Understanding Chemical Equations
Lesson 3 of 10
Objective: SWBAT identify reactants and products and identify the number of atoms of each element in a chemical reaction.
This lesson is all about applying new knowledge. The previous day we introduced:
- Reactants and Products
- Synthesis and Decomposition Reactions
- Conservation of Matter
- Signs of a Chemical Equation
- Symbology of chemical equations
Today students will be processing these concepts. It is important to circle back to continue to build memory and pathways.
We open referring back to Conservation of matter, then move on to reactants vs products and synthesis vs decomposition. We then move to a refreshing of a skill: counting atoms in formula, which is a precursor to balancing chemical equations. We wrap up tying it all together. Today is purely seat work, but it comes after a very active day and gives myself and the students time to clear up misconceptions and cement concepts.
This lesson is aligned with HS-PS1-2 through its use of simple chemical equations. We also access Science and Engineering Practice 2 by using chemical equations as models for the reactions they represent. The Energy and Matter Cross Cutting Concept is accessed through our focus on conservation of matter.
This lesson is designed for a 42 minute period, to reflect the shorter day for my district's professional learning late starts. For a full 50 minute period, I would augment the closing by demonstrating the dehydration of sucrose in the fume hood with concentrated sulfuric acid. Care should be taken with this demonstration, as the final carbon column will still contain the sulfuric acid. I always do this in a large beaker (250mL+), as the carbon column can appear very phallic in smaller beakers. This is a good chance to build even more on the signs of chemical reactions.
When students enter, I have returned their demonstration notes papers from yesterdays introduction to chemical reactions. Additionally, each table has a copy of Probe #3, Burning Paper from Uncovering Student Ideas in Science, Vol 4 by Page Keeley and Joyce Tugel.
The probe puts forth a simple problem. A piece of paper is in a sealed glass jar, and ignited using a magnifying glass and the sun. Would the mass of the jar be greater, less, or the same after the paper is done burning? This probe helps me assess if students conceptualized our discussion of conservation of matter the day before. I give each table two minutes to discuss and come to a conclusion.
When the time is up, I ask for a show of hands to see how they responded. No one chose greater, but it is a fairly even split between the same and less. I ask students who put less why they chose it:
- The paper is gone
- The paper turned to energy
- The ashes are lighter than the paper
Then I ask students who chose the same mass for their reasoning:
- Burning is a reaction, like yesterday. Atoms aren't created or destroyed in reactions
- Matter is conserved in reactions
- The paper changed to gas and ashes, but all the atoms are still there.
I then ask the class to put their hands up if they think the mass would be less. Then if they would be the same. After hearing the reasoning based on our experiments of the day before most students have changed to the mass being the same after the experiment. I mentally note the holdout students who aren't convinced that mass is conserved to follow up with during the next activity.
As I'm passing out the Reaction Basics paper, I tell the class that we will be applying what we learned yesterday from the chemical demonstrations. Students will be identifying the reactants, products and type of each reaction.
We have not taught nomenclature this year, so students are expected to write the chemical formula with state of matter for the reactants and products. (I will use the names below, since the BL site does not support subscripts) We do the first one together.
- Reactants: Hydrogen peroxide (aq)
- Products: Water (l) and Oxygen (g)
- Type of reaction: Decomposition
We discuss how students remembered where the reactants were, that they are the starting chemicals that react. I tell them that the arrow "Points at the Products", using alliteration to help remember how to determine the products. Then I ask a student to give us the definition from the day before for both decomposition and synthesis, applying the definition to our reaction students determine it is one chemical breaking down, so it is decomposition.
Students ask about the NaI over the arrow, and I ask if they remember the elephant toothpaste from the day before. They respond yes and I remind them of having to add the iodine to make the reaction happen, but how the bubbles had a brown tinge at the end. I explain that anything above the arrow is needed to help the reaction happen, but does not take part in the reaction.
I may or may not use the term catalyst at this point, depending on how the class responds and if I feel it would be overload. If a student asks about the triangle over the arrow on the third equation, I explain it means heat, like when we needed the candle to start reactions the day before.
I explain to students that they will turn the paper in when they are done, and get the next assignment from the front desk. I tell them this should take no more than 15 minutes to accomplish, and then circulate among the tables to check student work, provide feedback, and answer questions.
Students will struggle the most with the difference between decomposition and synthesis. I recommend to some students to do all the reactants first, then all the products, then to find all the decompositions, and then all the synthesis reactions. This focus on one task at a time is a useful scaffold to many students who are having difficulty learning while switching from task to task.
During this time, I check in with the holdouts from the opening activity, and ask them to pick an equation on the page and count how many atoms of each element are on both sides of the equation. When they begin to see that the number of atoms are still the same, I refer back to the bellringer and point out that if the jar is sealed, all the atoms are still in there, and ask why it would weigh less. This helps them overcome the resistance to the idea that gases are lighter. I point out that if the jar were open, they would be right, since gas would escape.
When students complete the Reactant and Product identifications, they move on to counting the number of atoms in a chemical formula. I use the Physical Science Worksheets from Instructional Fair as a resource here.
Page 51 is entitled Number of Atoms in a Formula, and students must break down chemical formulas to represent how many atoms of each element are in it. This is a precursor skill to balancing chemical equations, and something we did on a limited basis first semester when discussing chemical bonding.
For simplicity's sake, I crossed off the hydrates when copying the sheet. My focus is on simple formulas and those with parentheses around polyatomic ions. When students are ready, they pick up the sheet and work on it. I do not explain this at all upfront. If a couple students at a table are struggling, I explain it at that point. Once everyone has transitioned to this sheet, then I stop and we do two together on the white board.
We break down sulfuric acid, first by element: reminding the students that every capital letter begins a new element. So we have hydrogen, sulfur, and oxygen. Next we extend to the subscripts giving us the number of each: two hydrogen, one sulfur, and four oxygen atoms.
The next we break down is calcium phosphate. First breaking out the elements, Calcium, phosphorus and oxygen. Then dealing with the subscripts and the parentheses. I ask students what they do with parentheses in math class, and they reply "Distribute." So we distribute the subscript through the parentheses only, giving us three calcium, two phosphorus, and eight oxygen atoms.
Armed with these two examples, I let the students work on the sheet until there are 5 minutes left. If anyone is not done, they can take it home and finish it for homework. While students work, I circulate the room and check in with students and assist where needed.
Common mistakes are breaking apart two letter elements, for example, Cobalt gets split into carbon and oxygen. The reverse is also common, as students put three cobalts instead of one carbon and three oxygen for the carbonate ion. Students hesitate with the polyatomic ions and distributing through the parentheses, so they need some gentle prompting to treat it just like they would in algebra.
To wrap up, I write the equation for the dehydration of sucrose on the board using the chemical formulas.
Sucrose --> 12 carbon atoms and 11 water molecules
I then call on tables at random to answer the exit questions.
- What are the reactants? Sucrose
- What are the products? Carbon and water
- What type of reaction is it? Decomposition
- How do we know it is decomposition? Single reactant, multiple products
- Was matter conserved in the reaction? Yes
- What evidence is there that matter is conserved? Same number of each element on both sides of the equation.
- How might we know this reaction is happening in front of us? Energy given off, color change, bubbles form.
I inform students we will be quizzing on these topics in two school days, so they have time to study and prepare themselves. I ask for all completed work before they leave the class.