Reversible Reactions

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Objective

SWBAT use computer models to understand reversible reactions, and that one side -- reactants or products -- of the reaction may be favored.

Big Idea

Students can visualize reversible reactions using computer simulations.

Background

The previous lesson introduced students to reversible reactions and the concept of equilibrium.  Today's lesson will reinforce both concepts using a computer simulation to help us observe the particles as they "react".

We begin with a bellringer that I had planned to be the previous day's exit ticket.  Instead, I used the final part of their activity sheets to assess their understanding when they left the class.  Students understood that equilibrium meant equal exchanges, or rates, not equal amounts of reactants and products.  However, they struggled a little on the last two questions about how a reaction would proceed to equilibrium.

I am hoping that the PhET simulation used today will help answer that question, as some students will put the starting molecules in the reactant (A) side, and some will begin in the product (B) side.  If students do not change the activation energy slider or the energy levels of the reactants and products, they should reach the same equilibrium points given enough time.

This lesson is slightly shorter to fit in our 42 minute resource hour schedule.

This lesson aligns with the following standards:

  • High School Systems and System Models Cross Cutting Concept: Models can be used to simulate systems and interactions-- including energy, matter and information flows-- within and between systems at different scales.
  • Science and Engineering Practice 2: Develop and use models.
  • Science and Engineering Practice 4: Analyzing and interpreting data.
  • HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.

Bellringer

7 minutes

When students enter, I hand them the Introduction to Equilibrium Bellringer and ask them to complete it immediately.  While students are working, I take attendance, and sort the previous day's assignment by tables for quick return.

After 3 minutes, I project a copy of the bellringer using my document camera and ask for a student to explain what equilibrium means.  I got two common answers throughout the day:

  1. "The reactions are going at the same speed/amount"  "Which reactions?"  "Forward and backwards"
  2. "The amount of reactants and products becomes constant even though the reaction is still happening."

I ask which answer students think is the best answer, and they vote for the first one.

We then talk about how we know we are at equilibrium, again getting two common responses:

  1. "The amounts of each chemical are constant"
  2. "It looks like it stopped."

I like both of these, so I ask students to be sure they have both.  I ask what the difference is between question 1 and 2, and a student responds "One is talking about the molecules, one is what we can actually see."  This distinction is something I will put into the text of my bellringer questions for next year to make it even more explicit.

Last we review if equilibrium means equal.  Students are torn, with some responding yes and others responding no.  The difference is the remainder of their answer.  The students who said yes are referring to the equal rates of the forward and reverse reactions, whereas the students stating no are referring to the amounts of reactants and products.  Below are three samples of student responses.

I return their activity papers from the day before, and explain today's activity.  I instruct students with an asterisk by their name to bring their bellringers and activity papers to the front table for a quick conference with me.  Everyone else is to go back, log in to a computer, and get the simulation paper from the middle lab table.

PhET Simulation

35 minutes

As students transition to the back of the room, they pick up the Intro To Equilibrium PhET document.  The attached document is edited after my students performed it this year, so it will look slightly different than the student work.  These students begin logging into the computers and navigating to the PhET site.

The group of students I needed to touch base with come to the front of the room, and I ask to see what they wrote initially on their bellringers.  Most of these students either did not finish the activity the day before, or had difficulty still identifying the reactants and products and that the reactions are proceeding forward and backwards.  I show them the A <-->B model and walk them through how A turns into B in the forward reaction, but then B can turn into A in the reverse reaction.  After spending 2-3 minutes discussing their questions, I send them back to join their classmates on the computers.

The basics of the simulation are explained in this video.

Students gather the data pretty quickly, but get hung up on the ratio and the graph.  the revised assignment linked above removes the ratio, and runs the experiment for extra time to ensure students reach equilibrium.

Here is a student who recorded the data correctly, but didn't really work at making a quality graph.

This graph is more precisely done.

When finished, students are asked on the back to describe what it means to be at a dynamic equilibrium state.  Not all students got to this analysis question due to difficulties with creating their graphs.  Those that did were able to explain that it meant that the reaction was still occurring, with molecules changing (dynamic) but that the changes were equal, so it was at equilibrium.

When students finished, they turned in their work, some students took the assignment home to complete the graph due to the difficulties they had with it.