Lesson 3. Lesson 3: Energy Transfer When Bonds Break

Carole Namowicz, Shruti Researcher
Chemistry
100 min
High School
v5

Overview

In this lesson, students will utilize a simulation that shows the dissolving of an ionic compound in water. Water molecules must collide with the ionic compound in the proper orientation and with enough kinetic energy for the ionic compound to separate into constituent cations and anions.

The activity will force students to equate the reduction in the temperature of the water with the breaking of the ionic bond. The kinetic energy of the water is transferred to break the ionic bonds. This is an endothermic process. Students will only be able to recognize this when they view the simulation with a smaller number of water molecules present (~10).

After students develop the understanding that breaking bonds is an endothermic process, they will be asked to model bonds breaking by showing the changes in KE and PE of the system (ionic compound) and the surroundings (water).

Standards

Next Generation Science Standards
  • Physical Science
    • [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.
    • [HS-PS3-2] Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative positions of particles (objects)
Computational Thinking in STEM
  • Data Practices
    • Collecting Data
    • Creating Data
    • Manipulating Data
    • Visualizing Data
    • Analyzing Data
  • Modeling and Simulation Practices
    • Assessing Computational Models
    • Using Computational Models to Understand a Concept
  • Systems Thinking Practices
    • Communicating Information about a System
    • Investigating a Complex System as a Whole
    • Thinking in Levels
    • Understanding the Relationships within a System

Activities

  • 1. Temperature Changes of Ionic Compounds Dissolved in Water
  • 2. Exploring the Simulation
  • 3. Behavior of the Overall System
  • 4. Behavior of Individual Molecules
  • 5. Reviewing the Simulation

Student Directions and Resources


In this lesson, we will focus on the transfer of energy that occurs when bonds are broken. In the previous lesson, you added several different solid ionic compounds to water. When ionic compounds are added to water, ionic bonds are broken due to the attraction of the cations and anions that make up ionic compounds to the partially charged sides of water molecules.

You will need the following resources to complete this assignment.

1. Temperature Changes of Ionic Compounds Dissolved in Water


Let's review what occurred in lesson 2 by looking at Temperature vs Time graphs for three of the ionic compounds we tested.

         KCl: 

    NH4Cl:  

  NaNO3:  


Question 1.1

Use the graphs above to identify temperature changes from the three representative ionic compounds shown. Recall we also tested these same ionic compounds last class.



Question 1.2

Describe the temperature change that occurred with the ionic compounds tested.

2. Exploring the Simulation


Observe the simulation by hitting "setup" and then “go/stop”. After you feel you have learned all the features of the simulation move on to answer questions 2.1 and 2.2 below.


Question 2.1

What temperature scale (Celsius, Fahrenheit, Kelvin) is represented on the graph? Explain how you know this.



Question 2.2

What state of matter are the water molecules in the simulation? Explain how you know this.

3. Behavior of the Overall System


The simulation you were working with models dissolving KI in water. You have control over the exact number of water molecules represented in the “beaker”. You will begin by looking at the overall bulk system.

 

Directions:

  • Adjust the slider bar to the maximum number of water molecules possible. Then press "setup". Press "go" and observe the water molecules moving around the beaker in liquid form.
  • Now press “add KI” and observe what happens to the ionic compound and the temperature of the beaker.
  • Allow the simulation to run until all KI is split into K+ cations and I− anions.
  • You may repeat the procedure as needed to answer the questions below.


Question 3.1

Calculate the change in temperature of the system by recording the initial and final temperatures of the system below. (Hover your cursor over the graph to obtain exact initial and final temperatures.)



Question 3.2

What action in the simulation corresponds to the specific temperature drops shown on your graph? If you are not able to determine this, reduce the number of water molecules until you are able to observe how the molecular motion of both the water molecules and the ionic compound corresponds to the temperature drop.

4. Behavior of Individual Molecules


Now you will use the same simulation, but this time you will be observing individual molecules.

 

Directions:

  • Adjust the slider bar to 10 water molecules. Then press "setup" and "go".
  • Now press “add KI” and allow the simulation to run until all KI is split into K+ cations and I− anions.
  • You may repeat the procedure as needed to answer the questions below.


Question 4.1

What must occur for an ionic bond to break and KI to separate?



Question 4.2

It takes the KI noticeably longer to separate into separate cations and anions here. Describe why this is so.



Question 4.3

Calculate the ∆T of the system by recording the initial and final temperatures of the system below. (Hover your cursor over the graph to obtain exact initial and final temperatures.)



Question 4.4

Compare your answer to Question 4.3 to your answer for Question 3.1 (referenced again at the top of this page); are the temperature differences identical? Why or why not?



Question 4.5

Start the simulation over and press “watch an H2O molecule”. Wait for that molecule to collide with KI. What observation can you make about the relative speed of a water molecule before and after it collides with KI?



Question 4.6

Explain why you think the change in relative speed of a water molecule occurs. Where does the energy go?



Question 4.7

Based on your observations of the simulation, make a claim of what molecular interaction could cause the kinetic energy of the water molecules to increase (other than simply adding heat to the beaker via a hotplate or flame).

5. Reviewing the Simulation


A GIF of the simulation you have been working with is shown below. Observe that a water molecule slows down and seems to lose kinetic energy when it collides with the ionic compound KI. This collision causes the ionic compound KI to split into a K+ cation and an I− anion.

Where does the kinetic energy of the water molecule go? We will explore this question further in your homework and again in the next lesson.