Temperature, Heat, Exothermic, and Endothermic Reactions

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Students will be able to differentiate between temperature and heat as well as exothermic and endothermic reactions through taking notes, performing lab activities, and drawing models.

Big Idea

Temperature is a measure of the average kinetic energy of molecules and can change as heat flows between objects. This change in heat for chemical reactions can be considered endothermic or exothermic.


In this lesson students are introduced to the concepts of temperature, heat, exothermic reactions, and endothermic reactions.  This is done through notes, lab activities, a model, and a computer 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.
  • 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
    • 3:  Planning and Carrying out Investigations.  It does so because students are performing 2 lab activities.
    • 2: Developing and Using Models. It does so because students perform multiple models to show what happens when energy is transferred between objects.
    • 6: Constructing Explanations: It does so because students are expected to explain why the chemical reaction in the second lab activity is endothermic or exothermic.

For this lesson students need several resources:

  1. They need access to the internet through computers or tablets.
  2. They need supplies for the two lab activities. 
    • For the first activity each group needs two beakers, a hot plate, ice water, a stirring rod, and a thermometer
    • For the second activity each group needs a thermometer, a container of water, and a ziploc bag with about a spoonful of sodium acetate trihydrate.



10 minutes

In this first section of the lesson I give students a chance to think about what we will be learning during the lesson and get at their prior knowledge concerning temperature and heat. 

I show students the second slide of the PowerPoint and prompt them to read along at the top of their notes graphic organizer as I read out loud.

I then read the scenario about the temperature of four different objects after sitting out in the classroom overnight.  After reading the scenario I also show them examples of similar objects in my classroom (a block of wood, a sheet of metal, a ball of wool, and a square of glass).

I instruct students to think about the scenario and choose the answer that they think is best.  I also tell them to try to describe WHY they think that their choice is the best.  Finally I tell students that if they are not sure to still come up with an educated guess based on their prior knowledge.

As students work I walk around the classroom.  When the majority of students seem to have chosen an answer and wrote out WHY they chose that answer (usually about 5 minutes) I instruct students to share with their table partners.

I then take a poll of the class for each answer. I do this by saying "Okay, we are going to see what we think is the best answer.  Raise your hands if  you felt that A was the best answer."  I then wait for students to raise their hands and then go onto B through D.

Finally I show slide 3 of the PowerPoint and go over the answer of "D" and explain why it is correct in terms of ambient temperature and they should have the same average kinetic energy.  I tell them that I understand it is tricky with how the object "feel" with some feeling cold and some feeling hot, but that we will explore that idea more within this unit when we discuss specific heat capacity.

This movie shows how I review the answers in my classroom.


15 minutes

For the next section of the lesson I have students begin to think about what happens when hot and cold substances are placed together.  To do this I have them do two things:

First, I have students perform a quick lab so that they have a chance to visualize what happens when they mix hot and cold water together.  

  • Some students already understand how the temperatures will even out, but other students do not necessarily understand this.  Therefore, doing this activity gives them the chance to see what happens. 
  • The instructions for the activity are on the fourth slide of the PowerPoint and in the middle of the first page of their notes. I instruct students to make sure to put on their safety goggles and then perform the lab activity. 
  • If you do not have a lot of time or do not want to do the setup for each of the lab groups this can also be done as a demonstration. 
  • This usually takes students about 10 minutes to complete.
  • This is a movie of the students doing the lab with my questioning students.
  • This is an example of a student's notes with the data for the lab in the middle of the first page.

Second, I have students draw a model of what happens when they mix equal amounts of 0 degree and 100 degree water together. 

  • The instructions for creating the model are on the 5th slide of the PowerPoint and students are to draw the model on the modeling paper in the first box for initial model. 
  • I remind them that models should both show what happens at the macroscopic (visual) level as well as at the molecular level.  
  • This usually takes students about 5 minutes.  As they are getting done I encourage them to share their models with their partners.
  • The goal of this initial model is to simply get at students prior knowledge and understanding before the lesson.  Having students do the activity allows all students to understand what the scenario is referring to.  Furthermore, by allowing students to get their ideas down on paper they are more likely to deepen their understandings when they do their final model later in the unit.
  • This is an example of a student's initial model.  Notice how the student is not able to show why the temperatures balance out or what happens at the particle level.

Explain I

5 minutes

For this next section of the lesson I present notes to students regarding heat and the different measurements for heat.  These notes are found on slides 6 and 7 of the PowerPoint and students fill in their notes on the bottom of their notes graphic organizer.

While I teach students the various terms and equivalence statements I point out that they are located on the word wall at the back of the classroom.   Here is a picture of the word wall.

Here is a copy of a student's filled in notes for this section.

Elaborate Part I

5 minutes

In this section of the notes students have a chance to practice some energy conversions with two examples. 

The practice problems are on slides 8-11 of the PowerPoint and students fill in answers at the top of the second page of their notes graphic organizer.

Because students have done lots of dimensional analysis they find these problems pretty easy.  I make sure to reiterate that they should underline what they know, circle what they want, and then set up the problem to convert the given to wanted.

This movie shows how I help students to solve problem #2.

Here is a copy of a filled in graphic organizer.

Explain Part II

10 minutes

In this next session of the lesson I introduce the concepts of temperature and heat on slides 12 and 13 of the PowerPoint.  Students fill in their notes on the second page of their notes graphic organizer.  When I talk about heat I show students a second Heat flow PowerPoint which shows how energy is transferred between two boxes (one hot and one cold).  

This second PowerPoint shows what happens at the molecular and macroscopic levels.  I have students perform a model of the reaction which I help them to complete by demonstrating what they should have on the board.

Here is a copy of a student's filled in notes.  Notice how on the second page their model shows what happens at the particle and macroscopic levels and also shows why this occurs.

Elaborate Part II

15 minutes

In this section of the lesson students have another opportunity to work with energy changes in a PhET activity. 

  • This activity is found on the PhET website.
  • I introduce the activity to students on the 14th slide of the PowerPoint and they fill in answers for the activity at the bottom of the second page of their notes graphic organizer.
  • I have students go up to one of the class computers with their table groups.  As students are working I walk around to ensure that they are all participating and writing down data. 
  • This activity is nice because students have a chance to both visualize what happens with energy and see how energy flows (from the warmer to cooler object).
  • This is a copy of a student's filled in notes.


Explain Part III

20 minutes

In this final section of the notes I explain to students the concepts of thermochemistry, how energy changes in chemical reactions, and the difference between exothermic and endothermic reactions.

These notes are found on slides 15-21 of the PowerPoint and students fill in their notes on page 3 of their notes graphic organizer.

While I explain the notes to students I present many examples to help them understand the concepts.

Here is a copy of the filled in notes.

Elaborate Part III

15 minutes

In this part of the lesson students perform another quick lab to check their understanding of chemical reactions and the difference between endothermic and exothermic reactions. 

  • I give directions to students on the final slide of the PowerPoint (slide 22) and students fill in data and analysis for the lab on the last page of their notes graphic organizer.
  • For this lab I use sodium acetate trihydrate which causes the temperature to drop in an endothermic reaction, but there are LOTS of other reactions that you could do depending on what is available in your classroom (for example epsom salts -magnesium sulfate, borax, ammonium chloride, calcium chloride).  I also do this reaction in a plastic baggie so it is easier for students to feel the temperature change.
    • This is a picture of the lab set up consisting of a thermometer, bag of sodium acetate trihydrate, and a bottle of water.
  • Here is a movie of my students doing the lab.
  • While students are performing this activity I have them come up with an explanation using the claim, evidence, and reasoning approach.
    • Here is a video of my helping students come up with their explanations.
    • An example of a student's explanation is, "In this lab the reaction was endothermic.  My data showed that the temperature dropped from 230C   to 140C .  In an endothermic reaction the reaction absorbs energy so the surroundings get cold (drop in temperature) which is what happened to the water in this experiment, that is why the reaction is endothermic."
  • Here is a picture of a student's completed lab.
  • This year I chose to have the lab simply be part of the notes, but if you want to grade the lab individually this is the rubric that I have used in past years.



The final section of this lesson is students' homework.  The homework is found on the last page (page 4) of the student notes graphic organizer

The homework gives me a chance to test students understanding of the lesson.  There are questions related to various parts of the lesson. 

I have students complete the homework and stamp for completion the next day.  I review the answers using the practice key

The problems that students find the most difficult include:

#3 Where they are finding food Calories as Kilocalories (Cal).  Some students get confused with converting between calories and Kilocalories (Cal).

#4 With some students not showing WHY they become the same temperature and other student not showing what happens at the particle level.

#5 Some students aren't sure how to describe the difference so I make sure to tell them to use the definitions to help differentiate.