Reaction Rate Demonstrations

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Objective

SWBAT predict and observe rates of reactions of various chemical demonstrations, and explain their choices in terms of the Collision Theory of chemical reactions.

Big Idea

The rate of a reaction is controlled by the number of properly aligned collisions between reactants who have enough activation energy.

Background

This lesson comes two days after the Collision Theory Gizmo jigsaw.  In the day in between, students took their reaction rate quiz, and worked on revising their Alka-Seltzer investigation posters as needed.  That pause provided me a chance to assess where the classes were, and decide the next step.

Since the quizzes were sub-par, I decided to revisit reaction rates and collision theory via a series of demonstrations.

  • Mossy zinc with different concentrations of hydrochloric acid
  • Glow sticks in water baths at different temperatures
  • Lycopodium powder combustion
  • Decomposition of 30% hydrogen peroxide with and without a catalyst

I am hoping that the whole class talk through and a structured sheet to record their results on will help students break misconceptions and be ready for our unit exam the following Monday.

I have deliberately chosen to exceed the performance expectation below and include surface area and catalysts as I believe they are equally important to understand.  I do not include pressure as we have not dealt with gases at all this year, so students would not have the prior knowledge to process pressure efficiently.  Our district assessments are limited to discussions of temperature and concentration effects only, as per HS-PS1-5.

This lesson aligns to the following standards:

  • HS-PS1-5  Apply scientific principles and evidence to provide an explanation about the effects of changing temperature or concentration of the reacting particles on the rate at which a reaction occurs.
  • Science and Engineering Practice 6: Constructing Explanations
  • High School Scale, Quantity and Proportion Cross-Cutting Concept: Some systems can only be studied indirectly as they are too small, too large, too fast or too slow to observe directly.
  • High School Energy and Matter Cross Cutting Concept: Energy drives the cycling of matter within and between systems.

For each demonstration you need the following:

Concentration: 

  • 6 pieces of mossy zinc, approximately the same size. 
  • 3 large test tubes. 
  • Test tube rack. 
  • 3mL each of .1M, 1M and 6M HCl.

Temperature: 

  • 3 identical glow sticks
  • 3 400mL beakers to make an ice bath, room temperature water bath, and boiling water bath. 
  • 300mL water per beaker.
  • Hot plate

Surface Area:

  • Wire gauze with ceramic circle
  • Small pile of lycopodium powder (white flour works as well, but less dramatically)
  • 3cm Mg strip as fuse
  • Crucible tongs
  • Ring stand and Ring clamp
  • Large tray to catch excess lycopodium powder

Catalyst:

  • Two large test tubes
  • Test tube rack
  • 20mL of 30% hydrogen peroxide, 10mL per test tube
  • Microscoop of Manganese (IV) oxide

Reaction Rate Demonstrations

35 minutes

When the period begins, I return student quizzes, and explain that students can correct their quiz outside of class for half credit on the questions they missed.

When students ask how they did overall, I explain that there are some gaps we need to fill in prior to the unit exam on Monday.  I have a student pass out the Collision Theory and Reaction Rates Demo notes sheet to all students.  I ask students to immediately answer the top two questions, defining a rate and listing the three aspects of collision theory.

I enter attendance while the students are writing, and then we capture their responses on the board.  I explain that the students will see four reactions today, each representing a change that can be made to a chemical reaction to affect its rate.

I begin by refreshing students on the concept of concentration, that it represents how strong, or how much chemical is dissolved.  I then demonstrate the effect of changing concentration on the rate of reaction using mossy zinc and 1M and 6M hydrochloric acid.

When students are done with their observations, we connect it to the collision theory, and draw in how a chemical at high and low concentration would appear.  This is a student sample.

Next, I set up the demonstration to investigate the effect of changing temperature on the rate of reaction within glow sticks.

Again, we connect it to the collision theory together, and show how a higher temperature results in more molecules having the required activation energy.

To explore the effect of changing surface area on the rate of reaction, we have to go to the back of the lab.  I perform this demonstration under our exhaust fan, and over a large plastic tray to catch any stray lycopodium powder that escapes the flame.  This demonstration can be done with flour as well.

This student sample shows the temperature graphs, as well as their conclusions on how surface area changes connect to the collision theory.


I tend to talk more about surface area, as the inverse nature (large piece is low area compared to many small pieces) can confuse students.  The go-to example I use is human digestion vs snake digestion.  Since snakes swallow food whole, the stomach acid can only work on the outside, but since we chew our food into smaller bits, the acid acts faster and we digest faster.

Last, I demonstrate the effect of a catalyst (manganese dioxide) on the rate of the decomposition reaction of hydrogen peroxide.

Students can readily see how fast the reaction happened, and think since it was exothermic and steamed that catalysts provide extra kinetic energy.  I ask the students who investigated catalysts on the Gizmo to explain how it helps molecules fit together or break apart; improving the alignment of the particles in the reaction.

Test Review: Student Work Time

15 minutes

At the conclusion of the demonstration analysis, I pass out the Reaction Rate Review.  I give students the remainder of the period to work on it, as well as the first 15 minutes of the following day.

After I check in the reviews, we go over them as a whole class to ensure everyone is correct and understands each part.  Students are incredulous about the idea of surviving underwater due to low temperatures, so I share this recent story of a Utah toddler who survived a similar accident, although she was not submerged.  This article on the mammalian diving reflex is also good for students who are really interested in the topic.

I give the Kinetics Study Guide to students after going over the review. This allows students to organize their papers to study over the weekend, and provides a link to a set of online flashcards I made using the Flashcard Machine.  I made these to provide an additional study tool to help students prepare for the exam.