LAB: Is This Reaction Endothermic or Exothermic?

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Objective

SWBAT identify reactions in lab as either endothermic or exothermic.

Big Idea

Measured changes in temperature can help us classify chemical reactions as either endothermic or exothermic.

Why This Lesson?

This lesson asks students to determine if specific reactions/processes are endothermic or exothermic based on observations collected during investigation.  Students are engaging in SEP 3 - Planning and Carrying Out Investigations as they conduct their investigations.  The are also engaging in SEP 6 - Constructing Explanations by using their data to determine if their observations indicate endothermic or exothermic behavior.

In order to later address Performance Expectation HS-PS3-1, students will need a firm grasp on energy transfer.  This lesson is the first time in our class that students are observing energy transfer in the form of heat and temperature changes.

 

Chemicals to use for this lab can be modified based on what is readily available from the stockroom.  

This particular year, I used the following:

  • Ammonium nitrate dissolving in water  (about 1 spoonful for each group)
  • Calcium chloride dissolving in water  (about 1 spoonful for each group)
  • Vinegar plus baking soda  (about 25 mL and 1 spoonful for each group)

 

Others I have used in the past include:

  • Citric acid plus vinegar
  • Sodium hydroxide dissolving in water
  • Potassium hydroxide dissolving in water
  • Barium hydroxide octahydrate plus ammonium chloride (solids, not solutions)


Warm-Up

5 minutes

While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books.  I use warm-ups to either probe for students' prior knowledge about the day's upcoming lesson or to have them bring to mind and review what they should have learned previously.  (To read more about Warm Up and Reflection Books please see the attached resource.)

Today’s Warm-Up: “What is the difference between 'endothermic' and 'exothermic?'”

In this case, the warm-up is asking students to draw upon their prior knowledge and previous discussions in this class about endothermic and exothermic reactions and processes.  It is also preparing students for today's activity during which they will be making observations about different reactions and processes in order to determine if they are endothermic or exothermic examples.

If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy.  In order to speed things up, my students have been trained to pass their books into the center of the table rows and stack them so that I can quickly pass by and stamp.  On days when there is too much business keeping, I do not stamp.  Students have been told that warm-ups are occasionally immediately checked and other times not.  At the end of each unit, Warm-Up/Reflection Books are collected and spot-checked.  Today, I do NOT collect books to the middle and stamp because I want students to have enough time to observe all of the specified reactions/processes and to analyze their results in order to conclude whether they are endothermic or exothermic.  Instead, we spend about 2-3 minutes discussing answers.

During our short discussion, I make sure that students understand that endothermic means energy is being absorbed and exothermic means energy is being produced.

Pre-Lab

10 minutes

I begin by handing out Chemistry - Endothermic & Exothermic Reactions.  I ask students to answer the first question, based on what we had already discussed and perhaps even clarified for some students in our warm-up discussion.

After students record their answers, I ask them to think about ways to determine if a process is endothermic or exothermic.  I tell students to record some of their ideas on their handouts. Most students know that the way to tell have something to do with observing temperature differences, but some have difficulty articulating what that would look like in real life.  In order to prompt their thinking in the right direction, I pose the question, "If something is absorbing heat, what would happen if you touched it?"  I want students to arrive at the conclusion that the heat from a hand would be absorbed--that there would be a heat flow of energy from the hand to the endothermic reaction.  As heat leaves the hand, it would feel cold because it is losing energy.  I also prompt discussion about exothermic processes by explaining that as heat is being produced, it would flow towards the hand, therefore feeling hot.

Student Investigation

30 minutes

This particular investigation can be done using whatever chemicals are on hand that would produce observable endothermic and exothermic results.  I deliberately have left the reactions blank on this handout because the reactions/processes we do varies from year to year depending on what chemicals I have in stock.  

This particular year, I used the following:

  • Ammonium nitrate dissolving in water
  • Calcium chloride dissolving in water
  • Vinegar plus baking soda

 

Others I have used in the past include:

  • Citric acid plus vinegar
  • Sodium hydroxide dissolving in water
  • Potassium hydroxide dissolving in water
  • Barium hydroxide octahydrate plus ammonium chloride (solids, not solutions)

 

I write these reactions on the whiteboard so that students can copy them onto their handouts and know what they will be combining.  I direct students to the lab stations where chemicals are available, and remind them to wear goggles for the duration of this lab.  Students are accustomed to using the lab equipment available in their equipment baskets in order to complete their investigations.  

As they collect observations by assessing if heat is flowing into the system or out of it, they also will be determining if the results indicate endothermic or exothermic characteristics.  While students work to gather information, I roam the room asking them questions and prompting their explanations of how to determine endothermic versus exothermic behavior.

Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt.  Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  The responses also allow me to see if there are any students who are missing the mark in terms of understanding.  The collection of responses in the composition books can also show a progression (or lack thereof) for individual students. 

Today, I ask students to instead of recording their answers in their books, to record their answers on their activity handouts, simply because we were pressed for time.

Today's Reflection Prompt:  "How could we have made today's investigation more quantitative?”

Desired student responses should indicate that measuring temperature and calculating changes in temperature would have yielded quantitative results.

My students had no trouble identifying quantitative measurements of temperature as the desired answer.