Energy Diagrams

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Objective

Students will be able to describe and label energy diagrams through taking notes and performing a lab activity.

Big Idea

Energy diagrams shows the relationship between the progress of a reaction and the relative energy of reactants' and products' bonds during a chemical reaction.

Introduction

  • In this lesson students are introduced to the idea of energy diagrams.  This is done through notes and a lab activity.
  • This lesson aligns with the Next Generation Performance Expectation of HS-PS1-4: Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.  It does so because students are learning about how the energy of the bonds within reactants and products change in chemical reactions and we can visualize this change with energy diagrams.
  • This lesson aligns with the Next Generation Crosscutting Concept 5: Energy and matter.  It does so because students are thinking about how to measure energy and how energy flows into, out of, and within systems.
  • This lesson aligns with three NGSS Science and Engineering Practices
    • 2: Developing and Using Models. It does so because students are coming up with a final model to show what happens when hot and cold water are mixed.
    • 3:  Planning and Carrying out Investigations.  It does so because students are performing a lab activities.
    • 4: Analyzing and Interpreting Data: Because students are analyzing data from their experiments.

For this lesson students need several resources for the Magnesium Oxide Lab experiment.

  • Because students are weighing magnesium they need to have access to an analytical balance.  I only have two balances of the precision so I have students go to the back of the classroom to weigh their Magnesium when they are ready (rather than having a balance on each station).
  • Additionally each Lab group needs the following:
  • Bunsen burner
  • striker
  • metal tongs
  • magnesium ribbon (about 4 cm)
  • weighing boat

 

Engage

15 minutes

To begin the lesson I have students think back to the previous lesson where they were learning about what happens when two different substances with different temperatures are placed together.  In particular I have them go back to their model paper and come up with a final model of what happens when equal amounts of 0 and 100 degree water are placed together.  The goal of this section is to make sure that students are putting together the idea that in chemistry there are many phenomena that occur at the particle level which we cannot see so we must rely on what we can see at the macroscopic level to infer what is occurring with the molecules.

  1. I begin by instructing students to get out their modeling paper from the previous lesson.
  2. I then show the first slide of the PowerPoint for the day and tell students that they should come up with a final model of what happens when equal amounts of 0 and 100 degree Celsius water are placed together.  I remind students that it is important to describe what happens both macroscopically (at the visual level) as well as molecularly (at the molecular level).  I also tell students that it does not matter what they had for their initial drawing and that this should be based on what they now know about temperature and heat.  This movie shows how I explain this to students.
  3. As students are finishing their models (after about 5 minutes) I tell students to reflect on how their final model is different than their initial model and to share with the partners at their table. 
  4. I give students about 5 minutes to do this and then I call on several students to share out what their partner told them.  As they share out I take their partner's paper to show the class the partner's initial and final models.
  5. Finally I tell students that we are going to continue thinking about heat and temperature today with our exploration of energy diagrams.
  6. These are some examples of students' initial and final models (example 1, example 2, example 3, example 4, example 5)

Explain

20 minutes

In this section of the lesson I introduce students to the idea of energy diagrams.  This builds on their knowledge from the previous lesson where we discussed exothermic and endothermic reactions including how in exothermic reactions products have less energy; whereas in endothermic reactions products have more energy. 

I present the information on the unit6 lecture 3- energy diagrams PowerPoint while students fill in their unit 6 lecture 3- energy diagrams student notes graphic organizer.

The lesson begins with looking at energy diagrams in terms of the reaction progress and energy of reactants and products.  I also discuss the concept of Activation energy (slides 3-5).  When I tell students about activation energy I use examples like "the spark needed to start the Bunsen burners" and "why you need to turn the ignition to get your car started"

I then discuss how there is a change in overall energy for a reaction ∆H which allows one to determine exothermic versus endothermic reactions (slides 6-7).

I then go over an example of an endothermic reaction (H2O (l) --> H2O (g)) and an exothermic reaction (N2 + 3H2 --> 2NH3) (slides 8-9).  Here is a video of my reviewing the second example with students.

Here is a copy of one student's filled in notes.

Elaborate

30 minutes

In this section of the lesson I have students perform a lab activity to better understand energy diagrams and how energy changes during a chemical reaction.

  1. I begin by passing out the Magnesium Oxide Energy Lab paper and telling students to get out their Periodic Tables and calculators.
  2. I then instruct students to spend 10 minutes looking over the lab and to answer the prelab questions at the bottom of the first page of the lab.   As students are reading and working on the questions I walk around the classroom to ensure that they are on task and to help answer any questions that they may have.
  3. After most students are done with the prelab questions I spend some time going over the answers.
  4. I then quickly go over the materials and procedure with students highlighting the safety concerns of working with Bunsen burners and magnesium ribbon.  In particular I remind them to wear safety goggles and that they should be careful when lighting the Bunsen burners.  I also remind them that they should keep flammables away from the burner, and to not look directly at the Magnesium when it burns. 
  5. I then break students into cooperative groups and release them to their lab benches to work.  For more details about how I do cooperative groups see my reflection on cooperative groups in unit 1 lesson 9: Density Part 2 and Review for further details.
  6. As students are working I walk around the classroom to ensure that they are on task and to check for their understanding.  Some of the examples of the questions I ask include:
    • How do you know that the reaction was endothermic or exothermic?  This movie shows me asking students this question.
    • Why did the temperature have to be so high in order to make the reaction occur? This movie shows me asking students about the high temperature to make the reaction occur.
    • What are your reactants?
    • What is your product?
    • How did you determine the amount of energy (analysis #3).  Did you show your work?
    • How do you know what the diagram looks like?  Did you label all of the parts to your diagram?
  7. As students complete their labs I have them clean up their areas and make sure that the lab is complete.  If students do not have time to complete the analysis in class I have them complete for homework.  If students are done I have them turn the lab into my basket.

Evaluate

I grade the Magnesium Oxide Energy Labs for correct answers using my magnesium oxide lab rubric

Here is an example of a student's graded lab that received all points.

Some of the common mistakes that students make include performing the conversions to figure out the amount of energy given off by the reaction (#3), why the reaction needs to be heated to begin (#2), and how to do the proper sketch of the energy diagram (#4).  For more details on this see my reflection below concerning student understanding in labs.