The Energy of Phase Changes

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Objective

Students will be able to explain how energy plays a role in phase changes and be able to perform calculations using latent heat of fusion and vaporization through taking notes, working with partners, watching a video, performing a lab, and answering practice questions.

Big Idea

The phase of matter depends on the energy of the particles, and phase changed require either the absorption or release of energy.

Introduction

In this lesson students learn about how energy plays a role in phase changes.  This is done through notes, a video, practice questions, and a lab.

  • This lesson aligns with the Next Generation Performance Expectation of HS-PS3-1 Create a computation model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.  It does so because students are learning how to apply the equations of latent heat of fusion and latent heat of vaporization to compute the energy absorbed or released during phase changes.
  • This lesson aligns with the Next Generation Crosscutting Concept 5: Energy and matter.  It does so because students are learning about how energy plays a role in phase changes.
  • This lesson aligns with three NGSS Science and Engineering Practices
    • 3:  Planning and Carrying out Investigations.  It does so because students are performing 2 lab activities.
    • 4: Analyzing and Interpreting Data: It does so because students are analyze data from their heat of fusion lab.

For this lesson each lab group needs several resources:

  • ice
  • warm water
  • thermometer
  • 2 Styrofoam cups
  • plastic spoon
  • stirring rod
  • 100mL graduated cylinder

Engage

15 minutes

To engage students in the lesson I have them perform several quick review questions.

  • I show the questions to the students and give them 5 minutes to work on them on their own.  I tell them to pretend that the questions are a quiz and to try them on their own first.  I tell them that if they are unsure of the answers that they should go back and look at their previous notes and if they are still stuck to work with the other students at their tables.
  • After about 5 minutes I go over the answers with the students.  This video shows me going over the first question with students.  Many students get confused by the last question where I am simply expecting the answer "that to heat something up means to transfer energy from a warmer to colder substance"
  • Here is a copy of one student's filled in notes graphic organizer with their answers to the quick review questions.  

After the questions I introduce the lesson and then have students talk in their groups about these questions on slide 3 of the PowerPoint:

  1. If you want to make water boil, what do you have to do?  What do you think is happening at the molecular level?
  2. When we get out of the shower we feel cold.  Why is this?
  • I give students about 5 minutes to think about the questions and then have them share out what they have come up with.  
  • I then explain that they have to do with the fact that it takes energy to vaporize. 
  • This video shows me going over the answers with my students.  Notice how I call on students based on their groups.  Also I make sure to validate answers even if they are not correct to help reinforce students sharing ideas.

 

Explain

40 minutes

In this section of the lesson I present notes to students on the Phase Changes PowerPoint while students fill them in on their graphic notes organizer.

  • I begin by reviewing states of matter on slides 4 and 5 emphasizing how states of matter is dependent on the energy of particles.
  • I then talk about the phase change diagrams on slide 6 and how the various phase changes are exothermic or endothermic on slides 7 and 8
    • I make sure to show students how energy is added as one goes from left to right on the graph so those are endothermic processes and the opposite is true from right to left so those are exothermic processes.
  • I then show students a Changes of State movie and have them answer questions located on slide 9 as they watch the video.  When the video is over I give them a few minutes to check answers with their table groups and ask me for help if they missed any answers.
  • For the next portion of the notes I discuss the concept of enthalpy with students on slide 10 and how to calculate the amount of energy gained or released in phase changes with the latent heat of fusion on slide 11 and the latent heat of vaporization on slide 13.  As I go over these calculations I have students perform practice questions on slides 12 and 14.
    • I have chosen to only teach students the values for calories or joules per gram and not per mole.  I then teach students how they can convert from moles to grams if they are given moles in a question.
    • This is a movie of my going over the answers to the second practice question stressing how to solve problems in chemistry.
  • Finally I finish the lesson by showing students slide 15 and having students think back to my initial questions of "If you want to make water boil, what do you have to do?  What do you think is happening at the molecular level?  When we get out of the shower we feel cold.  Why is this?" I have students discuss them with their table groups to see if these concepts now make more sense. 
    • Students tend to better understand why these phenomenon make sense after having discussed them in the notes.
  • This is a copy of one student's filled in notes with answers to the video questions and practice enthalpy change questions.

Elaborate

50 minutes

To help reinforce the concepts that we learned in the lesson I have students perform a heat of fusion lab where they calculate the heat of fusion of ice.

  • I begin by passing out the Heat of fusion lab paper.
  • I then have students take several minutes to read over the lab
  • I then tell students that the lab's procedures are fairly straightforward and they should make sure to work with their groups to ensure that it is done properly.
  • I next break students into cooperative groups and have them go up to the lab stations.  I emphasize that they should take the roles seriously and to make sure that they are all answering the questions appropriately.  I also tell them that they should all have calculators to perform the calculations on their own and then check their answers with their lab partners.  Finally I tell them that when they are ready for ice that it is located at the front of the classroom and they should send one student over to collect it.
  • As students are working I walk around to help answer questions and make sure that they are staying on task.  In order to do this I ask students questions as they work.  Some examples of the questions I ask include:
    • How did you figure out the mass of the water without using a balance?
    • What is the goal of the lab?
    • What are you trying to do when you are stirring the ice/water mixture?  This quick movie shows my students responding to this question.
    • How did you determine the mass of your melted ice?
    • How did you determine the experimental value?
    • How close were your results?
    • What is a source of error?
  • There are several parts of the lab that students struggle with:
    • Some students forget that ml and g are the same for water because its density is 1g/ml so I have to make sure to help them as I walk around the classroom.
    • Some students have a final volume larger than 100mL so need to measure the water in the cylinder twice (fill to 100mL, pour out, then pour in the remaining and add together).
    • To make sure that they have units on all of their answers.  I make sure to remind them of this throughout their lab.
  • As students finish their labs I have them turn them into my basket and then have them start their homework. 

Evaluate

When students complete their Heat of fusion labs they turn them into my basket and I grade them using heat of fusion lab rubric.

There are several responses for sources of error including:

  • getting the ice out in a timely manner after the temperature reaches zero otherwise more ice melts than expected.
  • getting ice with the water for the final volume so the volume reads higher than it should.
  • having a final volume more than 100mL so not getting an accurate final reading.
  • Having a thermometer that does not properly work so not reading the proper temperature.

In terms of the goal of the lab I had expected students to write that they it was found that the heat of fusion of ice was whatever they had calculated and that evidence from their experiment that supports the claim was the data that they had collected.  However, I got various responses to this question including:

  • It was found that when ice is added to water that the water cools down.
  • It was found that it takes ---joules to melt ice.

Some of the common mistakes that student had in the lab include:

  • Not including units in their answers or having incorrect unit.
  • Not using the correct values in the analysis (for example in analysis #1 using a value besides 75g).
  • Having conclusions that do not make sense with the overall goal of the lab.

Here are several examples of graded student lab.  Note that they have different responses for the conclusions and that they have relatively low percent errors.

When students are done with their labs I instruct them to start their homework which is on the last page of the unit6 lecture 4 phase changes graphic notes organizer.

  • Students complete this homework at home and I stamp for completion the following class period. 
  • I then review the answers while students check their answers and make corrections. 
  • Here is an example of a student's homework with corrections made except for #6 where I was expecting students to write about how water evaporates so takes energy from our bodies so we feel cold.
  • Some of the common confusions of students include:
    • filling in the chart for question #1 with spacing and order.
    • the equations with Q=mcT where students had to solve for grams (#4) and where students had to first convert from mole to gram (#3).
    • Explaining what happens when we come out of the shower (#6)