Defining Collision Theory

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Objective

SWBAT define the collision theory of reactions and explain activation energy and activation complexes.

Big Idea

Chemical reactions only occur when molecules collide with sufficient energy and proper orientation.

Background

The previous day students were introduced to the concept of the collision theory via an ExploreLearning Gizmo.  In assessing the student exploration papers, I found a wide range of correct and incorrect answers to the question "What three conditions are necessary for a chemical reaction to occur?"

In response to this student data, I felt strongly that we needed to come together and lay out the basics of the Collision Theory of chemical reactions in order for students to create quality experimental design to speed up the rate of reaction of Alka-Seltzer with water.

The quickest way to get this information into students' hands to work with is via direct instruction.  In previous years, I included how temperature, concentration, surface area and presence of catalysts affect reaction rate.  I pared my notes way back, so as to not directly tie each part to lab conditions so students will be able to perform their Alka-Seltzer labs in a true inquiry setting.

This lesson involves the following standards:

  • HS-PS1-5Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
  • Cause and Effect Cross Cutting Concept:  Systems can be changed to achieve a desired effect.  Students will decide on how to change their Alka-Seltzer and water system to change the reaction rate.
  • Science and Engineering Practice 1: Asking questions and defining problems.  Students will have to reason through how the collision theory applies to the macro world in terms of how to change the reaction rate between Alka-Seltzer and water.

 

Opener

5 minutes

I begin class by returning the student exploration sheets from the previous day.  I explain to the class that we were all over the place in terms of determining the three necessary aspects for chemicals to react. I inform them that we will be taking notes today to fill in the gaps on what is taking place at the molecular level when two chemicals react.

While I am passing out the Collision Theory of Reactions Notes paper I praise them for playing around the Gizmo and correctly identifying that temperature, concentration, surface area and presence of catalysts all can affect the reaction rates.  In the past, students have not persevered through their incomplete understandings and have quit on activities.  This time it was evident that they kept working and finished the Exploration in a strong fashion.

I have chosen to provide students with the outline today to ensure they have the visuals from each slide to study from.  I think the graphs and diagrams are very helpful in understanding the concepts in this lesson.

Whole Class Notes

32 minutes

Once I have attendance entered into the computer, I project the Collision Theory of Reactions PowerPoint. 

On the first slide, I ask students what the three parts of the theory are, to allow those who got it correct from the Gizmo an easy chance to participate.  I always teach it as three parts: Proper Kinetic Energy, Proper Alignment, and an actual Collision.  The first year I taught this without stressing that the molecules had to actually collide, and many of my students forgot that critical requirement.  I refer back to their experience with the Gizmo to ask if they saw each scenario of the molecules bouncing without reacting due to a lack of energy or alignment.

On the second slide, I open asking students which molecule set is moving the fastest.  We discussed the relationship of molecular kinetic energy and temperature in our Intermolecular Forces unit back in first semester.  Students correctly identify the bottom left diagram as representing the most energy, as the longer arrows indicate they are moving the fastest.

With the third slide, I perform a quick demonstration.  I show them an unlit candle and ask, "Why isn't it reacting with the oxygen to burn?"  "You haven't lit it"  "What does the match or lighter provide that it needs to react?"  "Energy"  I light the candle with a match and we break down the term, students are familiar with "activation" from activating their phones, so activation energy is the energy needed to "turn on" a reaction. 

The fourth slide I explain to students is a difficult concept, because it happens so fast.  The formation of an activated complex is just beyond our base curricular expectations, but I want to expose students to the idea because it may occur in resources I use after this day.  After explaining the slide, I ask a student to come up front to help me demonstrate.  (A video of this is present in the reflection)

I tell the student to stand so everyone can see, and put their hand out to shake hands.  First I reach for their hands and miss repeatedly.  Above, below, left and right.  I ask students "What part of the collision theory is missing from our handshake reaction?"  "Collisions"

Next I collide with the student's hand, but back to back and top to bottom.  Again, I ask "What part of the theory is missing?"  "Proper alignment"

Before finishing, I stress that this is my biggest handshake pet peeve, and a student yells out "Limp hands?"  We laugh and I say yes, and let the student grasp my hand without me moving.  I ask "what is missing this time?"  "Enough kinetic energy"

So I now give the student a proper handshake, and compare it with the activated complex, "We collided with proper alignment and energy, and it lasts for just a second."  I hold on extra long to show how that makes the handshake awkward, and connect it to the speed of the activated complex forming and breaking down.

The final slide I have students review, and then take examples of activation energy from their life.  They refer to the toaster cooking their bread, or anything cooking.  Straightening irons to cook down and straighten hair.  Lighters on various objects, both school appropriate and whispers of school inappropriate items.  I make sure they think of spark plugs starting the reaction in cars, as that example is often on standardized tests.

Alka Seltzer Investigation Introduction

5 minutes

While we are discussing the examples of activation energy from real life, I pass out the Alka Seltzer Investigation Introduction. (cut into half pages)  I ask how many students have taken Alka-Seltzer for upset stomachs or cold and flu symptoms?  It is less than a quarter of each class.

I explain that one of the most practical reasons to speed up reactions is for medicine to be faster acting.  We chat informally about how it is terrible to be sick, take medicine, and wait for what seems like forever for it to start to help.  I ask a student with experience what happens when Alka-Seltzer is dropped into water.  "It dissolves and bubbles"  I explain that their job will be to decide if they want to speed up the reaction, or slow it down, and they will have to design a lab that tests a change of their choice to meet their goal.