Introduction to Chemical Reactions

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Objective

SWBAT explain that reactions involve the rearrangement of atoms into new substances.

Big Idea

Matter is conserved in chemical reactions; synthesis and decomposition are two types of chemical reactions.

Background

This is the first lesson on chemical reactions.  The key idea regarding chemical reactions in the NGSS is that matter and mass are conserved, and that atoms are merely rearranged.  The Performance expectation reads: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.  This is a shift in thinking from the past, where we taught the Law of Conservation of Matter, but didn't have it as the key idea.

This lesson relies on some modeling (SP2) done by the teacher and interpreted (SP6) by the students.  I am approaching chemical reactions from a constructivist mindset, showing examples of reactions and visual models of how the atoms are being rearranged.  I believe the demonstrations are essential here, because our focus in first semester on the atom, periodic table, IMFs and nuclear chemistry has meant that students have not done many reactions.  By performing the demonstrations, students have a frame of reference to understand the discussions about the chemicals.  In this activity, I am showing balanced equations and models, despite us not having yet discussed balancing chemical equations.

The four reactions need the following materials:

Reaction 1:

  • Bunsen burner
  • Match/striker/lighter

Reaction 2:

  • Hydrogen gas in a balloon, taped to a meter stick (we have a tank of hydrogen, but in the past I have generated the hydrogen via the reaction of zinc and hydrochloric acid.)
  • Candle taped to a meter stick
  • Matches/lighter

Reaction 3:

  • 30% Hydrogen peroxide
  • Liquid dish soap
  • Food coloring (optional)
  • Sodium or potassium iodide, solid or solution.

Reaction 4:

  • Ammonium dichromate crystals
  • Magnesium strip
  • Ring stand
  • Wire gauze
  • Ring clamp
  • Large tray
  • Bunsen burner and igniter

Opener

8 minutes

When the period begins, I have a question on the board: "What is a chemical reaction?"

I ask students to brainstorm on their own and write a definition in their notebooks, taking no more than two minutes.  While students are brainstorming, I enter attendance and collect paperwork (safety contract, syllabus agreement) from my new students for this semester.

After two minutes, I ask the students to share their definitions at their table, and create a table definition of a chemical reaction.  I give them three minutes to share out and synthesize a table definition.  While students are discussing, I circulate the room to listen in on their conversations and read their definitions.  Most groups have a variation on "its what happens when you mix chemicals."

On observing this, I realize that my first demo will need to address that misconception, so I mentally alter it while passing out today's Introduction to Chemical Reactions Investigation Sheet.

I explain our format today, that I will be doing some demonstrations with help from them, and that we are focused on three questions:

  • What is a chemical reaction?
  • How do we describe chemical reactions?
  • What types of reactions are there?

While students are putting their names on their papers, I get on my safety equipment for the demonstrations.

Reaction 1: Burning Methane

8 minutes

Before beginning, I ask students to look at the pictures and equation on their paper.  I ask someone to describe what is going to happen in the reaction based on the picture or the equation.

In two of my classes, students questioned my use of the word equation.  At this point, we reviewed the symbols, specifically that in chemistry the arrow indicates "equals" as well as "produces".  Some students wrote this information down, others simply put an equal sign over the arrow for reference.

A volunteer interprets the picture to mean that when we mix the methane and oxygen, it will react to make carbon dioxide and water.  I ask if the class understands why she said that, and ask her or another student to explain the diagram and their thinking.  They point out that on the left side the methane and oxygen are mixed, and that it creates carbon dioxide and water after the arrow.

I ask if the class is ready to mix the chemicals, and then when they are, I turn on the gas.  And I wait....and wait...and wait.  The students get frustrated and ask if I'm going to light it.  I shut off the gas and ask "What do you mean light it?  You only told me to mix them."  This slight pause allows us to talk about the misconception from the opener: that chemicals react automatically when mixed.

When students tell me they need to be lit, I re-open the gas and light the burner.  Now we begin to make our observations and interpret our results.  I ask how they know there is a reaction happening.  Students indicate the heat and light, as well as the sound of the burner.  I prompt them to find a term that includes heat, light and sound.  It may take a minute, and if we are stalled I'll draw a quick game of hangman on the board to get the term energy.

I shut off the burner, and ask students to look at the next two questions on their page using the drawings.  Some students will reply yes, because they are mistaking compounds for elements.  When they focus in on the atoms on each side of the equation, they realize that the atoms were not created or destroyed.

I provide students the terms reactants and products for their page, if no one can provide the terms from their own prior knowledge.

Reaction 2: Synthesis of Water

7 minutes

As we move on to reaction 2, I point out that this reaction will represent a type of reaction that is called either synthesis or combination.  I ask for a student volunteer to help me with the reaction.  If a student who is retaking the course volunteers, I choose them.  This is to help hook them in the class.  Knowing they are taking the class a second time, they will already be cool to the idea of chemistry so it is crucial to get them involved and active.

I provide the volunteer goggles, and explain the reaction to the class.  I point out that the balloon contains hydrogen gas, and ask where the oxygen will come from.  Students respond "the air around the balloon."  I ask if just popping the balloon would be enough to have them react.  The students hesitate, and I have them think back to the methane.  They state that it might need energy to start.

I explain to the student volunteer that they will provide the energy.  I light the candle, and hand it to the student volunteer and ask them to back up.  I hold the meter stick with the balloon and ask the class if they are ready.  This year more than any other, students have videoed my demonstrations for social media, so if students are getting their phones out, we pause to let them do so.

I have the volunteer bring the flame to the balloon.  The fireball it produces is both hot and loud. 

When students catch their breath, I ask them to write down how they knew it reacted.  This time, students write "Lots of energy" immediately.  I have them attempt to identify the reactants and products based on their example above.

Now I ask students what happened to the reactants, and ask them to think before responding or writing.  I won't accept "they reacted" or "they blew up".  I may project the diagram on the screen with the document camera if needed.  We look at the diagram and how the atoms are arranged on the reactant side.  I have students focus on just the hydrogen, and they point out that hydrogen is with hydrogen, and then oxygen is with oxygen.

I then ask how that's different from the product side, and they point out that they oxygen atoms split apart and the hydrogen atoms are attached to oxygen now instead of each other.  I refer back to the opening- "Have we created or destroyed any atoms?"  "No" "Then how would you describe what happened?"  "They got remixed/rearranged/split and reattached"

I seize on this idea and spin it into an analogy: legos as atoms.  You can take one lego set and build multiple things, but before you can make something new, you have to take apart the old structure.  It takes energy to pull the legos apart, and that's why we needed the candle, to get the hydrogen and oxygen apart so they could come back together.

I pause for questions, and then direct students to the bold writing: Synthesis or Combination.  I ask why this reaction would be called synthesis or combination.  A student replies "You combined the hydrogen and oxygen into water."  I ask them to then make a more generic definition, without the chemical names.  A student will volunteer "two chemicals become one chemical?" and I excitedly say yes, but alter it to be "two or more chemicals combine to make/synthesize one chemical."

Reactions 3 + 4: Decomposition of Hydrogen Peroxide and Ammonium Dichromate

17 minutes

We flip the paper over, and look at the term Decomposition.  I ask what it means, and get the following responses:

  • To rot
  • To decompose
  • To break down

I acknowledge all the responses, but pounce when I hear break down.  I ask students what they think of when they hear the term decompose, and they mention bodies.  I point out that rotting and decomposing are just other ways to say breaking down.

I ask them to look at the chemical equation, and to tell me what is happening.  Students are getting more comfortable, and they point out that the hydrogen peroxide is breaking down to create oxygen and water.  I ask how many have used peroxide to treat a scrape.  I mention that the bubbles they see are oxygen, and part of how peroxide cleans a scrape is to move the dirt out with the bubbles.

I then begin my back story for the elephant toothpaste demonstration.  The story is 90% fabricated, but draws students in to the demo, and allows for the reintroduction of periodic law from last semester. 

I begin by asking students if they are familiar with Lincoln Park Zoo on the north-side of Chicago.  I extol its virtues, being free, near public transportation, and the beautiful architecture of older buildings.  I mention that my wife's cousin's fiance had an internship there, so he would get us behind the exhibits sometime (the lie that underpins the story).  I explain he was a biology major and was working with the pachyderms and brought us into the exhibit to meet the elephants.  He had been getting under my skin that day, so I pushed him a little about the fact that the elephants tusks were yellow "Like they smoke three packs a day."  He gets huffy and challenges me to make elephant toothpaste since I was the chemist.

Not being one to back down from a challenge, especially from that guy, I went back to school and started brainstorming.  I ask the students "What is in whitening toothpaste?"  "Baking soda."  I talk about that, but how I was afraid the grittiness of the baking soda might irritate the elephant.  "Hydrogen peroxide" is the next response, and I mention how as a science teacher I can order 30% peroxide, versus the 3% at a drugstore.  I pour 30mL of peroxide into my 1000mL graduated cylinder.

Next I think about how to get them clean, and recalled that ivory was used to make expensive place settings, so I add some dish soap in to the peroxide.  Next, since we are the Cardinals, I have to add red coloring to my creation, and add 4 drops of red food coloring.

Finally, I can't figure out what I'm missing, and ask "What else is in toothpaste?"  "Fluoride!"  I reply, that's true, but fluoride can be a nasty chemical in its pure form, so "Remember how elements in the same group behave the same?  I skipped chlorine, because it can smell nasty, and bromine is a brown liquid, not suitable to cleaning yellowed tusks.  However, iodine, might do the trick."

As I hold up the bottle of aqueous sodium iodide, I tell the students "I finally had it, and I went back to the zoo to show him.  He was astonished that I followed through, and wanted to see it.  My competitive side got the best of me and I said 'I even hooked it up, since you're clearly too weak to squeeze out the toothpaste from an elephant sized container yourself, I made it self dispensing'" and I add the sodium iodide to start the reaction.  Students love both the spectacular reaction and the dovetail to my story.

I perform the demonstration for the class, but was unable to capture it on video.  This is a sample video from a newscast to demonstrate the reaction.  Flinn Scientific has a longer video explaining how to do the demonstration here.

When the demonstration is done, I go back to our signs of a reaction, and this time in addition to heat, students add "bubbles/gas" as a sign of a reaction.  I have them answer everything up to the definition of a decomposition reaction as a table, and then we define decomposition based on this reaction.  Students define it as "a chemical breaking down into two or more chemicals, the opposite of synthesis/combination"

The final reaction is one of my favorite demonstrations, and is excellent for demonstrating cinder-type volcanoes in an Earth Science class.  I perform this experiment under our room exhaust fan, as the product can flutter about otherwise.  It can be performed in a fume hood, but the hood limits visibility.

I have students gather around the center lab table and pour the orange ammonium dichromate crystals onto the wire gauze.  I ask them to describe the reactant, and they indicate that is is orange, solid and small crystals.  I then insert a small strip of magnesium into the peak of the pile to serve as our fuse.  I ask students about what happens when they burn pure magnesium, and someone remembers the bright white flame from last semester.  I ask for a student volunteer to light the magnesium, and caution the class to avert their eyes while the magnesium burns.

When the magnesium is done, the ammonium dichromate is decomposing, throwing off sparks and deep green, fluffy crystals of chromium (III) oxide.  When the reaction is over, we turn the lights back on and let students observe the product.  When we discuss signs of a reaction, students add color changes to their observation of energy.

Note:  The chromium (III) oxide is a pain to clean up.  I always do this demonstration over a large tray so I can just dump the product into the garbage can at the end.

Drawing Conclusions

10 minutes

Now I give the students the remainder of the period to answer the summary questions at the bottom of the page.  Today they learned quite a bit, and still may need to struggle through interpreting the symbols of chemical equations.  When students are struggling with that part, I have them think about each demo, and why would one chemical have been labeled with a G, vs an L or an S?

I have to explain the use of the cursive L to stand for liquid, so that students won't misinterpret it as a capital I.  Writing the lower case L in cursive was a trick my high school chemistry teacher insisted upon, and I insist upon it still, especially to avoid confusion with the symbol for chlorine versus carbon iodide as we learn more about writing and balancing reactions.

When the period ends, I collect their sheets.  I am mostly checking to see how they followed the lesson, or if they stopped writing.  The next day, I will follow up with some students about how these will be functioning as their notes, and that it is important to be sure to get all the information as we go so they can study later.

The content I am most interested in is their summary and the definitions of synthesis and decomposition reactions.  The student above substitutes one of the characteristics of synthesis reactions as a general rule for all reactions.  The next day, I needed to really hammer home the idea of atoms being re-arranged in reactions, and that there are different ways they might be re-arranged.  One of the key concepts in our district is differentiating between reaction types, so having a good definition now will help them later.