Lesson 7. Lesson 7: Making and Breaking Bonds

Carole Namowicz, Shruti Researcher
Chemistry
100 min
High School
v5

Overview

The following endothermic reaction is demonstrated for students:

2Ba(OH)2 + NH4Cl → 2BaCl2 + NH3 + 4H2O

They will easily identify it as endothermic as the temperature will drop rapidly after the solid reactants are mixed together. Students will answer some introductory questions about breaking and making bonds and what they believe occurred here.

Then students will use paper cut-outs of the atoms involved in the chemical reaction. This will allow them to indicate the initial bonds present in the reactants, to determine which bonds need to break to allow the rearrangement of atoms in the products, and finally what new bonds must be created to form the products. The goal is to determine if more bonds are broken or formed in the overall reaction, and to connect this with the entire reaction being endothermic or exothermic.

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. The Chemical Reaction
  • 2. Describing the Energy Transfer
  • 3. Modeling the Reaction With Paper Cutouts
  • 4. Determining Endothermic or Exothermic
  • 5. What if the number of bonds broken and formed are identical?

Student Directions and Resources


Most chemical reactions involve both breaking and making bonds. Whether or not energy is absorbed or released in a chemical reaction can be understood by looking at the difference between the number of bonds broken versus the number of bonds formed.

You will need the following resources to complete this assignment.

1. The Chemical Reaction


Your instructor will demonstrate the following chemical reaction using 32 grams of solid barium hydroxide, Ba(OH)2, and 10 grams of solid ammonium chloride, NH4Cl:

 

Ba(OH)2 (s) + 2 NH4Cl (s) → 2 NH3 (g) + BaCl2 (s) + 2 H2O (l)


Question 1.1

Record the temperature data from the chemical reaction.



Question 1.2

Is this an endothermic or an exothermic reaction? How do you know?

2. Describing the Energy Transfer


In order to describe the energy transfer in the reaction, first we must define both the system and the surroundings in the reaction as demonstrated.


Question 2.1

Define the system for this reaction.



Question 2.2

Define the surroundings for this reaction.



Question 2.3

Using the terms system and surroundings, describe the movement of kinetic energy from the start of the reaction to the end of the reaction.



Question 2.4

Using the terms system and surroundings, describe the movement of potential energy from the start of the reaction to the end of the reaction.

3. Modeling the Reaction With Paper Cutouts


  • Get a large whiteboard and whiteboard markers for your table group.
  • Create the starting molecules with the paper elements given. You should use all the paper materials you have (no elements should be left over). Draw bonds between atoms on the whiteboard.
  • Now, rearrange the molecules you have in order to create the products by breaking as few bonds as possible. Again, draw bonds between atoms on the whiteboard
  • Once you have a solid grasp on which bonds are not broken during the process, you can tape the atoms that stay bonded throughout the reaction together.

Question 3.1

Once you determine which bonds remain throughout the entire equation and which do not, count the total number of bonds that break and record that below.



Question 3.2

Now determine the number of new bonds that are formed and record that number below.



Question 3.3

Compare the total number of bonds broken versus the total number of bonds formed. Discuss this with your table group and the class to determine how this connects to your answer for question 1.2

4. Determining Endothermic or Exothermic


Comparing the total number of bonds broken to those formed is the first step in helping us determine if a reaction is endothermic or exothermic. On the previous page, you were provided with atom cutouts to better enable you to determine where bonds are located in the reactants and which bonds must be broken to form the products.

Drawing atom models is another way to go about this same process. In the questions below, you will be asked to draw atom models for both the reactants and the products in each chemical reaction. Then you will count both the bonds present in the reactants that must be broken in order to form the products, and the number of new bonds that must be formed to create the products.

You will then use the difference in the number of bonds broken vs. formed to determine if the overall chemical reaction is likely endothermic or exothermic. More bonds broken than formed correlates with an endothermic reaction as more energy must be added to the system to break the bonds, while fewer bonds broken than formed correlates with an exothermic reaction. An exothermic reaction is one is which energy is being released from the system.


Question 4.1

Draw a complete atom model for the following reaction:

2NaHCO3 → Na2CO3 + H2O + CO2

Note: Draw your sketch in the sketchpad below


Question 4.2

Count the number of bonds broken in the reactants versus the new bonds formed in the products to determine if the reaction is endothermic or exothermic.

2NaHCO3 → Na2CO3 + H2O + CO2



Question 4.3

Draw a complete atom model for the following reaction:

2H2O2 → 2H2O + O2

Note: Draw your sketch in the sketchpad below


Question 4.4

Count the number of bonds broken in the reactants versus the new bonds formed in the products to determine if the reaction is endothermic or exothermic.

2H2O2 → 2H2O + O2



Question 4.5

Draw a complete atom model for the following reaction:

2Al + Fe2O3 → 2Fe + Al2O3

Note: Draw your sketch in the sketchpad below


Question 4.6

Count the number of bonds broken in the reactants versus the new bonds formed in the products to determine if the reaction is endothermic or exothermic.

2Al + Fe2O3 → 2Fe + Al2O3

 

5. What if the number of bonds broken and formed are identical?


As it turns out, all bonds are not created equal. Some bonds have more energy associated with them than others. A stronger, more energetic bond requires more energy to be broken, and releases more energy when it is formed. The end of the video below shows an example chemical reaction where two bonds are broken and two bonds are formed.