands for gas, (l) for liquid, (s) for solid, and (aq) for aqueous, or dissolved in water. I remind them of the lab the day before, that they started with solid crystals, and then dissolved them in water before reacting with the aluminum.
I ask them to look at the two reactions in bold on the top of the paper. I ask what they have in common that is different from the synthesis and decomposition reactions we have already learned. If students are struggling, I encourage them to get out their practice paper from three days ago to review how synthesis and decomposition reactions look.
Once students have the papers side by side, a student notices that in the replacement reactions, there is a plus sign on each side of the arrow. I ask the rest of the class to focus on that, and ask how many chemicals are on each side of the reaction in the replacements. Students respond "two" and I ask them to write that on the first line. I repeat the sentence, that replacement reactions always have two chemicals on each side of the reaction (a local limitation we impose on our instruction and assessments, more complex replacement reactions with more than two products are left for AP where they are classified as Acid/Base, Precipitation, or Redox reactions), whereas synthesis and decomposition reactions will have a single chemical somewhere on the reaction.
I then start the aluminum and copper II chloride reaction that students did the day before. While it reacts, I ask them what they notice about patterns in the chemical equation. Students struggle until I pass around the waste beaker from yesterday's lab and ask what is in it. Students recognize the solid copper product now, and the solid aluminum excess. A student first states that there is a solid at the start and at the end. I respond that while that is true here, it's not true of ALL single replacements.
Another student refines the observation that there is a plain element by itself on both sides of the reaction. I stop the class and point out that this is the KEY to single replacements. A single element replaces another single element. Since we are close to the winter dance, I provide the analogy of a couple (copper II chloride) going to the dance. When they get there, chloride notices how much hotter aluminum is than copper. Chloride leaves copper and hooks up with aluminum, kicking out copper. The kids laugh and make some side comments about chlorine's character.
Next I focus them on what type of element aluminum replaced. I ask them to look on the periodic table and tell me what type of element aluminum is. Students who still have their color coded periodic table quickly reply that it is a metal. I have them also find copper, and they realize that metals replace other metals. I have them extrapolate and ask what they would expect to replace a non-metal, and they reply non-metals. I ask them to check that they have completely filled in their single replacement portion of the notes.
Now we focus on the double replacement reaction. I use the winter dance analogy again, and describe two couples coming to the dance, and leaving with each others' dates. Kids laugh, and some side comments are made, but again, the analogy helps them make sense of the reaction. So we look at the equation and try to figure out who silver switched with. Students notice it is now with chlorine, so it must have switched with potassium. I ask someone else to prove that correct, and another student points out that potassium ends up with nitrate, who came to the reaction with silver.
I then show students the chemicals. I used sodium chloride instead because it was pre-made in my storeroom, but I don't bother telling each class since the visible results will be the same. I ask them to describe each chemical, and they say "clear, like water." I then ask a student to come up and mix them. When they mix, the bright white silver chloride precipitate forms. Students get confused, and I pass around the test tube for them to see it.
I ask what kind of chemical was made, and someone says "solid" so I ask what they are seeing based on the equation, "Which chemical is solid?" "AgCl"
As students are passing the test tube around, the precipitate is settling to the bottom. I ask students what is happening and they say "its falling to the bottom." I ask what the generic term is for when rain or snow falls from the sky and they respond "Precipitation". I write "Precipitate" on the board and explain that the word means "to fall," so our precipitate is a solid that forms out of solution and falls out of the liquid. I point out that bubbles forming or precipitates are classic signs for double replacement reactions.
We move down to the practice. I have all the students write the reactants and products of the first reaction. I ask them if they have single elements in their reactants or products. They respond that they do, so I ask "Does that mean a single or double replacement if we have SINGLE elements?"
Students begin to link the single:single pattern, and label the reaction as a single replacement. I ask if a precipitate formed: a solid from only solutions. Some students respond yes, and others no. Students who said yes indicate that there is a solid in the product. Students who said no point out that both reactants were not aqueous. I agree with the second group, and point out that since the reactants were not solutions, we don't consider the silver in the example to be a precipitate.
I give them time to work on the second equation, and we check it in. Students correctly note there are no single elements, so it is a double replacement, and that the silver chloride is a solid product from aqueous reactants, so it is the precipitate.
I give them the next twenty minutes to work. While they work, I am circulating the room, especially to check on the students who confused reactants and products on the quiz and ensure that they are doing them correctly today.
As expected, students are struggling the most with the concept of precipitates as seen below. Tomorrow's lab will show them a good example, so I am hopeful that with the visual from lab, they will be able to seize better on the concept.
With five minutes left, I pass out the Replacement Reactions Lab to the class. I tell them that we will be doing four experiments involving the replacement reactions in stations tomorrow, or whenever we come back (as we are expecting -40 degree wind chills and expect to be closed). I ask them to read each procedure, come with any questions about the procedures and to be ready to work in lab the next time we have class.